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Musings of an Energy Nerd

How to Build an Insulated Cathedral Ceiling

Whether you decide to make it vented or unvented, get the details right — because every cathedral ceiling offers opportunities to make big mistakes

Image 1 of 3
A leaky ceiling doomed the roof sheathing. The ceiling of this ten-year-old house was cut for an electrical box, and the oversized hole was never caulked. Moisture-laden air entered the rafter bays through these cracks. When the moisture encountered cold roof sheathing, it condensed, leading to moisture accumulation and rot. (The rubberized membrane in the photo was used as a temporary patch for a hole in the roof, and did not contribute to the failure mechanism.)
Image Credit: Peter Yost
A leaky ceiling doomed the roof sheathing. The ceiling of this ten-year-old house was cut for an electrical box, and the oversized hole was never caulked. Moisture-laden air entered the rafter bays through these cracks. When the moisture encountered cold roof sheathing, it condensed, leading to moisture accumulation and rot. (The rubberized membrane in the photo was used as a temporary patch for a hole in the roof, and did not contribute to the failure mechanism.)
Image Credit: Peter Yost
If you want to install fluffy insulation between the rafters of an unvented cathedral ceiling, you must also install enough rigid foam insulation on top of the roof sheathing to keep the sheathing above the dew point. The only other acceptable approach would be a flash-and-batt installation using closed-cell spray foam on the underside of the roof sheathing.
Image Credit: Fine Homebuilding
This insulated sloped roof assembly locates all of the insulation on the exterior side of the roof sheathing. A ventilation channel is provided between the top of the insulation and a second layer of roof sheathing.
Image Credit: Fine Homebuilding

UPDATED on August 31, 2018 with information on vapor diffusion ports.

Although the GBA website already contains many articles on the topic, we continue to receive frequent questions about the best way to insulate a cathedral ceiling. It’s therefore time to pull together as much information on the topic as possible and publish it in one place, to clarify the building science issues and code requirements governing insulated sloped roofs.

In this blog, I’ll attempt to answer the following questions:

  • Does a cathedral ceiling need to be vented?
  • What’s the best way to build a vented cathedral ceiling?
  • What’s the best way to build an unvented cathedral ceiling?
  • What do building codes require?
  • What risky practices should be avoided?

What is a cathedral ceiling?

This article will discuss insulated sloped roofs. The methods described here can be used to build an insulated cathedral ceiling over a great room, a section of sloped roof above a kneewall, or any similar section of insulated sloped roof.

This type of roof differs from an uninsulated roof over an unconditioned vented attic.

A brief history of cathedral ceilings

Insulated cathedral ceilings are a relatively recent phenomenon. The craze for insulated cathedral ceilings (and great rooms) really took off in the 1970s and 1980s, when examples began popping up like mushrooms after a warm rain. In those days, most builders stuffed cathedral ceiling rafter bays with fiberglass batts. Sometimes they included flimsy Proper-Vents between the fiberglass and the roof sheathing, but often they just specified thin batts to ensure that there would be an air space above the batts for ventilation.

The cathedral ceilings of the 1970s and 1980s were thermal disasters. In most cases, these ceilings leaked air, leaked heat, created monumental ice dams, and encouraged condensation and rot. In many cases, roofers tried to solve these problems by improving ventilation openings in the soffits and at the ridge; these “improvements” often made every symptom worse.

Fortunately, most builders have learned a few lessons from these disasters.

Minimum R-value requirements

Energy codes establish minimum R-value requirements for roofs and ceilings. There are several possible code compliance paths; most builders choose the prescriptive path, which sets forth minimum R-values for roofs and ceilings in a prescriptive table. This prescriptive table is known as Table N1102.1.1 in the IRC; in the IECC, the identical table is known as Table R402.1.2.

The minimum prescriptive requirements for ceiling (roof) R-value haven’t changed in years; the requirements in the 2018 code are the same as those in the 2015 and 2012 code. These requirements are:

  • In Climate Zone 1, a minimum of R-30;
  • In Climate Zones 2 and 3, a minimum of R-38;
  • In Climate Zones 4 through 8, a minimum of R-49.

Green builders usually try to meet or exceed these minimum R-values. That said, the code provides several loopholes allowing builders (in some cases) to get away with lower ceiling R-values than required in the prescriptive table. For more information on these loopholes, see Three Code-Approved Tricks for Reducing Insulation Thickness.

Does a cathedral ceiling need to be vented?

Until recently, building codes required that insulated sloped roofs include ventilation channels directly under the roof sheathing. Many builders still follow this time-tested technique.

As building scientist Bill Rose has shown, code requirements for roof venting were never based on research or scientific principles. In a well documented JLC article on roof venting (“Roof Ventilation Update”), Rose explained, “For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. … Once this is accomplished, roof ventilation becomes pretty much a nonissue.”

For more information on venting roofs, see All About Attic Venting.

Because of their unscientific origins, code requirements for venting roofs are often misunderstood. It’s worth establishing a few basic facts:

  • Roof ventilation cannot be used to lower indoor humidity levels.
  • Builders should not encourage the migration of water vapor through a cathedral ceiling.
  • During the summer, roof ventilation does not significantly lower the temperature of asphalt shingles or other types of roofing.
  • While roof ventilation can lower the risk of ice damming, it’s essential for builders to limit the flow of heat into roof ventilation channels by including one or more ceiling air barriers and by installing thick insulation, so that as little heat as possible escapes from the home.
  • While roof ventilation can help dry out damp roof sheathing, it’s essential to limit the flow of water vapor escaping from the home so that the roof sheathing never gets damp in the first place.
  • In the absence of an airtight ceiling, roof ventilation can do more harm than good, since air movement in rafter bays can encourage indoor air to leak through ceiling cracks.

How do I build a vented cathedral ceiling?

A vented cathedral ceiling only makes sense if the geometry of your roof is simple. You need a straight shot from the soffits to the ridge. That’s relatively easy on a gable roof without any dormers or skylights, but if the geometry of your roof is complicated — with features like hips, valleys, and dormers — it’s impossible to assure air flow through all of your rafter bays.

If you’re trying to insulate a roof like that, consider building an unvented roof.

Ventilation channels are created by installing a material that can maintain a separation (an air gap) between the insulation and the roof sheathing. This building component is known by a variety of confusing names, including a ventilation (or vent) baffle, a ventilation (or vent) chute, a ventilation (or vent) channel, or a Proper-Vent (a brand name).

The first vent baffles to hit the market — the classic Proper-Vent of the 1970s and ’80s — were inexpensive, flimsy items made of thin polystyrene. Polystyrene baffles have several disadvantages: being thin and flexible, they can’t resist the pressures from dense-packed cellulose or blown-in fiberglass; they don’t ventilate the entire width of the rafter bay; and as usually installed, they allow air to leak out the top of the insulated assembly.

Eventually, manufacturers began offering stiffer alternatives that are better able to resist the pressures of dense-packed insulation. These products come and go, and many are no longer available. At one time or another, it was possible to buy baffles made of polystyrene, cardboard, vinyl, and compressed cellulose fibers. These days, the best available vent baffle is probably the SmartBaffle, which is made from polypropylene.

Site-built ventilation baffles

Some builders aren’t satisfied with commercially available vent baffles, so they make their own site-built baffles. (For more on this topic, see Site-Built Ventilation Baffles for Roofs.)

According to section R806.3 of the 2006 International Residential Code (IRC), “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.” Such a vent space can be created by installing 1 inch by 1 inch “sticks” in the upper corners of each rafter bay, followed by stiff cardboard, thin plywood, OSB, fiberboard sheathing, or panels of rigid foam insulation. (If you use rigid foam for your baffles, it probably makes more sense to choose thin EPS or XPS rather than foil-faced polyisocyanurate, to allow a bit of outward drying, however slow, by diffusion. A thin layer of EPS or XPS is somewhat vapor-permeable, while foil facing is a vapor barrier.)

Many experts advise that 2-inch-deep vent cavities are even better than 1-inch-deep cavities; if that’s the route you want to go, size your spacers accordingly.

As with all types of vent baffles, it’s a good idea to pay attention to airtight construction methods, especially if you will be installing air-permeable insulation in the rafter bays. Seal the edges of each panel with caulk, and tape the seams between panels with a high-quality tape. (If you are installing air-permeable insulation like fiberglass, mineral wool, or cellulose, the ventilation baffle isn’t optional; it’s required. Air-permeable insulation materials need to be enclosed by an air barrier on all six sides. If you don’t install a sealed ventilation baffle above the insulation, the thermal performance of the insulation will be degraded by wind washing.)

Creating vent channels above the roof sheathing

If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. If you decide to do this, make sure that any roofing underlayment that you install above the roof sheathing is vapor-permeable — for example, #15 asphalt felt, VaproShield SlopeShield, or Solitex Mento — and that your local building department accepts this approach to roof venting. If you install ventilation channels on top of a vapor-impermeable synthetic roofing underlayment, the flowing air won’t be able to help dry out the roof sheathing.

If you plan to install ventilation channels above your roof sheathing, it’s best to choose a roof sheathing that is vapor-permeable (for example, fiberboard). If you use plywood or OSB, there’s a small chance that the sheathing can still accumulate worrisome amounts of moisture over the winter; this is especially true for north-facing roofs.

You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels.

In some cases, these ventilation channels are installed above a layer or two of rigid foam. It’s also possible to purchase nailbase (a type of SIP with OSB on one side instead of two) that includes integrated ventilation channels between the OSB and the rigid foam; one brand of these panels is Cool-Vent from Hunter Panels.

If you are choosing to build a vented roof assembly, don’t forget to include soffit vents and ridge vents.

How do I know if my soffit vents and ridge vents provide enough air flow?

As I noted earlier, researcher Bill Rose has exposed the unscientific nature of code requirements and formulas for calculating roof ventilation openings. Unscientific or not, these code requirements must be followed.

Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.

Manufacturers of soffit vents and ridge vents usually specify the net free vent area of their products on product packaging or in specifications available online.

Are my rafters deep enough?

Most rafters aren’t deep enough to accommodate the insulation needed to meet minimum R-values required by code, especially if the rafter bays include a ventilation channel. For example, 2×10 rafters are 9 1/4 inches deep, so they only provide room for about 8 1/4 inches of insulation — in other words, about R-30 of fibrous insulation — if the rafter bay is ventilated. This is less than the minimum code requirement in colder climates.

Builders solve this problem by furring down or scabbing on additional framing below the rafters to deepen the rafter bays. Another technique is to add a layer of cross-hatched 2x4s, 16 inches on center, installed beneath the rafters. It’s also possible to specify deep open-web trusses or to use deep I-joists for rafters.

Another way to add R-value to your roof assembly is to include one or two layers of rigid foam in the roof assembly — either above the roof sheathing or below the rafters. In addition to improving the R-value of the roof assembly, a layer of rigid foam has another benefit: it interrupts thermal bridging through the rafters.

Remember: if you choose to install rigid foam on top of your roof sheathing, don’t install ventilation channels under the roof sheathing; these two practices are incompatible.

Can I build an unvented roof assembly?

It is quite possible to design an unvented insulated roof assembly that performs well, as long as you get the details right. In recent years, most building codes have begun to allow the construction of unvented insulated sloped roof assemblies. Many such roofs have failed over the years, however, so don’t get creative. Follow the rules.

First of all, you can’t use air-permeable insulation (for example, fiberglass batts, mineral wool batts, dense-packed cellulose, or blown-in fiberglass) to insulate an unvented roof assembly unless the roof assembly also includes a layer of air-impermeable insulation (either spray polyurethane foam or rigid foam panels) directly above or directly below the roof sheathing.

The 2009 IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” Although spray foam insulation and rigid foam insulation meet this standard, fiberglass batts and dense-packed cellulose do not.

If you want to use just one type of insulation in unvented rafter bays, you are limited to spray polyurethane foam. Another possibility, of course, is to build your roof with structural insulated panels (SIPs).

The code restrictions on the use of air-permeable insulation between rafters were developed to prevent the roof sheathing from rotting. When fiberglass batts are installed in unvented rafter bays, the batts allow moist indoor air to reach the cold roof sheathing. That leads to condensation or moisture accumulation in the sheathing, followed eventually by sheathing rot. Since spray foam prevents air movement, it almost eliminates this problem.

It’s important to note, however, that recent research suggests that closed-cell spray foam is much less risky than open-cell spray foam in this location. For more information, see Open-Cell Spray Foam and Damp Roof Sheathing.

To summarize, there are four ways to build an unvented roof assembly:

  • Install closed-cell spray foam against the underside of the roof sheathing, and no other type of insulation. Be sure that the thickness of the spray foam is adequate to meet minimum code requirements. Remember that open-cell spray foam is risky in all climate zones, and if open-cell spray foam is installed in this location in a cold climate, the underside of the cured foam must be covered with gypsum drywall that has been painted with vapor-retarder paint. Vapor-retarder paint is ineffective if it is sprayed directly on the cured foam.
  • Install rigid foam insulation above the roof sheathing and air-permeable insulation between the rafters. This type of assembly is designed to dry to the interior, so the assembly should never include an interior polyethylene vapor barrier. If you choose this method, it’s possible (though not necessary) to install vent channels between the top of the rigid foam and the top layer of roof sheathing by installing a series of parallel 2x4s — one above each rafter — extending from soffit to ridge. (For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.)
  • Install a layer of closed-cell spray foam against the underside of the roof sheathing, and fill the rest of the rafter cavity with an air-permeable insulation. Again, this type of assembly is designed to dry to the interior, so the assembly should never include an interior polyethylene vapor barrier. (In this case, the closed-cell spray foam prevents the roof sheathing from drying toward the interior if the sheathing gets damp. But wintertime condensation is theoretically possible on the interior side of the cured spray foam, especially if the spray foam layer has thin areas. Because of this possibility, it’s best to allow inward drying.)
  • Install a vapor diffusion port at the ridge. This approach, which is only permitted in Climate Zones 1, 2, and 3, was recently approved in the 2018 International Residential Code. For more information, see “Vapor Diffusion Ports.”

What about the cut-and-cobble method?

Cut-and-cobble is an insulation method used by some homeowners, but never by insulation contractors. It involves cutting rigid insulation into narrow rectangles, and inserting the rectangles between rafters or studs. In most cases, the perimeter of the each rectangle of rigid foam is sealed with canned spray foam or caulk.

When it comes to cathedral ceilings, here’s the rule: the cut-and-cobble method can be used for vented cathedral ceilings, but not for unvented cathedral ceilings. (There have been several reports of moisture problems in unvented cut-and-cobble cathedral ceilings.)

For more information on this issue, see Cut-and-Cobble Insulation.

If I use a combination of foam and fluffy insulation, how thick should the foam be?

If you want to install a combination of rigid foam on top of your roof sheathing and air-permeable insulation between your rafters, you need to be sure that your rigid foam is thick enough to keep your roof sheathing above the dew point. Guidelines to achieve that goal are included in the 2009 and 2012 International Residential Code (IRC).

According to section R806.5 of the 2012 IRC, “Unvented attic assemblies (spaces between the top-story ceiling joists and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted” as long as a number of conditions are met.

If you want to combine air-permeable and air-impermeable insulation, there are two possible ways to proceed. One option (according to the code) requires: “In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.”

Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly) . The table calls for a minimum of:

  • R-5 foam for Climate Zones 1-3,
  • R-10 for Climate Zone 4C,
  • R-15 for Climate Zones 4A and 4B,
  • R-20 for Climate Zone 5,
  • R-25 for Climate Zone 6,
  • R-30 for Climate Zone 7, and
  • R-35 for Climate Zone 8.

After you have installed at least the code-mandated thickness of rigid foam above your roof sheathing, you should install the balance of your required insulation (in most cases, by installing an air-permeable insulation material like fiberglass, cellulose, or mineral wool) below the roof sheathing. Note that both types of insulation — the rigid insulation above the roof sheathing, and the fluffy insulation below the roof sheathing — need to be in direct contact with the roof sheathing.

For more on this topic, see these two articles:

If you want to install a combination of closed-cell spray-foam on the underside of the roof sheathing and air-permeable insulation between your rafters — an approach sometimes called “flash and batt” — the building code requires that spray foam (or, arguably, rigid foam insulation) be “applied in direct contact with the underside of the structural roof sheathing” and that the foam insulation meet the requirements “specified in Table R806.4 for condensation control.” These are the same minimum R-value requirements mentioned above, ranging from R-5 in zone 1 to R-35 in zone 8. Moreover, “The air-permeable insulation [for example, fiberglass batts or cellulose insulation] shall be installed directly under the air-impermeable insulation.”

Can I use dense-packed cellulose as the only insulation for an unvented roof assembly?

In a word, no — the code explicitly forbids this method. Cellulose can only be used in an unvented roof assembly if there is an adequate layer of rigid foam above the roof sheathing or an adequate layer of closed-cell spray foam under the roof sheathing. Cellulose alone won’t work.

However, in some areas of the country, especially in the Northeast, insulation contractors have been dense-packing unvented rafter bays with cellulose for years. Because the method has deep roots in New England, many building inspectors accept such installations.

If you’re building a new house, however, here’s my advice: if you want to insulate with cellulose, make it a ventilated roof by including ventilation channels under your roof sheathing. Leaving out the ventilation channels is risky.

Do I need to install an interior vapor barrier?

While vented roof assemblies are designed to dry to the exterior, unvented roof assemblies are designed to dry to the interior. That’s why an unvented roof assembly should never include interior polyethylene.

If a building inspector insists that you install some type of interior “vapor barrier,” you can always install a smart vapor retarder like MemBrain to satisfy your inspector. For more information on the theory behind roof assemblies and wall assemblies with exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Do I need to install an air barrier under the insulation?

Yes, of course — especially if you are using fluffy insulation like fiberglass batts, blown-in fiberglass, or dense-packed cellulose. (If you insulate your ceiling with spray foam, the spray foam should create an air barrier, as long as the installer does a good job.)

If you are building a cathedral ceiling, the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall. A board ceiling is notoriously leaky, and this type of ceiling is often associated with roof sheathing rot.

What about recessed can lights?

Recessed can lights should never been installed in insulated rafter bays. Period, full stop, end of story.

Recessed can lights take up room which should be filled with insulation; they give off heat, creating thermal hot spots in your insulated roof; and they leak air. They should be removed from your ceiling and deposited in front of a moving steam roller.

A good roof has airtight details and thick insulation

Now you know how to build an insulated sloped roof. To sum up:

  • Make sure the roof assembly is as close to airtight as you can make it. If you are using fluffy insulation, you need two air barriers: one below the insulation, and one above the insulation.
  • Make sure to install insulation that provides at least the minimum code requirement for ceiling R-value. Insulation that exceeds the minimum code requirement is even better.
  • If possible, include a ventilation channel above the top of your insulation layer. The ventilation channel will provide cheap insurance against moisture build-up, and will lower the chance of ice damming.
  • Remember, an insulated sloped ceiling isn’t always a good idea. Sometimes a good old-fashioned unconditioned attic is the best way to cap your house.

Last week’s blog: “More Energy Myths.”

266 Comments

  1. John Brooks | | #1

    Vented Over-Roof Hybrid
    For Affordable High Performance...the closing sentence in Martin's Article is worthy of a Rant.
    THE MOST UNDERAPPRECIATED BUILDING ASSEMBLY
    Joe Lstiburek: "A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science."

    If you do not care for affordable or buildable....
    This BSC article is helpful
    http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting
    same article at Fine Homebuilding
    http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx?langtype=1033
    here is a good video and discussion about roof ventilation
    http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs
    I like Joe's comment #53 where he mentions the "vented unvented roof hybrid"
    I think this is how many/most of the European Compact Roofs are Built
    I think the same strategy(vented over-roof) could be used to improve the Marc Rosenbaum type roof.

  2. John Brooks | | #2

    Image 1
    Martin, I really,really like Image 1
    (I also like that it is High Resolution)
    GBA has posted it before for other articles.
    The image shows an almost perfect storm (including the Ice & Water Sheild)
    Do you know the original source for this photo and if it is possible to view other photos of this fascinating failure.
    It is hard to tell exactly where the cellulose was and wasn't before the demo.....
    but it appears that the areas that were packed with cellulose(lower left) show less damage.

  3. Doug McEvers | | #3

    Rafter vent channels
    I would add to Martin's list of materials for rafter vent channnels, fiberboard sheathing. I use I joists for vented cathedral ceilings and staple 1/2" fiberboard to the bottom of the top cord of the I joists. It can be ripped on a table saw and installs quickly, fiberboard does have a high perm rating and may allow small amounts of moisture to pass through into the vent space. I like a vented eave and ridge and have never had a problem with this assembly. A cathedral ceiling can be energy efficient if built properly with high levels of insulaton. You also get full insulation over the outside wall top plate which eliminates ice dam potential providing the building is airtight.

  4. User avater GBA Editor
    Peter Yost | | #4

    other photos of this roof failure
    Hi John -

    This project was in Vermont in a ten-year old home and as the caption indicates, the main culprit was leaky can lights in the cathedral ceiling. But the situation was exacerbated by an attached greenhouse, which communicated with the rest of the home, and a clothes dryer vented to the indoors during the winter. Interior relative humidity in the winter was running about 60 - 65%. The roof framing cavities were filled with cellulose; the cellulose was air permeable enough to allow the can light penetration and wiring run lead the leaking moisture laden air up the rafters to the hip.

    I am including a photo of the solution: spray foam held down from the top edge of the rafters so that air can be drawn up from soffit vents to gable vents in the truncated attic cavity. This roof cavity is actively ventilated with a fan, not because the remodeler who did the fix was not confident about his new roof assembly, but because the owners were so gun shy of keeping their roof assembly dry.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Doug McEvers - & a video for cold-climate builders
    Doug,
    Thanks for your suggested addition to the list of materials for making site-built vent baffles. I have edited the text of my blog to include your suggestion.

    * * * *

    On an unrelated topic: The Cold Climate Housing Research Center has produced a video (below) showing one method of building an insulated cathedral ceiling. The method shown in the video is particularly appropriate for builders in climates subject to ice dams.

  6. Doug McEvers | | #6

    Dense pack cellulose in vented athedral ceilings
    Martin,

    It seems if one were to use dense pack cellulose in a vented cathedral ceiling the vent channel material should be rigid enough to withstand the pressure. This would be a good insulation choice especially when using I joists, batt insulation is typically sized for dimension lumber and does not fill the space completely without modification.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Doug McEvers
    Doug,
    You're right. In How to Install Cellulose Insulation, Bill Hullstrunk said that he has had good success with AccuVent baffles, which are evidently rigid enough to withstand the pressure of dense-packed cellulose.

  8. User avater GBA Editor
    Martin Holladay | | #8

    A comment from Bill Rose
    [Editor's note: Bill Rose just sent me the following comments in an e-mail; he included his permission to post the comments.]

    A few things come to mind:

    1) Simple roof geometry always permits venting, tortured roof geometry does not. So if we permit non-simple geometry, then we need solutions that do not rely on venting. Or we strictly forbid complicated roof geometry, and blame future problems on designers of non-simple roofs.

    2) The lead photo shows ugliness right at the ridge. There are a lot of phenomena occurring there. A ridge is actually a corner, with all the effects of corner geometry (change in film resistance, radiant reflections, shape factors, etc).

    3) Ugliness right at the ridge also prompted an inquiry about still-air stratification. How do lighter molecules like water vapor stratify against the heavier air molecules? I'll leave that as a homework exercise (it's a neat one). Short answer is that buoyancy plays a big role in air movement, but stratification of still air does not.

    4) Have there ever been moisture problems in any roofs caused by effects other than, first, roof leaks, and second, air flow of one kind or another? My limited experience (honest!) is that airflow always plays a role in problematic roofs. And if the answer is that air flow from below is always a partial culprit, then are we so sure of our roof-venting skills to make sure that our vent strategies provide pure dilution and play no role in inducing flow?

    I forget where you leave things in the post, forgive me. But the "tipped-up commercial foam-based, no air-path sandwich" is a good approach, a keeper. I stop taking bets on cathedral ceilings that use venting in other than simple, straight-run, air-tight-underneath conditions.
    -- William Rose

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Bill
    Bill,
    Thanks for your perceptive comments. I will edit my blog to reflect your point about roof geometry.

  10. User avater
    Ken Levenson | | #10

    Air tightness
    As Bill Rose notes, air tightness of the assembly, both inside and out, is of critical importance - we need to stop the air flow. Therefore I'd propose an assembly that utilizes materials meant to provide air tightness while also maximizing the drying potential of the roof assembly. The detail illustration I've attached shows such an approach. Solitex Mento 1000 membrane is used on the outside of the insulation - a vapor open, yet water and air-tight material - with vented cavity above - allowing for maximum drying out and preventing any wind/air penetration of the insulation. On the inside is Intello Plus, an air-tight membrane with vapor control intelligence - being vapor closed in winter and vapor open in summer - and protected with a service cavity. (Full disclosure, we are now supplying these products to the American market at http://www.foursevenfive.com.) A robust air-tight insulation assembly that doesn't require foam.

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Ken Levenson
    Ken,
    Thanks for posting the detail. Two comments:

    1. Your suggestion is only a slight variation on a detail that is already discussed in my blog, namely cellulose installed with a vent channel between the top of the cellulose and the roof sheathing. You’re just using an air barrier material instead of cardboard, plywood, or fiberboard sheathing to separate the cellulose from the ventilation gap.

    2. I think your proposed detail would be difficult to build. How easy is it to balance on the rafters as you install the membrane, presumably with staples, and then install the 2x2 spacers, presumably with nails or screws, all without putting a foot between the rafters? I wouldn’t want to do it.

  12. User avater
    Ken Levenson | | #12

    vent channels and installs
    Martin,
    My sense is that if we are worried about air leakage compromising the insulation's effectiveness, vent channels are problematic.
    Regarding installation: While it's been successfully installed in Europe for many years, we've just gotten the material, and so we look forward to getting it on a job site soon and actually installing it - and of course, reporting back how it went.

  13. Doug McEvers | | #13

    Vent channels
    Ken,

    I am with Martin on this one, your detail is labor intensive for little if any gain in my opinion. The vent channels we describe can be done after the roof is on from the underside (dry side). For those of us building in a cold climate this is a huge advantage, we spend as little time as is needed on the roof. A lot of new products out there looking for a home with some tough construction details to make them work. Your system as I see it would negate the use of dense pack insulation.

  14. User avater
    Ken Levenson | | #14

    new products
    Doug,
    Admittedly these are new products for Americans but they've been used successfully in Europe for years - so not really new. It will be interesting to see how the installation is adapted to our market. I can say we are not selling these products at arms length, we are working closely with the architects and builders on these projects. Like I mentioned, we look forward to reporting back from the field.
    Regarding dense pack cellulose: not sure why dense pack would be negated - I think it perfectly suited.

  15. J Chesnut | | #15

    thanks
    Thanks for the summary of this condition. Makes a good reference point.
    I like the detail provided but I wouldn't call that a cathedral ceiling condition as it includes ceiling joists spanning at the bottom of the roof rafters. I would call this an insulated attic.
    Yes a cathedral ceiling detail would be very similar but the challenge of many cathedral ceilings is the added structural elements, electrical boxes, and the desire for wood board ceilings all which start compromising the drywall layer which in your detail is used at the air barrier.

  16. Dan Kolbert | | #16

    Where are failures
    I have built dense-packed unventilated roof assemblies several times without problem. Of course I haven't torn into them either. I would like to see more examples of failures in these kinds of assemblies if they're out there.

  17. Doug McEvers | | #17

    Vented ridge only
    I have to wonder if the failure in the photo would have occured if the hip and ridge was vented and everything else the same?

  18. Jesse Smith | | #18

    Dense-packing accuvent
    I was skeptical of Bill Hulstrunk's claim that AccuVent baffles could withstand being dense-packed in an enclosed cavity. So at Bill's suggestion Jim Millhouse (also from National Fiber) and I ran a couple of 'tests'. Bill was right, and the results weren't ambiguous. During the process the baffles showed few signs of deflection.

    To be clear, we tested the AccuVent cathedral ceiling extension, not the standard AccuVent shown in Joe Lstiburek's Fine Homebuilding article on roof ventilation. Also, a small amount of material ended up on the wrong side of the baffle, which probably means folks would do well to follow Martin's advice on sealing them during installation.

  19. User avater GBA Editor
    Martin Holladay | | #19

    About "Where are the failures?"
    Dan,
    I agree with you: I'd like to see more information on this issue. I'm not aware of any failures that have occurred in properly dense-packed unventilated rafter bays. The Building Science Corp. people refer to a failed roof that, if memory serves me, was a low-slope or flat roof with many peculiarities -- not a typical installation.

    I suspect that Bill Rose is right: all of these failures involve moisture that is carried by moving air. These are not diffusion failures. If the rafter bays really are properly dense-packed -- a big if -- the chance of air movement is very low.

    I think that people who write building codes are concerned that, if the code allows unvented rafter bays to be insulated with cellulose, failures will occur due to sloppy cellulose installation. Some installers will fail to dense pack every cavity, and there will be rafter bays with air movement, and we'll be right back to having problems with sheathing rot.

    But if the installer is conscientious, and the rafter bays really are dense-packed, my guess is that everything will be fine.

  20. Aj Builder, Upstate NY Zone 6a | | #20

    Why not just do it right plus
    Why not just do it right plus build with pressure treated roof sheathing. Even standard plywood can handle wetting cycles many times more than our number one super inexpensive OSB.

    Any home being built better than the absolute minimums should at the very least not use OSB for roof sheathing. Imo

  21. John Brooks | | #21

    Concerning post #19
    Martin,
    Perhaps your "memory" is from Joe's Don't Be Dense Insight and Bill Hulstrunk's comments

    see post #9
    http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19174/joe-lstiburek-says-dont-dense-pack-unvented-c?page=1&cm_BOqHX%24B7W8pUB8eKwgDiDDMa_B7W8pUxeh-0=1319665667

  22. John Brooks | | #22

    Plywood is better than wuzwood
    It won't/didn't save a bad design like Image 1

  23. Garth Sproule 7B | | #23

    Thanks
    John
    Thanks for finding the thread above...note that in post #17, Robert Riversong talks about the "vented over roof". I agree with you that this method deserves some attention for cathedral ceilings.

  24. Aj Builder, Upstate NY Zone 6a | | #24

    Looking at the fatal pic I
    Looking at the fatal pic I feel there might be a simple though unorthodox solution. All the rot from warm wet air meets cold wood is right close to where a vented hip may have averted all and WITHOUT vent chutes.

    So a hybrid approach needs to be investigated. Dense packed cathedrals with warmside ridged foam to add R, taped, no recessed lighting or other air paths and then belt and suspenders... Add venting even though there is no normal 1" vent path.

    Doug... For some reason I did't see your post #17. Looks like two of us at least think there might be a very easy inexpensive solution.

  25. Doug McEvers | | #25

    aj builder
    The main flaw in this roof looks to be the ice and water membrane at the highest point in the rafter bays. Would just regular felt paper have allowed enough moisture to dissipate through the roof sheathing avoiding rot?, I sort of doubt it. Plywood clips do give a bit of spacing between 4 x 8 sheets and depending on the location may have helped with regular roofing felt. There had to be some major air leaks in the air barrier to show this kind of damage. I am a plywood devotee and never use OSB anywhere but I think under the same conditions plywood would have failed also.

  26. Deniz Bilge | | #26

    Whew, I'm confused now.
    Hello all,

    I thank you for this impeccably timed blog, as I am currently framing my semi-basic cathedral ceiling hip roof (with short ridge). I would like to describe my detail and try to understand where I am on the right track and more importantly, where I will have failures. Hope I can amuse some of you...I feel like the more I try to address a concern, the more of a concern it becomes, and I certainly don't want to take great time, care, and detail to facilitate the demise of my roof.

    The existing: hip rafters, ridge, continuously connected top plate of wall perimeter are all i-beam construction, welded together & to each adjoining member. Remaining space is filled with dimensional 2x12 @24" o.c.

    The plan: Part A: I came across 1" Dow rigid foam backed 5/8CDX for the same price as the bare CDX, and I plan to double it up like a SIP, installing the bottom sheet foam side up and the next sheet on top (staggered of course) with the foam facing down, giving me 2" of insulation between the plywood.
    Part B: Install ice/water shield over the whole roof.
    Part C: Install treated 2x4's from eave to ridge, long bolting into each respective rafter.
    Part D: Either notch the treated 2x4 spacers for fitting 1x4's horizontally across them in order to ...Part E: Install metal roof over this.

    My understanding as I read this blog is that I do not have to worry about creating an air channel above the sheathed portion because of the rigid foam. So simply install the metal roof on top of the ice/water ??? Will the properties of the metal cause condensation on the backside without sufficient airflow?

    Thanks for your input.

  27. User avater GBA Editor
    Martin Holladay | | #27

    Response to Deniz Bilge
    Deniz,
    1. You are building an R-10 roof. That's not much insulation; as far as I know, it won't meet code anywhere in the U.S. Where are you located?

    2. It sounds like you are using steel beams to frame your hips. As I'm sure you know, these steel beams are thermal bridges. All insulation has to be on the exterior side of the steel.

    3. Using continuous Ice & Water Shield is sometimes risky, but it's OK if you are following the PERSIST approach and putting all of your insulation on the outside of the peel-and-stick. The method can work, but you need much more foam on the exterior of your roof.

  28. Aj Builder, Upstate NY Zone 6a | | #28

    Doug I agree, too many wrongs
    Doug I agree, too many wrongs "overthinking" and overdoing. I once had to resupply a doctor building an aircraft with dozens of AN aircraft bolts because they were tightened to the point of breaking the bolts. Less is more sometimes... Along with the KISS rule.

    Back to the vent idea. I really think venting a roof that is airsealed in the interior well and built without vent chutes is possibly the best way to build cathedral ceilings. I think vented roofs have too much airflow and aid interior air getting out in the first place wasting energy and moving moisture to where we don't want it.

  29. TJ Elder | | #29

    Response to Deniz
    If your climate is not too severe it's probably okay to have just the R-10 foam over the structural steel, although there's some risk that the steel could sweat in cold weather. I'm assuming you also intended to insulate the 2x12 joist bays to get your total R-value to a reasonable level. I'd suggest using asphalt felt rather than impermeable membrane, so the upper plywood has some ability to dry if needed. You are wise to include the vent space below the metal roofing.

  30. James Martin | | #30

    Science-code disconnect
    What a timely discussion for me!

    I am building a forensic laboratory constructed in Massachusetts, and am having a difficult time reconciling applicable building code with my read of the literature pertaining to insulation and roof deck condensation.

    We decided to forego ridge and soffit vents during framing and roofing, trusting advice that an unvented roof insulated with spray foam was the better option. However, subsequent research and first-hand experience examining aged polyurethane foam (in a different context) that reverted to a liquid phase due to improper mixing 20+ years ago leave me extremely reluctant to use spray foam insulation.

    Heres a snapshot of the roof: 2 x 12 fir rafters (12/12 pitch), 3/4 inch tongue and groove plywood underlayment, Grace membrane near eaves/felt elsewhere, asphalt shingles. The only ceiling and roof penetration will be a plumbing vent (no can lights, ductwork, etc.). Radiant heat, room air conditioners, and dehumidifier to contain RH at or below 40% (a must for one of my spectrometers).

    My read of the literature suggests a low to moderate risk of roof deck condensation for my building, which could be reduced and managed using airtight drywall installation and a low perm primer or paint.

    However, the applicable code does not appear flexible, with respect to buildings like mine.

    I would greatly appreciate recommendations from experts: bite the bullet and risk of spray foam insulation, or send the builders back up on the roof in December to cut ridge and soffit vents?

    Thank you,

    James

  31. Malka Van Bemmelen | | #31

    What about an insulated nailbase product like Cool-Vent?
    Thanks for this great discussion!

    I am designing a house and we are using cellulose in 2x12 roof rafters, and I came across a product that looked great. It is an insulated nailbase called Cool-Vent by Hunter Panels. It provides up to 25.6 R-value in foam insulation (in thicknesses up to 6" which include a 1" airspace and plywood) while providing a cooling vent under the shingles, and easy installation for the roofing with the nailbase. Is this a product anyone has used? And do you think it is a better solution than not providing any venting, but using layers of poly iso over the 2x12 rafters filled with cellulose - a hot roof?

    Thanks,
    Malka

  32. TJ Elder | | #32

    Response to James Martin
    Sounds like you have the framing and roofing in place. Your options are described in the article above--you can add venting by installing baffles below the sheathing to hold back the insulation, or you can install at least R-20 of air impermeable insulation. If you go with spray foam, choose an experienced installer who understands the effects of temperature and other factors that can lead to failure. There are many reports on this website about what happens when foam is installed improperly, and it isn't pretty.

  33. Kim Shanahan | | #33

    Flat roofs, spray foam and vents
    Many spray foam roofing contractors in our market are touting their ability improve efficiency on old hot tar and graveled flat roofs by adding 3 to 4 inches of closed cell spray foam. Most of these old homes are flat roofs joisted with dimensional lumber, with parapets and have stucco vents cut into the rim joists. It has always been questionable to me as to the effectiveness of the occasional stucco vent to ventilate solid lumber joists, but we see little to no rot failure in homes even up to 60 years old.

    As the head of our local HBA, I have strenuously pointed out that foam over a vented flat roof assembly does not provide the insulation values promised. Some builders have suggested they could simply seal and stucco over the stucco vents and eliminate the ventilation. The problem is the rim joist is not insulated above the level of the ceiling's batt insulation and therefore creates a major thermal bridge, which could lead to condensation problems from the rsing moist air stack effect. Then they point out that the 3 inches of foam over the roof deck would keep the deck and joists warm enough to mitigate condensation.

    I want to promote cost-effective efficiency in my market, but not at the expense of roof failure. We are a 5B climate zone with 6000 HDD, heating from mid-October to mid-April. We are also extremely dry at 7000 ft above sea level.

    So the question is: seal the stucco vents and add the spray foam roof and don't worry about condensation? There is very little low-hanging fruit in our type of construction, especially on existing homes, which is why people are being sucked into the spray foam roof scam.

  34. Katy Hollbacher | | #34

    Condensation control
    Martin, I think your note re: 2009 IRC table R806.4, min impermeable insulation levels req'd for condensation control in unvented roof assemblies per climate zone, needs clarification; you stated
    "After you have installed the code-mandated thickness of rigid foam above your roof sheathing, you’re free to install as much fluffy stuff as you want between the rafters."

    However, I presume those min levels are based on the assumption that total roof insulation levels are around code requirements given the climate zone, e.g R-38 for CZ 5 (roughly double the R-20 impermeable ins. above roof deck req'd). So if one were to create an R-50 assembly, they better be providing R-26 impermeable insulation above the roof deck. In other words--it's the proportion of above vs. below roof deck insulation that matters, not absolute R-value provided above, that determines condensation potential.

  35. James Brown | | #35

    alternatives to spray foam underneath sheathing
    for an unvented construction with no insulation above the sheathing, how about using wood fibreboard between the rafters directly below and in contact with the sheathing? All gaps between the boards and the rafters filled with expanding foam / sealant. You could then cover the rafters with rigid foam to bring your insulation up to required R level and reduce the risk to the rafters and sheathing, and then install your airtight drywall underneath this. Also you could install T&G OSB onto the underneath of the rafters with all joints / edges bubble glued before your rigid foam and drywall go on. This would be another air barrier in your construction, perhaps more reliable than the rigid foam and drywall alone.

  36. Albert Rooks | | #36

    Take another look at the no foam option:
    Ken's detail (#10) does a good job of cold climate safety. It has what we are looking for: Good airsealing at the interior, fluffy stuff in the middle, and good drying channels for water vapor at the top. By changing from a low permeability sheeting to a high permeability membrane (at top) with venting above, there is no more concern for roof rot while at the same time wind washing is prevented in the bays by the air & water tight membrane with taped edges and seams. It appears to me to be a very durable assembly.

    Changing the Intelo membrane to glued & taped OSB or Plywood with a service cavity below made of framing lumber does make the assembly much easier to build. You can now walk on the Plywood/OSB as you work since it is glued (thats why glue + Tape rather than just tape) and also supported by the service cavity (via perpendicular framing lumber).

    This assembly was recently built just south of Portland OR. I posted it a couple of weeks ago. Here it is again (with the builders comments) I think it's a pretty straight forward build once you get past the idea of adding service cavities to your build.

    "Albert, Mark and Michael,

    Yesterday we blew the insulation and installed the SIGA Majcoat on the CAPACES Leadership Institute. There was more than 8,000 Cubic feet of insulation to install, so our insulators had 2 of their blower trucks and a good crew on site. They were blowing cellulose as fast as they could and the the rain was heading our direction.

    Less than 45 minutes after the insulation was completed we had finished the Majcoat and were busy taping seams. Because there is an airspace above the insulation, I am confident that the small amount of moisture that got on the insulation will be able to dry to the exterior.

    I have attached a few photos showing the installation process and the dried in roof. The roof sheathing plywood is going down as I type and the roofers will be here on Tuesday to install the membrane for the living roof."

  37. User avater GBA Editor
    Martin Holladay | | #37

    Response to James Martin
    James,
    Q. "Should I bite the bullet and risk of spray foam insulation, or send the builders back up on the roof in December to cut ridge and soffit vents?"

    A. Either approach will work, as long as you get at least the minimum code R-value for your roof, and as long as you use construction techniques that pay attention to airtightness. Most homeowners are happy with their spray foam jobs. Your decision depends in part on how important it is to you to avoid spray foam.

  38. User avater GBA Editor
    Martin Holladay | | #38

    Response to Malka van Bemmelen
    Malka,
    Using ventilated nailbase panels is a perfectly good way to build a cathedral ceiling. The same rules apply as when using SIPs: be sure that the seams are sealed to be as airtight as you can make them, and be sure that the total R-value of your roof assembly meets or exceeds minimum code R-value requirements.

  39. User avater GBA Editor
    Martin Holladay | | #39

    Response to Katy Hollbacher
    Katy,
    You wrote, "It's the proportion of above vs. below roof deck insulation that matters, not absolute R-value provided above."

    From a building science perspective, you are, of course, correct. However, the code does not use building science methods. The code requirements are as I reported them. The proportion of R-value above and below the roof sheathing are not cosidered by the code.

    The old Canadian rule of thumb -- at least 2/3 of the total R-value on the exterior side of the vapor barrier -- is a rough-and-ready way to address this question; it's also possible to approach the question by performing a dew-point analysis or a WUFI analysis.

    In the real world, however, we're all interested in determining the actual risk and hearing failure stories. I haven't heard any failure stories related to the problem of "too much fluffy insulation below the sheathing in a roof assembly with 2009 IECC levels of rigid foam above the sheathing."

    Your caveat is appropriate and conservative. For the vast majority of builders, however, the code requirements will keep roofs free of problems.

  40. User avater GBA Editor
    Martin Holladay | | #40

    Response to Kim Shanahan
    Kim,
    My advice would be to seal the existing vents.

    If insulation contractors in your area are installing 3 to 4 inches of closed-cell spray foam above the roof sheathing, you don't have to worry about condensation or moisture accumulation in your sheathing. You're certainly right, however, that you don't want to have any ventilation under roof sheathing that has spray foam above it.

  41. User avater GBA Editor
    Martin Holladay | | #41

    Response to James Brown
    James,
    Q. "For an unvented construction with no insulation above the sheathing, how about using wood fibreboard between the rafters directly below and in contact with the sheathing?"

    A. Your suggested approach probably meets code requirements, since wood fiberboard insulation isn't air-permeable. However, high-R wood fiberboard is not yet manufactured in North America, so the only way to get enough R-value using your approach would be to import the insulation from Europe. That's expensive.

  42. User avater GBA Editor
    Martin Holladay | | #42

    Response to Albert Rooks
    Albert,
    I think that most contractors would worry about the buildability of your approach. The photo shows a low-slope roof over a flat ceiling; this allows workers to stand on the ceiling joists. If the roof were steep and high, however, they would have to balance their feet on the rafters while installing a membrane. This membrane needs taped seams. That's tricky, fussy, work to do without any roof sheathing. If you add fall protection equipment, it becomes even trickier.

  43. John Brooks | | #43

    Buildability
    Albert,
    Martin makes a good point.
    the example you posted does look to be tricky and labor intensive.
    Buildability and WORKER SAFETY matter.
    Of course a high pitch roof is problematic with other Not-So-Buildable stratagems too.

    I would rather be the contractor on Albert's roof ... at least I would probably survive the fall

  44. Albert Rooks | | #44

    Martin & John: Yep. Your right of course...
    Magic bullet status is not achieved. It's a design for a low slope roof.

    It's very much along the lines of Doug McEvers use of high perm fibre board to create vent channels above dense packed cellulose. The idea is that it might be a "quicker build" to stretch a membrane across the top, tape seal it, and then cross batten, vent, and then sheet for roofing.

    Like Doug's design, the focus is on creating a high perm layer for drying to the exterior, that will still allow venting while blocking wind washing in the fluffy stuff.

    The specific design shown was for a Passive House. The roof thickness & use of a service cavity might appear to be overkill for most projects, but if you want an airtight lid, a service cavity will get you there - permanently... And allow you to wire and light the interior.

  45. John Brooks | | #45

    Not really Vaulted
    Albert, It looks like your example is a Flat Ceiling project ?
    It looks like it could be used with vaulted ceilings... (scissor truss)
    It looks like a lot of trouble for a flat ceiling....
    You mention wind-washing ... but isn't that mostly an issue near the soffit area?
    Thorsten Chlupp is in a much more challenging climate and he seems to be doing fine with simple loose fill on the attic floor....of course he is using "ample" amounts (cellulose galore)and he has baffles near the soffit.

  46. Albert Rooks | | #46

    Vaulted, compact, & sealed but drying
    John,

    Sure the example shown is for a low pitch (not flat) roof. But change it to a pitched roof less than 6-12 and the details are still a good method (I think). Regardless of how you assemble it, the idea is airtight on the bottom, fluffy stuff in the middle, and a high perm layer at the top that seals from the wind (and rain, and animals (to a degree), and bugs...) with venting on top of that.

    It's certainly not limited to an R 80 Passive House roof. Take out the parallel chord trusses and thin it up.

    Sure it's a lot of trouble. It looks like less trouble that the detail posted as example in the blog above and uses no foam... Which is a "biggie" for me.

    Since you brought up Thorsten Clupp, think of it as his Arctic Wall on a pitch (new picture below & still no exterior sheeting. Just a membrane, in this case Siga Maijcoat... Amazing...)

    He gets a bit more snow that needs to be shed from the roof than you might in Texas, so of course he builds a high pitched roof and puts enough fluffy stuff on the roof deck that we could both have a soft landing after jumping from Mt McKinley.

    Since the point of the string is making roofs that don't fail, there are going to be assemblies that really work well but are a bit of a departure. If we had the absolute best practices that we could ever imagine, already in hand and on job sites, we wouldn't be here (at GBA) and Martin would have far less to do... I'm sure he would enjoy writing about facial cream since the building stuff has been all figured out!

    JB -Greetings from Olympia!
    -a.

  47. John Brooks | | #47

    aha.....Parallel Chord trusses
    Ok. Albert.... I understand now
    I thought I was looking at conventional raised heel trusses with a flat ceiling.

  48. Albert Rooks | | #48

    Yeah... I could be more clear
    JB- I really ought to post sketches as Ken did. It would probably save some trouble. For me this was harking back to the compact roof string.

  49. Dan Reams | | #49

    Experience in Yukon
    I have built log homes in the Yukon Territory in northern Canada for 30 years. Most log buildings have cathedral ceilings. Most of the roofs we have built are as follows (from the bottom up): log purloins, nominal 2" (2x6) t&g decking, 6mil poly, 1 1/2" extruded polystyrene foam, 2x10 rafters on 24" centers, 7" fiberglass, 2x4 strapping, 7/16 OSB, underlayment (15 lb paper, 30lb felt, or now we would even use ice and water shield). There would be about an inch of air space below the strapping for both vertical and horizontal air flow. All roofs are vented with strip vents at the eaves and vents in the soffit (2x6 t&g) at the peak at both ends. I would disagree that venting is not needed if you stop the airflow on the warm side, for 2 reasons. First it is very difficult to stop all airflow, especially with conventional traditional practices (ie chimney and plumbing vent penetrations through the roof) We have several techniques developed over the years to seal the vb at the eaves and gables, but there are still nail penetrations through the vb. Overall I think we had very tight seals but one could never say airtight. (Then blower door tests would be pressurizing the building and seeing if it dropped at all, like testing your plumbing for leaks. The fact that we are talking air exchanges says we are not air tight.)If the warm side air barrier is compromised so much that venting creates a bigger problem then the horse is already out of the barn. When it is 70 to 80 degrees inside and -60 outside there is a pressure differential, a vented roof will let even the frost evaporate a little during the winter and will dry what little moisture there might be there out with the warm days of spring. The second reason is for keeping a cold roof surface. With 3 feet of snow (good insulation, as long as you keep it below freezing) and no ventilation just where is the freezing point going to be? Do you have 3 feet of insulation in your roof? No, so there will be times when the freezing point is above the roof, if this happens enough you will have an ice dam and a leaking roof. Even with 3 feet or more snow on the roof the only time one of our roofs will have and icicles is in the spring when the warmer days will start the snow melting and the nights are still freezing the water as it runs under the snow to the edge.
    Unless one uses extreme care with the warm side air barrier and this includes knowing that all fastener penetrations are sealed as well you should assume some air flow. The installation referred to here with the ice and water shield under the shingles obviously made a better air barrier than whatever was on the warm side. But if there had been no recessed lights would it just have taken longer? IF it was tight enough that the minimal amount of moisture was able to evaporate and move out of the roof enough to keep humidity levels low enough to stop any mold and decay growth than it may have lasted for ever. But to be properly vented above the insulation would certainly help with the big IF.

  50. John Walker | | #50

    Better check the walls too
    If they were venting the dryer inside (code violation?) and have an attached greenhouse I bet the walls are moldy too. Start in the laundry room with its perfect moisture point source "dryer".

    Can I get some more commentary on the "whole roof" of Ice & Water Shield? I think this had very little to do with the failure. The reason why I ask is I have a similar "whole roof" design coming up, HOWEVER, the rafter bays are all straight runs with soffit vents, continuous vent baffles (1/2" plywood) and continuous ridge vent. Dense pack cellulose, 2" XPS below 2x10's, air tight drywall (XPS could be sealed up too).

    Thanks,
    John

    [Editor's note: Click "page 2" to continue reading comments.]

  51. User avater GBA Editor
    Martin Holladay | | #51

    Response to John
    John,
    I used to work as a roofer, and roofs that are entirely covered with Ice & Water Shield make me nervous. I see this type of design as lazy and potentially problematic. A well designed roof shouldn't need a continuous layer of peel-and-stick, and the peel-and-stick introduces several new problems: the plywood has zero ability to dry outward, and future sheathing repairs become difficult.

    I know that many people note that drying to the exterior doesn't happen with most types of roofing, and that's true. (The exceptions are slate, clay tiles, concrete tiles, and cedar shingles, as well as steel roofing over purlins.) But I remain convinced that the peel-and-stick is just one more way to push a roof assembly toward danger.

    So, I believe that the best roof assemblies don't have peel-and-stick (except at eaves and valleys) and do have ventilation. These two measures provide safety.

  52. Ken Watts | | #52

    Snow Country Cathedral Roofs
    Dan Reams, thanks for your comments. I also live in snow country near Yellowstone National Park. I often have well over 3 feet of snow on my cathedral roof. I believe the dew point is often in the snow rather than in the roof. The R value of the snow is often near the R value of the roof. This can lead to massive ice dams. A simple thermal analysis shows that you cannot put a practical amount of insulation in the roof to prevent the ice dams and move the dew point into the roof. A vented cold roof is the only solution. Most roofers in the area use 100% ice and water shield. Of course, stopping air leaks is critical. A solution that works in areas with little snow may not work in snow country. Many articles I read in Fine Home Building and Journal of Light Construction do not adequately address deep snow on the roof.

  53. John W | | #53

    Unvented cathedral questions
    My home built in 63 here in Ohio has a cathedral ceiling with no ventilation. In fact there many cross-supports running perpendicular to the main joists and other framing elements that would make this unfeasible. So I want to make it an effective unvented roof.

    Half of the roof has 2x8 joists and the other half has 2x12s. Right now there is a membrane covering (rubber) that's worked well for over 20 years but is showing signs of deterioration and might need replacing soon. Having done remodeling on the inside and walking on the roof I know that a lot of the sheeting needs to be replaced along with the little insulation that is there now. We've had condensation issues in the past so I now there are air infiltration and bridging issues. Any insulation will have to be done from above when I take the sheeting off.

    After reading this article I have some questions. I was thinking that I would spray in cellulose from above, then put on 3/4 sheeting, then rigid foam, then the 1/2 OSB required for membrane roofing, then the membrane.

    I read this "First of all, you can’t use air-permeable insulation (for example, fiberglass batts, mineral wool batts, dense-packed cellulose, or blown-in fiberglass) to insulate an unvented roof assembly unless the roof assembly also includes a layer of air-impermeable insulation (either spray polyurethane foam or rigid foam panels) directly above or directly below the roof sheathing."

    Since I have the rigid foam above the cellulose should my idea work OK?

  54. User avater GBA Editor
    Martin Holladay | | #54

    Response to John W.
    John,
    Your plan will work fine, as long as the layer of rigid foam on top of your roof sheathing has an R-value of at least R-20. If you are using XPS, you need at least 4 inches of foam. If you are using polyisocyanurate, you need at least 3 inches of foam.

    You are in climate zone 5. I explained the minimum R-values for the approach you are taking in one of the paragraphs above, in which I wrote, "According to ... the 2009 IRC, “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” The table calls for a minimum of ... R-20 for Climate Zone 5..."

  55. John W | | #55

    OK, I was figuring that the
    OK, I was figuring that the total of the rigid foam + cellulose had to be a certain R-value in combination and had been thinking of using less rigid foam.

    I've been reading about different kinds of cellulose. Since I'm installing from above would something like stabilized cellulose work since it has a little water added? I would imagine loose fill could blow all over the neighborhood! Also in the case of my 2x12 section of roof, would 11.25 inches of cellulose cause any sagging in the ceiling drywall? Does that cavity need to be filled all the way?

    I notice the IRC you mentioned called for a vapor barrier for Zone 5. How could I handle this?

    Thanks for your response Martin.

  56. User avater GBA Editor
    Martin Holladay | | #56

    More answers for John W.
    John,
    Q. "Since I'm installing from above would something like stabilized cellulose work since it has a little water added? I would imagine loose fill could blow all over the neighborhood!"

    A. You should only use an installation method that your insulation contractor is familiar with. Here in the Northeast, many contractors would probably fill your rafter bays using the dense-pack method after your new roof sheathing was installed. For more information on dense-packing rafter bays with cellulose, see How to Install Cellulose Insulation.

    Q. "Also in the case of my 2x12 section of roof, would 11.25 inches of cellulose cause any sagging in the ceiling drywall?"

    A. Again, talk to your installation contractor. In most cases, you should be fine -- but the answer depends on the skill of the installer.

    Q. "Does that cavity need to be filled all the way?"

    A. Ideally, yes.

    Q. "I notice the IRC you mentioned called for a vapor barrier for Zone 5. How could I handle this?"

    A. There is a code requirement for a vapor retarder in some types of assemblies, but there is no requirement for a vapor barrier. Your layer of rigid foam is already a vapor retarder. No further vapor retarding layer should be installed.

  57. John W | | #57

    Thanks again for the answers.
    Thanks again for the answers. Now I just need to find an installer in this area who knows what they're doing (so far it's been iffy)!

  58. Jim Peck | | #58

    Zip Roof over 16" Heeled Trusses
    Martin,
    We had a Zip Roof installed and a metal Roof placed over it. The roof assembly is not vented and taped all around the soffits. Ideally I'd like to pack the entire cavity with blown cellulose. This string of conversations is giving me pause for thought. If I dense pack the ceiling right to the sheathing, will I incur condensation problems? I'm right on the Zone5/6 line in Southern NH. My intention is to control Indoor Humidity with a Panasonic WhisperSense Bathroom Fan With Motion & Humidity Sensors - FV-11VQC5. Can condensation be dried to the Interior?

  59. Jim Peck | | #59

    Zip Roof Assembly
    As in this.

  60. User avater GBA Editor
    Martin Holladay | | #60

    Response to Jim Peck
    Jim,
    Your proposed assembly is a code violation. If you want an unvented roof assembly, you need to keep the roof sheathing warm with spray foam or rigid foam. Cellulose insulation is classified as an air-permeable insulation and can't be used in such a roof assembly.

    The danger is that interior moisture will migrate to the cold sheathing, leading to condensation or moisture accumulation.

    To comply with the code -- and to adopt a building system that is less risky -- you need to either include ventilation channels or you need to install a layer of spray foam on the underside of the roof sheathing, following the guidelines explained in the article on this page.

    Now that I have explained the right way to do it, I will also let you know that many people have done what you propose. A few cellulose insulation dealers, including Bill Hulstrunk, claim that you can get away with omitting the ventilation channels if you do a perfect job of dense-packing the cellulose.

    Why risk it, though, with a new construction job?

  61. Jim Peck | | #61

    What formula to calculate the
    What formula to calculate the amount of ventilation?

  62. User avater GBA Editor
    Martin Holladay | | #62

    Response to Jim Peck
    Jim,
    Here's the standard answer for those who need to comply with most building codes (the answer comes from an article by Mike Guertin):

    "The standard roof ventilation requirement used in building codes and asphalt roofing manufacturers calls for 1 square foot of "net free ventilation area" for every 300 square feet of ceiling area (attic floor area) when vents are evenly divided — half of them high on the roof at the ridge, gable, or a high roof, and half low on the roof at the eaves. The alternative is to vent only with low (soffit) vents, by doubling the "net free vent area" to 1 square foot per 150 square feet of ceiling area.

    "All manufactured roof venting products (soffit vents, venting drip-edge, gable vents, roof vents, and ridge vents) are labeled for "net free vent area." In the case of soffit vent strips and ridge vents, this capacity is listed in square inches per linear foot. For fixed-sized units, the capacity is published for the entire unit. Often it's necessary to convert the square footage of vent area to the published net free vent area in inches to make sure you are meeting the requirement.

    "For example, a house with a 28-foot by 40-foot ceiling has an area of 1,120 square feet. With soffit vents only, you'd divide that area by 150 square feet to come up with the "net free vent area" in square feet (1,120 / 150 = 7.47 square feet). But because the net free vent area for most vents is listed in square inches, I find it easier to convert the calculated square foot vent area into square inches. There are 144 square inches in a square foot (12 inches x 12 inches), so I multiply 7.47 x 144 = 1,075.68 square inches of net free vent area. Using a strip vent product listed for 9 square inches per linear foot requires you to make only one simple division calculation: 1,075.68 / 9 = 119.5 linear feet of vent."

  63. Jim Peck | | #63

    Venting
    Thanks Martin,
    I seem to recall this but, it has been awhile. Seems there are several options for us here. Thanks for all your help and the refresher. Once again I celebrate the useful dissemination of knowledge in this technical age vs hunting for a book.

  64. Michael Collins | | #64

    Unvented Cathedral questions
    Martin,
    Excellent article and discussion. I have a question about an existing unvented attic assembly. The rafters are 2x4's and the bays between them are completely filled with closed cell spray foam. I would like to add two 1in layers of polyiso towards the interior of attic and then cover with a vapor open membrane to airseal the assembly. This assembly seems like it would have poor drying potential and as such taping the seams of the polyiso seems like a bad idea. The interior covering would be either T&G planks or plywood. I am located in zone 6.
    Thanks for your posts and articles

  65. User avater GBA Editor
    Martin Holladay | | #65

    Response to Michael Collins
    Michael,
    You're right that this type of roof assembly won't dry to the interior. But it isn't supposed to. I would tape the polyiso -- the tape won't cause any problems.

  66. Michael Collins | | #66

    Thanks so much
    Thanks

  67. Daniel Grade | | #67

    Advice Needed - Retrofit Solution for 1915 House Zone 5/6
    I have a series of questions that incorporate many of the topics that you have covered in these articles, but I am trying to apply them in a retrofit situation.

    We're working on upgrading the energy efficiency, including added/updating insulation, for our almost 100 year old 2-1/2 story balloon frame house (stucco over lathe exterior). The walk-up attic will be utilized space. The location is southeastern Wisconsin, which puts us close to the borderline of zones 5 and 6. Ground snow load here is about 30lb/sq.ft.

    Did I mention that money is tight?

    Rafters are 2x4, 12/12 slope, and the roof is complicated with intersecting ridge lines, valleys and dormers. (The house originally had a cedar roof. The deck is the traditional spaced fir planks.) Due to the construction, there is almost no chance of getting adequate air intake via soffit vents. (On top of the layout issues, there are problems because all the spaces between rafters where they meet the top plate were bricked in, apparently at time of construction). Proper traditional flow-through underdeck roof ventilation will be difficult.

    When we first looked into this project, the building inspector asked us to still use vent channels (cheap polystrene was OK) to capture whatever ventilation we could. (Note: Re-roofing is on the horizon. We anticipated installing ridge venting at that time, and maybe figure out a way to get around the soffit problems.) Rafters were furred down with 2x2's so we could fill the rafter bays with R-11 fiberglass batts against the vent channels, and then we planned to layer the underside of the rafters with 2" XPS to build up to a nominal R-21 and help limit thermal bridging.

    I've started on that work, but I'm beginning to doubt whether this is the best way to proceed. I'm looking to improve things.

    Q-1: Confusion over the relative merits of low/medium/high density fiberglass batts.
    Would high density R-15 fiberglass batts, or even medium density R-13, be a significant improvement over the low density R-11 (beyond the nominal difference of their R ratings).
    This is a hard one to figure out from the reading I've been doing. The question relates to how much (if any) the low density fiberglass loses insulation value due to convection during moderate to severe cold (wind-wash), and whether the medium (R-13) and/or high density (R-15) are really any better. The high density fiberglass seems like it should be substantially less prone to wind-wash, as it is substantially more dense, but of course just because that seems logical doesn't make it true! (It definitely costs a lot more, though!)

    (Side Question - couldn't you just compress two layers of the R-11 together when filling a 3-1/2" deep space and achieve R-15 that way? Per the R-Value charts for compressed fiberglass, it seems like that should work, and then you might have a less wind-wash prone space? I ask because I've got a lot of R-11 on my hands that might not end up being used if R-15 is a worthwhile upgrade.)

    Q-2: To vent or not vent the underside of the roof deck.
    (Side note - as mentioned below, we hope to re-roof in the next year, so options open up then.)
    If I'm already not going to get adequate under-deck ventilation due to the inability to get adequate soffit vent air intake, might it make sense to just skip the attempt at roof ventilation and maybe instead put 1" XPS in place of the vent channel on the underside of the current roof deck? (It's labor intensive, but you can achieve a pretty tight fit between the rafters.) It's a tempting idea, because that would bump the nominal R Value of the assembly to R-30. (From the top down that would be R-5 for the 1" XPS against the underside of the roof deck, R-15 for the fiberglass batts (assuming a switch to high density fiberglass makes sense) which will fill the remainder of the rafter bays, and then another R-10 from the 2" XPS spanning the underside of the rafters.)
    From reading, it appears that trapping water vapor becomes a potential serious issue to be concerned about. The 1" XPS has a perm rating of somewhere between 1.1 and 1.5 (depending on what source you refer to.) The 2" XPS has a perm rating of around .7 If moisture can't easily get in from either the topside or underside, but can basically exit just as readily as it can enter, is that a more or less neutral situation?
    Another issue that comes up in that scenario is whether that could promote condensation on the underside of the 1" XPS when it is cold enough outside because the underside of the XPS could be below the dew point?

    Q-3: What to do when we re-roof.
    As mentioned above, we hope to replace the roof this next year. If so, at that time I'd like to upgrade to either an unvented roof or if necessary a cold roof (a vented over-roof). (I don;t know that it matters in the equations, but we're looking at buying and installing stamped metal shingles. The goal would be to get an exceptionally long life span out of the roof, and to do so while putting much less weight up on those 2x4 rafters than would be the result if we used asphalt dimensional shingles.)
    Something I saw in the Building Science dot Com article "A Crash Course in Roof Venting" by Joseph Lstiburek (see http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting ) looked like it might be applicable to our situation. In the article Mr. Lstiburek had a side bar titled "Master Class - Unvented Roofs", where he discussed three possible unvented roof approaches. In one of those he illustrated using a combination of insulation above and below the roof deck to achieve your total insulation requirements. His illustration shows a roof with 2" R-10 XPS on the top side of the deck, and R-30 under the deck (in the rafter bays).
    That looks very appealing: go with R-30 on the underside of the deck at our house using a combination of fiberglass batts and XPS as I described above, and then add an additional R-10 (2" XPS) on the top side when we tear down and re-roof? Unfortunately that sounds like it's too easy.
    Can we (does the code allow it)? Does it make practical sense? Are there pitfalls due to our climate? How might we have to modify that to fit it to out situation?
    In the alternative, if we can't do a ventless roof that way, can we achieve our total insulation needs by combining the above and below deck R-Values if we top it all with a vented over-roof?

    Thanks for your time and your advice! Dan

  68. User avater GBA Editor
    Martin Holladay | | #68

    Response to Daniel Grade
    Daniel,
    You've got a lot of restraints, and those restraints are leading you to some convoluted thinking. Instead of advising you to adopt a series of compromises, I'm going to explain the right way to address your roof, even if the right way is expensive.

    1. You should get an engineer to evaluate the safety of your 2x4 rafters. "Furring down with 2x2s" doesn't improve the ability of your 2x4s to bear the snow load. It's possible that an engineer will advise you to sister 2x8s or 2x10s onto your existing 2x4 rafters.

    2. You have described a roof that is hard (or impossible) to vent, for two reasons: complicated geometry, and the impossibility of installing soffit vents. That means you need an unvented roof assembly.

    3. According to the building code, an unvented roof assembly needs R-20 of rigid foam above the roof sheathing for Climate Zone 5, or R-25 of rigid foam above the roof sheathing for for Climate Zone 6.

    4. Once you have R-20 or R-25 of rigid foam on top of your sheathing, you can safely insulate below your sheathing with almost any kind of insulation you want to use to bring your insulation up to minimum code requirements.

    Good luck.

  69. Daniel Grade | | #69

    Response to Martin Holladay
    Thanks for the input. I see a visit with a structural engineer and 35 square of polyiso in my near future.

    On the subject of fluffy insulation, are there any sources you can refer me to which address the differences between low, medium and high density fiberglass batt insulation from the standpoint of loss of R-Value due to wind-wash (whether due to air infiltration or convection as a result of significant temperature differences front to back)?
    Most of the information I run across seems to be from the cellulose industry that claims wind-wash is a big problem with fiberglass; and from the fiberglass industry which asserts that the cellulose group overstates the issue and that to the extent wind-wash occurs it is really inconsequential.
    I have a lot of walls to re-insulate, and once the roof is dealt with I'll have rafter bays to insulate, and it would be helpful to have some insight into which type of fiberglass gives you what benefit for its cost.
    The high density fiberglass is so much more dense than the medium and low density fiberglass that it seems like it should reduce the potential for convection within the batts, but at 2 to 3 times the cost of medium density fiberglass batts the gain may be nominal and not worth the added cost.

    Thanks for sharing your expertise,
    Dan

  70. User avater GBA Editor
    Martin Holladay | | #70

    Second response to Daniel Grade
    Daniel,
    Ideally, you want to have an airtight barrier above and below your fiberglass batts. If you can achieve those two air barriers, then wind-washing isn't an issue in a roof like yours. (It can be in a vented unconditioned attic, of course, especially near the soffits.)

    If you intend to build a ventilated insulated sloped roof assembly, choose a type of ventilation baffle that can be installed in an airtight manner. Site-built baffles are best.

    I don't have any data from thermal performance tests of fiberglass batts that attempt to make a wind-washing comparison between (for example) R-11 and R-15 batts. But it stands to reason that denser batts will perform better. You're right that stuffing a fat fluffy batt (or two fat fluffy batts) in a small cavity can mimic some, if not all, of the benefits of a dense batt.

  71. James Brown | | #71

    Response to Martin
    Thanks for your response Martin. Would you still be happy to use that unvented method for a renovation situation where there is non-breathable membrane (eg bitumous 'felt') above solid board sheathing? So the build up is, from outside to in; tiles, battens, aforementioned felt, solid board sheathing, rafters with tight fitting wood fibre board between and below, airtight drywall.

  72. User avater GBA Editor
    Martin Holladay | | #72

    Response to James Brown
    James,
    It sounds like you have an existing roof with tile roofing. I'm guessing that you're writing from Europe. It sounds like the insulation consists of "tight-fitting wood fibre board" between your rafters -- thickness and R-value aren't specified.

    If you want to create an unvented insulated roof assembly, you probably don't want to disturb the tile roofing. That means that the most appropriate insulation would be spray polyurethane foam. Of course, the existing insulation should be removed before you install the spray foam.

    However, I think you are wrong about your asphalt felt; I doubt if the felt is impermeable to water vapor. Most types of asphalt felt are somewhat permeable -- typically 5 perms when dry and up to 60 perms when wet. Since your roof assembly can dry to the exterior, you may be able to use cellulose or mineral wool insulation between your rafters, if local building codes permit the practice.

  73. Jaime Masick | | #73

    Proper vapor barrier for vented?
    This article was EXTREMELY helpful, Thank you! I have one question about creating a vented assembly by making an airtght system with accuvents. If I do that then install fiberglass, then an interior vapor barrier (this is for zone 5-6) am I creating the risk of trapping moisture between the two barriers? What type of interior vapor barrier should I use, if any?

  74. User avater GBA Editor
    Martin Holladay | | #74

    Response to Jaime Masick
    Jaime,
    U.S. building codes do not require the use of interior vapor barriers, even in cold climates. What they do require is an interior vapor retarder. This requirement can be satisfied with kraft facing on fiberglass batts or vapor-retarder paint.

    It is far more important to have an interior air barrier than an interior vapor retarder. Be sure that you have created a good air barrier by sealing all penetrations and cracks.

    For more information on vapor retarders, see Vapor Retarders and Vapor Barriers.

  75. Michael Meyer | | #75

    vented cathedral ceiling in climate zone 3, edge to zone 2
    I'm opening up my house and installing cathedral ceilings, I just wanted to confirm my plan since I got second thoughts reading more and more about it. My original decision was to go with a vented roof system and R38C batts. I read a lot about the different methods and it still appears to me that this is the best. Livint in Atlanta GA we have to deal with both hot humid summers and cold winters, so humidity control is a problem, that's why I believe the vented approach is the best. Anyhow, I'm down to the rafters and roofdecking and furred out the existing 2x6 with 6" metal studs, so I have about 11.5" for insulation. Mt plan was to create a continuous ventilation under the roofdeck, my question goes towards to use baffles, xps board build baffles or insect screen nailed at furring strips to allow for the airspace. This is followed by either R38C or R30C depending on the baffle design. Next I want to cover everything with 1/2" XPS and taped seams to add insulation and guarantee air tightness as well as vapor barrier. No penetrations due to air tightness, so I go with indirect lights. Now the inside XPS will help with thermal bridging and help in the winter months to stop humidity to enter the insulation and condense at the roof decking, at lest by not using the baffles. If I use the baffles I would crate a condensation plane and trap any moisture in the fiberglass insulation. I've read a lot of problems that the baffles create. Will the air tightness on the inside that I'm planning prevent these problems? Should I rather use my insect screen baffle idea to let the insulation breathe? What will happen to this assembly in the hot humid summer, where it should be the other way around here in zone 3?
    Should I use airtight baffles or not, I don't see a way to have moisture escaping in a assembly with airtight baffles, fiberglass and xps on the inside, no matter what the dew point will be inside this assembly and possibly create condensation that cannot evaporate without ventilation. It appears to me that XPS baffles or any baffles that are airtight just worsens the problem, am I missing something?

  76. User avater GBA Editor
    Martin Holladay | | #76

    Response to Michael Meyer
    Michael,
    The point of the ventilation gap is not the encourage moisture to flow out of your ceiling. The point is to try to handle any moisture that makes its way there in spite of your best efforts to prevent any moisture from getting there.

    You don't want to use insect screen, because fiberglass batts need to be enclosed by an air barrier on both sides. When the top side of fiberglass batts are subject to wind-washing, the moving air degrades the performance of the insulation. During the winter, the moving cold air pulls heat out of the fiberglass batts as it moves by. You don't want that.

    So, whatever baffle material you choose, make your installation airtight. You can't make insect screen airtight. So use an air barrier material like plywood, rigid foam, or fiberboard sheathing.

    If you want a vapor-permeable material that is still an air barrier, choose fiberboard sheathing. But use caulk around the perimeter, and tape at the seams, to be sure that the material is installed in an airtight manner.

  77. User avater GBA Editor
    Martin Holladay | | #77

    Response to Kimbark Smith
    Kimbark,
    Q. "Does open-cell foam qualify as an air-permeable insulation?"

    A. Open-cell spray foam is vapor-permeable but air-impermeable.

    Q. "If I execute the unvented roof as designed and put 4 inches of polysio on the roof deck, can I then use open-cell foam between the rafters?"

    A. Yes.

  78. Kimbark Smith | | #78

    Does Open Cell foam qualify as an air permeable insulation
    Zone 5/6, I am trying to upgrade my cathedral ceiling and only have 2 x 4 (real)construction. Rather than extend the rafters, if I execute the unvented roof as designed and put 4" of polysio on the roof deck, can I then use open cell foam between the rafters?

  79. Kimbark Smith | | #79

    thnx
    I assume using closed cell would create the dreaded sandwich?

  80. User avater GBA Editor
    Martin Holladay | | #80

    Response to Kimbark Smith
    Kimbark,
    Q. "I assume using closed cell would create the dreaded sandwich?"

    A. Yes indeed.

  81. PJ PR | | #81

    Hot humid climate-all metal cathedral roof-must use R-11 batts
    Incredibly helpful (5*) thread. So, if you must use fiberglass insulation batts, and you dont have access to spray foam or closed cell boards, what do you do? I guess the key is to make it unvented and add in other air-impermeable layers.

    I am about to insulate a new all-metal cathedral pitched gable roof in the Caribean. The plan is to fill the cavity between the 4" Z-purlins with R-11 batting (vapor side facing the living space). For additional heat conduction resistance we plan to roll another thinner (filleted?) layer of this batting over the top of the purlins (ie between the purlins and the sheeting). I am thinking of installing this upper batt with its facing towards the metal sheeting. We also plan to seal the underneath of this "insulated purlin cavity" with a poly vapor barrier sheet and then screw in sheetrock (to the bottom of the purlins).

    The overall house is *continuously* well ventilated with lots of louvered shuttered openings (No AC). There is a 4 x 3 vented window high up on the back cathedral gable, allowing hot air to excape. Basically the set up is very different to your typical northern-US home, but could be relevant to a non-airconditioned "eco-green" South Florida dwelling.

    This is where I got the above idea. It's the "Double Layer banding system."
    http://steelbuildinginsulation.com/highr.html

    Many of my neighbors here have cathedral ceilings, no attics and absolutely no insulation (basically they have aluzinc roofs on plywood with pine rafters). These high roofs are pretty cool, which is good as most of us cant afford to A/C. We wanted a metal roof specifically to avoid the termites that plague the latter roofs eventually.

    1) Is my plan OK?
    2) Will we need more air barriers (how about above the top layer of batting, just below the metal sheeting)?
    3) (bonus) How would you go about installing this roof?

    Answers to any of the above are very appreciated.
    PJ

  82. User avater GBA Editor
    Martin Holladay | | #82

    Response to PJ PR
    PJ,
    I don't have experience building in the Carribbean or building steel buildings, so I'm not the best person to comment. But here are a few reflections:

    1. You don't want an interior vapor barrier. In a hot humid climate, the vapor drive is always from the exterior toward the interior (assuming your building is air conditioned). If your building isn't ever going to be air conditioned, the vapor drive will still be from the exterior to the interior, but there won't be much vapor drive. If you want any kind of vapor retarder -- and I'm not saying you need one -- it would go on the exterior side of the insulation, not the interior.

    2. Whenever you install fiberglass batts, air barriers are much more important than vapor control. You need a good air barrier on both sides of your fiberglass batts. A material like MemBrain (with taped seams) is one possibility. Other materials that can be used to create an air barrier are plywood with taped seams or gypsum drywall.

  83. PJ PR | | #83

    Hi Martin,
    Thanks for the

    Hi Martin,
    Thanks for the feedback. We will never A/C the building (it will be solar powered). The wind is pretty constant, but it is a humid environment, so moisture control is certainly on my mind.

    You say "You don't want an interior vapor barrier," but you also say that "you need a good air barrier on both sides of your fiberglass batts." The latter would turn out to be an interior vapor barrier not so? Ie. If I install a polyethlene sheet between the bottom of the batting and the sheetrock [it would be attached to the purlins], wouldn't this be an interior vapor barrier? Maybe I'm not getting what you mean by an interior vapor barrier.

    The other question I have is will the gypsum sheetrock be enough (do I need the polyethlene)?
    Cheers ,
    PJ

  84. User avater GBA Editor
    Martin Holladay | | #84

    Response to PJ
    PJ,
    Q. "You say 'You don't want an interior vapor barrier,' but you also say that 'you need a good air barrier on both sides of your fiberglass batts.' The latter would turn out to be an interior vapor barrier not so?"

    A. No. Not all air barrier are vapor barriers. For example, gypsum drywall is an air barrier, but it is not a vapor barrier. Similarly, MemBrain is an air barrier but not a vapor barrier. For more information on these issues, see these two articles:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    Q. "If I install a polyethlene sheet between the bottom of the batting and the sheetrock [it would be attached to the purlins], wouldn't this be an interior vapor barrier?"

    A. Yes, it would. I don't recommend the installation of interior polyethylene in your climate.

    Q. "Will the gypsum sheetrock be enough (do I need the polyethlene)?"

    A. If your goal is to install a good air barrier, then drywall is certainly adequate, as long as you seal all penetrations to avoid air leakage. For more on installing drywall in an airtight manner, see Airtight Drywall.

  85. PJ PR | | #85

    Light on regarding vapor & air barriers
    Aha....I get it now.

    Our cooler mornings may make the interior of my metal roof sheeting to sweat slightly since it will be in contact with the warmer unvented fiberglass-insulated purlin "cavity" underneath. Once our hot Caribbean sunshine starts to kick in I want this moisture to dry and/or diffuse out of this cavity, through the gypsum sheeting, into the cathedral living space below, to then be blown out of my 4x3 gable window. A vapor barrier would completely retard this process. Finally, the reason for installing an air-sealed gypsum drywall is to LIMIT the amout of humid air that gets into the purlin cavity in the first place. Consequently, as much as possible of the hot daytime humid air is directed out of the interior building space by the Caribbean breeze flowing through the continously open shutters that I installed. How does that sound?

    Aside: I wish I had access to foam spray or the foam boards. But we can only get our hands on double bubble or R-11 batts (the latter arrived today).

    Finally (I promise this is my last topic). Regarding my fiberglass batts: Will the facing that comes with it retard the drying out process that I described above (does it matter what side I put it on within the purlin cavity?) and will 2 layers of R11 be too much to compress in my 4" purlin space (I realize that codes in the US call for more insulation, but I will have a good amount of cool breeze flowing through my house). The folks at http://steelbuildinginsulation.com/highr.html say I should filet an inch off the upper batt, concur?
    Thank you Jedi masters at greenbuildingadvisor.....PJ

  86. PJ PR | | #86

    1st layer of R11 going on ...
    Well the roof is going on with the first layer of insulation (on-top the purlins, under the roof sheeting). There appears to be enough space left in the 4" purlin cavity for the second layer of R11 to be installed without any modification. I realise however that, having been compressed for so long in these big rolls, the fiberglass may "swell up" over the next few days... We shall see.

    Any thoughts about the paper facing? (see my post immediately above). Obviously (from the photo) the first layer was installed facing up. The second layer I propose to install face down.

    The plan is to install it (from the inside of the house) about the same time the drywall is put up. I will instruct my crew to make sure there are no air pockets in the purlin cavities & no air flow into the latter. I will supply them with cans of great stuff to make sure they get to all the gaps.

    Thoughts? ideas? PJ

  87. Bryce Nesbitt | | #87

    Flat roof cathedral ceiling
    We're going to give it a try. Existing is a fourteen year old roof with 2" of closed cell roofing foam with an acrylic elastomeric coating. Below is 1x decking and 2x4 rafters. Our plan is to put rigid board up between the 2x4's, caulk the edges, and then hang sheetrock. We'll air seal the one electric junction box for the room light. And cross our fingers.

    This is in the San Francisco Bay area fog zone (a heating climate).

    The one question: should we vent the space above the sheetrock to the interior?

  88. User avater GBA Editor
    Martin Holladay | | #88

    Response to Bryce Nesbitt
    Bryce,
    Q. "Should we vent the space above the sheetrock to the interior?"

    A. No, that won't be necessary.

  89. Thomas Kral | | #89

    Old cottage cathedral ceiling
    I have a 50 year-old seasonal cottage in a northern climate. The simple 6/12 gable roof is composed of actual 2x6 rafters (24" oc), sheathed with 1/2" plywood and covered with asphalt shingles. There is no insulation in the ceiling and I suspect little to no effective insulation in the 2x4 walls. In the summer the heat gain inside makes living almost unbearable. I suspect that the biggest bang for the buck would be to address the ceiling insulation issue first, but would like to maintain the interior appearance. Would it be reasonable to strip off the shingles, lay down (in order inside out) 6mil VB, high density foam board (6 to 8"?), roof deck and shingles? Would it be better to strap under the new deck with 2x's from eave to peak to vent the deck? Is a separate VB necessary with that thickness of foam board? Any advice would be appreciated. Thanks.

  90. Melissa MacMillian | | #90

    Question: Bunkhouse
    Client wants exposed cathedral ceiling (cool moist climate - Zone 5-6).
    I looked at the unvented schematics and I am not sure that fits the bill for up here (Canada - west coast).
    My feeling is that the dew point profile would end up in the XPS foam board, hence a VB should be put on first deck.

    In order of assembly:
    Rafters
    1/2" ply
    Blueskin (peel'n stick rubber membrane - VB)
    2 layers of 2.4" XPS glued and taped
    Furring strips 1/2" over XPS aligned with rafters held in place by 8" screws into rafters
    1/2" ply
    Roof felt
    Asphalt shingles

    Bug screen for vented space between XPS and ashpalt deck, and ridge cap vent.
    Any thoughts would be appreciated. Melli

  91. User avater GBA Editor
    Martin Holladay | | #91

    Response to Mellisa Macmillan
    Mellisa,
    Condensation cannot occur in the middle of a chunk of XPS because the XPS is neither air-permeable nor is it particularly vapor-permeable. More information on that issue can be found here: Are Dew-Point Calculations Really Necessary?

    The peel-and-stick membrane is still a good idea, but its main purpose is to act as an air barrier, not as a vapor retarder.

    Your assembly calls for about 5 inches of XPS, for a total R-value of only R-25. That's not much. If you are going to all that trouble to build a complicated roof assembly, you should aim higher. U.S. building codes call for a minimum of R-38 ceiling assemblies in a climate like yours; that would require at least 8 inches of XPS.

  92. Melissa MacMillian | | #92

    Thanks Martin! Yeah, I agree
    Thanks Martin! Yeah, I agree that with XPS being closed cell, it's ability to hold vapour is limited. As far as the R-value is concerned, it is the clients wish, not mine...I'd have the cavities filled...
    I have a thing for blueskin, as a both an air and vapour barrier, but also it will act as a seal for the screws that hold everything down. In reference to the vaulted schematics on this site, it shows vapour/air barrier on top of insulation, which got me wondering. As good as I am installing materials, nobody is perfect, and an air leak may develop between XPS seams. Once ANY moisture gets down to lower sheathing, it is game over...imho. Being on the wet coast, it means mold.

    I've been pouring over the Dow site looking at compressive strength of XPS - 30psi. I've heard through a suspect grapevine that XPS, after repeated compressions, loses it's shape. Some folks have taken to putting in furring strips between XPS boards to support XPS. This seems counterproductive (thermal break). I think I found that confirmed on the Dow site, but their application instructions lost me...lol

  93. Jim Bohrer | | #93

    fiberboard
    I'm a homeowner in Seattle. Have a newly-framed, vented cathedral ceiling to insulate. Straight bays, 2x12, 24" o/c. The current plan is to use batts of some kind in the bays, take care to air seal the interior surface, avoid penetrations, air seal the vent channel too (bays fully sealed), double prime the inside surface for vapor resistance. I have a couple questions about baffling the vent channel:

    1. Some of the building professionals I'm working with feel the jury is still out on baffles and are recommending leaving the batts open to the channel except near the vents, assuming the 1-2" air channel can be maintained. Concern is condensation on the interior surface of the baffle. Is this really still an open design issue? I thought wind-washing would degrade the performance of the batts. Is Seattle's moderate climate at play? Do we need to take extra steps for vapor resistance on the inside surface?

    2. Assuming the baffle is still necessary, any recommendations on fiber board? From a web search I see there are many brands. Bildrite's site says their product is vapor permeable (good), but I don't know about the others. Will Homosote work? What about hardboard? - that's cheap at Home Depot and seems water would run off pretty well.

  94. User avater GBA Editor
    Martin Holladay | | #94

    Response to Jim Bohrer
    Jim,
    The thermal degradration in the performance of batts due to wind-washing is a much bigger problem, in my opinion, than any risks due to moisture accumulation. If you pay attention to airtightness, you shouldn't have any moisture worries. So, yes, you need a top-side air barrier.

    All fiberboard products that I am aware of, including Bildrite fiberboard and Homosote, are vapor-permeable. I don't know about the vapor permeance of Home Depot's hardboard, however.

  95. Bo Jespersen | | #95

    Closed cell foam and a roof leak
    We have been recently installing a lot of closed cell foam directly to the roof sheathing with great results. However I wonder if you have heard about what happens when the inevitable roof leak occurs? I imagine it builds up hyrdostatic pressure until it finds a way out, but that could take years and could rot a great deal of the roof before anyone notices. On top of that, many roofs are being covered with very low permeable underlayments. This concerns me...any thoughts?

  96. User avater GBA Editor
    Martin Holladay | | #96

    Response to Bo Jespersen
    Bo,
    You are not the only person with this concern. For those who (like you) are concerned about the issue, there are two choices:

    1. Stop using spray foam.

    2. Install sturdy site-built ventilation baffles in each rafter bay before you spray the foam. That way you end up with a vented cathedral ceiling.

  97. Bo Jespersen | | #97

    I like it
    Right on! Venting the slope protects the sheathing from being caught in between two vapor barriers/retarders and gives the water a place to go when there is a leak....a technique I will start using immediately.

    Thank you very much Martin.

  98. Joe Lethert | | #98

    Insulating existing ceiling
    ok, so I'm a little worried reading this thread. I have a great room that is paneled on top of the rafters, which are 4x8, and I installed sloped cans and was going to insulate and finish ceilings. My primary purpose of doing this is that the roof is directly on the beams and it gets very hot in the summer, winter not so much of problem as I live in San Diego and temps rarely dip below freezing. Since it is existing and has no vents, and I don't want to tear my roof off at the moment, looks like I have to go with unvented system. Looking for any suggestions or opinions on what my best option or two looks like?

  99. User avater GBA Editor
    Martin Holladay | | #99

    Response to Joe Lethert
    Joe,
    Your description is confusing. I'm going to try to see if I understand what you are saying.

    I think you mean this: Your cathedral ceiling is presently uninsulated. When you are indoors looking up, you can see your rafters. Above the rafters you can see a layer of boards. Is that correct?

    If I understand correctly, you have just finished installing some recessed can lights, and now you want to know how to insulate around and above these can lights. Your intention is to install a new finish ceiling, under the rafters, once the insulation is installed. Is that correct?

    Assuming I got that right, here is my advice.

    1. Remove the can lights and throw them away. Recessed can lights should never be installed in an insulated cathedral ceiling.

    2. Go to a good lighting showroom and choose some surface-mounted ceiling fixtures that you like.

    3. Install electrical boxes at the locations where your surface-mounted fixtures will be installed.

    4. Insulate the rafter bays with spray polyurethane foam, as described in the article.

    5. Install your new ceiling.

  100. Erich Riesenberg | | #100

    unvented attic and rigid foam in rafter bays
    Thank you for this detailed article and discussion.

    First, is it easy to identify whether a roof is ventilated? If no soffit vents or side vents are visible on the roof from the ground, does that mean the roof is not ventilated?

    Second, can you please confirm this: "If you want to use just one type of insulation in unvented rafter bays, you are limited to spray polyurethane foam."

    I have been using rigid breadboard style expanded polystyrene in the rim joists and thought the same could be put under the sheathing between the rafter bays, and with any gaps sealed with a urethane spray foam such as Fill and Seal. Is there a quality which makes spray foam acceptable and rigid foam unacceptable?

    Thank you.

    [Editor's note: An answer to this question has been posted on Page 3. Click the box below to reach the next comment page.]

  101. User avater GBA Editor
    Martin Holladay | | #101

    Response to Erich Riesenberg
    Erich,
    Q. "Is it easy to identify whether a roof is ventilated?"

    A. Not really. One of the problems is that many U.S. roofs are "kind of" ventilated. The builder may have included polystyrene baffles, but didn't realize that the valleys and dormers interrupted the ventilation. Or maybe there used to be a ridge vent, but when new roofing was installed, the roofer just ripped out the ridge vent and roofed over the ridge. At that point, the rafter bays are "kind of" ventilated, but not really.

    It's safe to say that an existing house doesn't have good roof ventilation if there aren't any soffit vents or ridge vents. It's also safe to say that, even if there are soffit vents and ridge vents, the roof isn't really ventilated if it includes valleys, hips, dormers, or skylights.

    Q. "I have been using rigid breadboard style expanded polystyrene in the rim joists and thought the same could be put under the sheathing between the rafter bays."

    A. This is a gray area in the building code. It's safe to say that no commercial insulation contractor would use that technique, which many people call "cut and cobble." It is fussy, time-consuming work, and it is hard to be sure that everything is properly air-sealed. It's the kind of approach that is used by a homeowner with lots of spare time and the idea that they may save a little money.

    If you want to use rigid foam, the best place to put it is on top of your roof sheathing, not under your roof sheathing.

    That said, if your local building inspector approves of your plan, and if you have lots of time on your hands, and you are capable of doing a meticulous job of air sealing, you can go ahead and try the cut-and-cobble approach.

  102. Erich Riesenberg | | #102

    cobble is the right word
    Thank you Martin. Installing about 100 linear feet of rigid foam in the rim joists, 2 layers at 2 inches each layer, has taken perhaps 35 hours. It is tedious but this is my first work with foam and it does get easier with experience. As to cost, the foam is probably $35 and the spray foam and caulk to seal about $30.

    Having spent all my life behind a desk working on this is a bit of a break. The roof is going to be done in perhaps 4 sections so may try rigid foam and spray foam to compare what works. Rigid foam is tedious but minimal sticky residue. Thanks.

  103. Mark Fredericks | | #103

    unvented spray foam + polyiso inside
    Martin, you wrote:
    "To summarize, there are three ways to build an unvented roof assembly:
    - Install spray foam (either open-cell foam or closed-cell foam) against the underside of the roof sheathing, and no other type of insulation. Be sure that the thickness of spray foam is adequate to meet minimum code requirements."

    This is an option I'm considering in my house to save the previous owners newly installed roof shingles, but the depth of the rafters cannot accomodate enough insulation to meet code. I'm wondering if I should furr out the rafters to get the desired thickness of spray foam, or if installing a few inches of polyiso foam to the underside of the rafters, after the spray foam, could work? I'm not certain which is cheaper, but the added foamboard on the underside of the rafters would reduce thermal bridging, so I'm leaning towards this option but I'm not sure I understand your quoted advice above. Suggestions?

  104. User avater GBA Editor
    Martin Holladay | | #104

    Response to Mark Fredericks
    Mark,
    Q. "I'm wondering if I should furr out the rafters to get the desired thickness of spray foam, or if installing a few inches of polyiso foam to the underside of the rafters, after the spray foam, could work?"

    A. As I wrote in the article, "Most rafters aren’t deep enough to accommodate the insulation needed to meet minimum R-values required by code, especially if the rafter bays include a ventilation channel. ...
    Builders solve this problem by furring down or scabbing on additional framing below the rafters to deepen the rafter bays. Another technique is to add a layer of cross-hatched 2x4s, 16 inches on center, installed beneath the rafters."

    If you install 2x4s (or 2x3s on edge, installed with long screws) perpendicular to the rafters, you will address most of the thermal bridging concern without the need to add a layer of rigid foam.

  105. Steve Knapp CZ 3A Georgia | | #105

    Engard in the rafters
    I'd like to do a unvented cathedral ceiling but don't want to use any spray foam. So I guess my options are (1) SIP roof or (2) exterior foam with cellulose on the second floor ceiling. The roof has a 12:12 pitch, so we are actually dropping the ceiling through the second floor. There will be plenty of room to pump in cellulose. One other option I was wondering about was whether Engard batts could be use in between the rafters with rigid exterior foam. Does anyone know if spun polyester is considered an air barrier.

  106. User avater GBA Editor
    Martin Holladay | | #106

    Steven Knapp
    Steven,
    EnGard insulation is a type of fibrous insulation that resembles a fiberglass batt. It uses polyester fibers instead of glass fibers. It is definitely not an air barrier. If you intend to install EnGard insulation batts in an unvented cathedral ceiling, you'll need to have a significant layer of rigid foam insulation above the roof sheathing.

  107. Milo Shubat | | #107

    Commercial unvented roof question
    Martin,
    I'm dealing with a little different condition that I'd love some input on. I'm building a winery tasting room in a masonry building (8" monolithic concrete walls) in 4C/5B climate zone in So. OR. I've got an exposed 2x10 T&G ceiling over GLB purlins @ 5'oc, so I'm building it from the bottom up and the last layer to go on is the sheathing under the metal roofing. I had planned to install an air barrier over the T&G deck (taped felt) then frame 2x8 @ 24"oc with high density batts to fill the cavities, over which is the sheathing under the metal roofing. I can't really spray foam the underside of the top sheathing layer, so would I be achieving the same effect with rigid foam over the sheathing / under the roofing? local code for commercial only calls for R-30, so I can get that in the 2x8 with high-density batts. Can I just tape the top sheathing layer? Thanks for your input

  108. User avater GBA Editor
    Martin Holladay | | #108

    Response to Milo Shubat
    Milo,
    You don't explain whether this is new construction or the renovation of an existing building. I rather suspect it is a renovation -- or else one might think that the designer of the building would also have provided a design for your roof.

    You describe your proposed air barrier as "taped felt." I don't think asphalt felt can be successfully taped, and I don't think that asphalt felt makes a very good air barrier.

    As I explained in the article on this page, you can't use fluffy (air-permeable) insulation in an unvented roof assembly unless you include rigid foam or spray foam above the fluffy insulation. So you will need R-10 or R-20 (depending on your climate zone) of rigid foam insulation on top of the fluffy stuff.

    Most commercial roof assemblies don't bother with the fluffy stuff. They just meet 100% of the R-value requirements for the roof with rigid foam (either EPS or polyiso) and skip the batts entirely. That's the way I suggest that you proceed.

    Any commercial roofer in your area should be familiar with this type of roof assembly.

  109. Milo Shubat | | #109

    Commercial unvented roof question
    Martin,
    Thanks - this is new construction, I'm the designer / builder, and the more I look at it, I'm liking all rigid - less fussy. The top layer of rigid over fluffy makes my eave too thick anyways. The air barrier for the T&G ceiling is actually the sandwich of felt and taped 3/8" CDX.

  110. User avater GBA Editor
    Martin Holladay | | #110

    Response to Milo Shubat
    Milo,
    Taped CDX plywood makes a good air barrier. It sounds like you are all set.

  111. Annemarie Belteu | | #111

    Unveiled air space
    Situation similar to Eric, above. If closed cell foam is sprayed or "cobbled" from xps, do you have to fill the entire space, or only to thickness for desired R value? Can space between rigid foam and drywall be left empty? Or must it be filled with fluffy stuff or more rigid foam the he rafters are a full 8" deep. The ceiling is in zone 3.

  112. User avater GBA Editor
    Martin Holladay | | #112

    Response to Annemarie Belteu
    Annemarie,
    I don't know what you mean by "unveiled." In zone 3, you need at least R-30 of insulation (which means 6 inches of XPS). Once you have installed R-30 of spray foam or XPS, you're done. You can leave an air space between the bottom of the foam and the drywall if you want.

    Of course, you'll still have thermal bridging through the rafters. But you won't have any moisture problems.

  113. Joell Solan | | #113

    polyiso facing question
    I have an unfinished attic, with cathedral ceiling, no insulation at this time. in 5-10years we will be adding dormers, but in the meantime i would like to add insulation and drywall in the simplest, most reversible/salvageable manner. My plan is to hang 2 layers of 2 inch polyiso under the rafters, taped with staggered seams leaving the rafters empty, so essentially a vented assembly. I have added blocking to the floor joists and will make the polyiso continuous in the knee walls. I thought i would add furring strips on top of the insulation then hang drywall, thinking that the furring strips would create an airgap, allowing the foil to act as a radiant barrier. While this won't provide ideal R-value it should make a significant difference for a reasonable cost, $ and labor. 1) does this make sense or is there a better option? 2) i've recently gained access to cheap roofing polyiso--black backing on both sides. My feeling is that putting this backing against the rafters in contact with that vented air might be inviting a mold problem, and if i face it towards the room i lose the radiant barrier. Please advise and thank you!

  114. User avater GBA Editor
    Martin Holladay | | #114

    Response to Joell Solan
    Joell,
    Q. "Does this make sense or is there a better option?"

    A. It makes sense to you because it meets your needs. As you are aware, it's not an ideal way to insulate your roof assembly. But you have described it as a temporary stop-gap measure.

    Q. "I've recently gained access to cheap roofing polyiso -- black backing on both sides. My feeling is that putting this backing against the rafters in contact with that vented air might be inviting a mold problem, and if i face it towards the room i lose the radiant barrier."

    A. The polyiso will not create a mold problem. While you are correct that black facing does not provide a radiant barrier, the difference in performance compared to foil-faced polyiso would be minor.

  115. James Anderson | | #115

    Bringing ducts "inside" and duct condensation
    I have a partial cathedral ceiling in the finished attic of my house in zone 4 (TN). Each slope of the roof is divided roughly in thirds by 3 different situations: knee walls at the outer thirds, "cathedral" ceiling in the middle thirds, and a vented "attic" in the middle third with a ridge vent - a common situation with finished attics, at least around here. I have a big problem in the summer with overheating. The insulation isn't working well because it was poorly insulated with just R-19 fiberglass batts between the rafters without a ventilation baffle or adequate soffit venting and only r-19 blown into the "attic space". Moreover, my upstairs HVAC ducts are run in the really hot "attic" space under the ridge.

    Based on what i haved learned from several articles here, my planned solution is to cathedralize the entire roof, eave to ridge: install continous air-tight site-built 2" vent channels using foil-faced polyiso board (as suggested in http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting), installing continuous soffit vents to drastically improve the airflow from soffit to ridge, creating an air-tight ceiling below the rafters, and, most importantly, bring the ductwork inside the conditioned envelope. I plan to fill the rafters with rigid polyiso to yield r-30, but might opt to fur out the rafters and use fiberglass or cellulose instead. My plan is to leave the ductwork "exposed", hanging below the ridge and above the collar-ties that run between the rafters. (The idea to dress up my round ductwork with a clean white insulated wrap and disguise the collar ties as solid beams by wrapping in a plywood box.) Once that is done, knee-walls and room partition walls can go back inside the new air-tight ceiling. Hopefully this is a sound plan... Let me know if I missed anything.

    Other than asking you readers for a review of my plan, I have another question brought about by concerns expressed by a local builder. He is concerned that once the AC ducts are inside the conditioned space that the ducts will be prone to sweat and will drip all over my furniture. He has suggested that I will need either expensive internally-insulated ducts (like some restaurants use) or I will have to make sure that my duct wrap will be sealed enough to "hold" any condensation.

    If the air inside my house was humid unconditioned air, I would be equally concerned. But won't the inside air be relatively dry due to the on-going air conditioning? Should I be concerned out condensation forming on my AC ducts if I cathedralize my ceiling?

    Thanks, Jim

  116. User avater GBA Editor
    Martin Holladay | | #116

    Response to James Anderson
    James,
    Your plan to install a stack of rigid foam rectangles in each rafter bay is called the "cut-and-cobble" method of insulation. It is a method often chosen by homeowners, but never chosen by insulation contractors. Spray foam would make more sense, because cut-and-cobble is fussy work that takes a very long time to do well. (The perimeter of each piece of rigid foam needs to be carefully sealed with caulk or canned spray foam.) Good luck.

    I live in northern Vermont, and I don't have enough experience with air conditioning to answer your question about condensation on interior ducts. I suspect that you won't have a problem; but if you want a more informed opinion, you should post your question on our Q&A page, where more GBA readers are likely to see your question.

  117. S. Chapman | | #117

    Cathedral Ceiling repair work
    Dear Experts:

    I am a homeowner with cathedral ceilings that were added during remodeling and expansion of my home in the late 90s. Only the ridge beam is exposed. The rest of the ceiling is covered with drywall. Through the years there has been some condensation leak down the side of the ridge beam on some mornings in March/April.The home is about 4 miles from the coast in Southern Califonia, so zone 3b. There are 4 recessed lights in the ceiling on the side of the beam that gets the condensation, but there are also lights on the side that is always dry. There is no air conditioning in the home. Heat is used during the night only about a dozen nights a year, not during the condensation. Usually the windows are open. It seems the condensation shows up on mornings when there is no marine layer (no clouds in the morning) and the temperature heats up fairly early after a very cool night. The problem ceiling faces east/west and the condensation is on the east facing side of the beam. The roof is black asphalt shingle. I know that there is no caulk/sealant between the side of the beam and the place where the drywall butts against it, so there is a small air gap there. My memory is that fiberglass insulation was used. There is definitely no vent in the roof, but I seem to remember that the insulation didn't go all the way to the beam, a gap was left as would be if there were to be a vent.

    My theory is that on those mornings that heat up fast the moist air that got inside and cooled the beam at night heats up fast with the sun shining on it and the beam stays cool for a few hours and "sweats." I've been contemplating a way to fix this without tearing open the whole roof or ceiling. I'm getting some scaffolding to acces it, it's a 14 foot ceiling, and I want to cut open a small access hole in the drywall to make sure the framing is not rotting from the occasional moisture. After that I want to find a way to get some insulation into the void around the beam for the whole length of the beam. The joists are 2x8 and the beam is 6x14. I figure the gap between the beam and drywall needs to be sealed and I also want to clean up the look of the beam by sanding off the water stains. Is there any way for me to insulate around the beam without opening the ceiling, like drilling holes and spraying some kind of insulation inside? Hope I didn't leave out anything important.

    S.Chapman

  118. User avater GBA Editor
    Martin Holladay | | #118

    Response to S. Chapman
    S. Chapman,
    Here is what you wrote: "I know that there is no caulk/sealant between the side of the beam and the place where the drywall butts against it, so there is a small air gap there. My memory is that fiberglass insulation was used. There is definitely no vent in the roof."

    You are describing a cathedral ceiling that incorporates all possible mistakes. There was no attention to air sealing. The worst possible insulation was used (fiberglass). And the roof assembly is unvented. So it is no surprise that you are seeing condensation.

    I doubt that the condensation is occurring against the cold ridge beam. Rather, I think it is likely that condensation is occurring on the underside of your ridge shingles or the underside of the asphalt felt underlayment.

    To prevent moist interior air from contacting cold surfaces near the ridge, you need a real air barrier and a real insulation layer. The right way to fix this problem is to expose the rafter bays -- either from above or below, it's your choice -- and pull out the fiberglass insulation. Put the fiberglass in a dumpster. Then fill the rafter bays with spray polyurethane foam.

    If you want to attempt a halfway solution that may or may not work, you can cut out some of the drywall near the ridge, and install spray foam insulation on the underside of the roof sheathing, just in the area near the ridge beam. (From the ridge to perhaps 12 inches down from the ridge on both sides.) Then patch your ceiling. This may solve your problem.

  119. S. Chapman | | #119

    Thanks for the confirmation
    Thanks for the confirmation and advice. Since everything is dry this time of year and the home was recently tented for termites, I will insulate by the beam and seal up the obvious air entryways. Once I can see inside I think I'll have a better idea how much moisture has been in there over the years. I can also see if there is improvement next spring. Then when it is time to reroof I'll have to take care of the rest. That gives me some flexibility to plan the timing.

    Ironically, my former husband was the contractor/engineer who did the work, and he now works as an expert in construction defect litigation. I wonder if I should mention this problem to him? lol
    SC

  120. Milo Shubat | | #120

    Venting an arched cathedral roof
    Martin,
    I'm a bit stumped on this one - I'd like to vent this roof assembly, but I can't say whether there will be a natural convective flow as there would be with rafters sloping with the roof pitch. My rafters are running level but they sit on parallel curved top plates. I can use 12" rafters and make 10" of batt work, leaving plenty of ventilation space. I can drill the tops of the engineered joist flanges to allow ventilation 'up-slope' (perpendicular to the rafters) as well as in the rafter bays, and I can provide continuous perimeter ventilation (Cor-A-Vent strip) at my roof fascias (eave, ridge and gables). Can I assume that any exterior air movement will promote a natural convective draw through the assembly without an otherwise typically sloped rafter layout? I am maintaining a neutral air pressure in the assembly, which leads me to believe the moisture won't otherwise be trapped in the roof.

  121. Milo Shubat | | #121

    Venting an arched cathedral roof
    Here's a graphic for the roof framing

  122. User avater GBA Editor
    Martin Holladay | | #122

    Response to Milo Shubat
    Milo,
    I think you should take one of two approaches: either:

    (a) design this roof as an unvented roof, using spray foam or rigid foam above the roof sheathing, or

    (b) install curved ventilation channels above the roof sheathing, perpendicular to the rafters, with a second layer of sheathing on top of the ventilation channels.

  123. Milo Shubat | | #123

    Venting an arched cathedral roof
    Martin,
    Thanks for the reply. For reference, it's in a 4C (mixed marine) climate zone.
    I don't have a lot of room above or below the roof structure for insulation / venting. Would a flash / batt unvented assembly work under the single sheathing deck? My insulator has done quite a few of these locally.
    Are you thinking the 'flat' vented cavities won't provide the assembly with an exhaust path for moisture?

  124. User avater GBA Editor
    Martin Holladay | | #124

    Reponse to Milo Shubat
    Milo,
    Needless to say, sometimes you need to plan ahead to be sure that your rafters are sized adequately to allow for insulation and venting. For more information on acceptable ways to insulate low-slope roofs, see this article: Insulating Low-Slope Residential Roofs.

  125. Milo Shubat | | #125

    Venting an arched cathedral roof
    Thanks Martin - Now I see why my original plan for a SIPS envelope was so elegantly simple. It looks like min R-10 sprayed under the roof sheathing and then a full-fill of the bays with batt / fluffy, and a vapor sealed drywall lid will get me the 'hot' (unvented) roof system the SIPS was from the get go.

    With this system , there's no problems with a full peel-stick roof underlayment on top of the deck is there? I have a metal roof on this project and the underlayment will be fully adhered to the roof sheathing.

  126. User avater GBA Editor
    Martin Holladay | | #126

    Response to Milo Shubat
    Milo,
    I'm not a fan of full peel-and-stick roofs, for a variety of reasons. But your plan will probably be fine. Just make sure that your roof sheathing is dry on the day you install spray polyurethane foam -- because the sheathing won't be drying very much in either direction after it's encapsulated between peel-and-stick membrane and spray foam.

    If you choose to install open-cell spray foam instead of closed-cell spray foam, your roof assembly will be able to dry somewhat to the interior. Just make sure that the spray foam layer is thick enough to keep your rafter bays above the dew point during the winter.

  127. Daniel Cabe | | #127

    Cathedral ceiling question
    I am changing my ceiling to a cathedral ceiling. I am at a mile high in southern california so I get some cold winters, but very dry mild summers and 13 1/2 inches of precip at year.

    I want to properly insulate my cathedral ceilings and was thinking to spray 2 inches of closed cell foam to sheathing, then fill with dense packed cellulose. I am wondering if I can attach craft paper and pine paneling to the 2x8 rafters and then fill the bays with cellulose. I.e. can I do it with out drywall? I don't want to add the extra weight and I figure that if the cellulose is dense packed, it will reduce fire spread similar to how drywall would serve that concern.

    Any thoughts or suggestions?

  128. User avater GBA Editor
    Martin Holladay | | #128

    Response to Daniel Cabe
    Daniel,
    Code interpretations are subject to the determination of your local building inspector. Ultimately, it doesn't matter what I think -- what matters is what your local code official thinks. So ask him or her.

    It wouldn't surprise me if your building official insisted on a layer of drywall.

  129. John C | | #129

    response to milo
    sips are great but in the coldest of climates they have encountered problems. google problem applications in alaska - apparently a group of homes there were built but not enough attention to detail for the plumbing and roof vents were taken into account (absolutely air tight foamed in penetrations), mated panel seams and fastener punctures caused the top level osb to rot. needless to say the whole roofs needed replacement.

  130. Hari Kamboji | | #130

    What to do with ridge vent in unvented assembly
    We plan to convert to an improperly vented cathedral ceiling to an unvented one. Sealing up the soffit vents is a no-brainer. What's the best way to deal with the ridge vent? Thanks!

  131. User avater GBA Editor
    Martin Holladay | | #131

    Response to Hari Kamboji
    Hari,
    Air sealing methods are similar in all areas. Big gaps are filled with solid materials (rigid foam, plywood, or OSB). Somewhat smaller gaps are filled with spray foam. Narrow gaps are filled with caulk.

  132. Eric Matsuzawa | | #132

    WSU for Valleys/Eaves/Low-Slope on Unvented Cathedral
    It makes sense to make sure the top layer of sheathing can dry to the exterior when using rigid insulation on an unvented cathedral ceiling. But what about the recommended waterproof underlayment for valleys, eaves, and low-slope sections of roof?

    I have gotten to the stage of laying down the tar paper for the unvented cathedral ceiling on my self-built house and don't know if I should add waterproof shingle underlayment to the valley, eaves, and a 2 in 12 section of roof. I see that it is added in the drawing detail found here:
    http://www.greenbuildingadvisor.com/cad/detail/unvented-roof-cold-climate-cathedral-ceiling-asphalt-roofing

    Is it okay to add peel-and-stick membrane to select areas of the roof and keep the rest breathable with tar paper?

    I attached two photos—the two taped layers of polyiso, and the current stage of the roof

  133. User avater GBA Editor
    Martin Holladay | | #133

    Response to Eric Matsuzawa
    Eric,
    Most types of roofing are vapor-impermeable, or close to it -- the exceptions include cedar shingles, slate, and concrete tiles -- so outward drying of roof sheathing is usually impossible.

    If a builder really wants roof sheathing to be able to dry outward, it's always possible to create ventilation channels above the roof sheathing. In your case, however, you'd end up with three layers of roof sheathing if you went that route. That's complicated and expensive.

    The bottom line is that, as long as your roof sheathing is relatively dry when the roof is closed in, you should be fine, even without outward drying. I would go ahead an install the peel-and-stick membrane in the valleys and at the eaves without worrying.

  134. Eric Matsuzawa | | #134

    Response to Martin Holladay
    Thank you for your advice, Martin. Green Building Advisor has been the best for answering specific questions I've had from the basement all the way to the roof. I wouldn't be able to build my own house solo like this without you and other readers helping me out with information.

  135. John Vickery | | #135

    not a perfect science
    I just opened up a roof, a-frame, that was built and insulated in 1977 with the cheap pink crap. The house is in summer shade and no shade winters. Climate is hot springs, Arkansas...humid summers!!!

    The roof was shingles on t&g pine decking. Insulation was right up against that pine decking since 77 and is 2014. Stuff looks like it was installed yesterday. Not a bad spot anywhere, no roof vents completely unvented roof. However between the first and second floor there was no sound batts or insulation and those 2x10 cavities did have some mold in them. I just wonder why this roof is in such good shape after all these years.

    Also, why not zip panels taped and then roxul batts with drywall over that. Roxul is awesome stuff I have been testing for years in crazy ways trying to see its abilities. I think that zip panels, taped properly, with roxul batts will last as long as the roof!

  136. User avater GBA Editor
    Martin Holladay | | #136

    Response to John Vickery
    John,
    I'm glad to hear that your insulated roof assembly has no rot. That's good news.

    Just because a roof assembly doesn't follow the rules established by the building code or recommendations of building scientists doesn't mean the roof has to fail, and your roof is proof of that.

    Let's say that a certain building practice results in a 20% failure rate. That practice would be totally unacceptable to builders, who don't want callbacks or headaches. But that practice would still result in 80% of customers being satisfied.

    Remember: just because your roof assembly is dry, doesn't mean it is well insulated. Your assembly could have massive air leaks that act as thermal bypasses. (Or not. It's hard to tell without a site visit.)

    The last time you posted comments on your roof (on 4/8/2014, when you posted Comment #6 on the Q&A thread titled Insulating unvented cathedral ceiling), Dana Dorsett and I provided several possible explanations for why your cathedral ceiling is dry.

    In my answer to your last post, I wrote:

    "Every house is unique. The factors that may have saved the roof sheathing of the house you describe could include:
    (a) It's not in a cold climate, so the roof sheathing never gets very cold.
    (b) The roof is steep, so snow doesn't sit on the roofing for long.
    (c) It's possible that the indoor relative humidity is relatively low -- you never know until you measure it.
    (d) There may be enough air leakage at the "soffit" and ridge to provide air flow that acts the way a ventilation gap ordinarily would.

    Of course, I'm speculating. The proof is in the pudding -- the roof sheathing isn't rotten. That's good."

    Dana Dorsett provided the following comment:

    "R-19 batts are very air permeable, and on long unobstructed rafter bays you have quite a stack effect. It doesn't take a huge amount of air leakage to provide some drying there. Also, if the batts are unfaced or kraft-faced, the drying toward the interior would be substantial. And a T & G plank roof deck is far more tolerant of wintertime moisture drives than OSB, not to mention that those moisture drive in Arkansas climate zone 3 aren't nearly what it is in cold climates."

    The air leakage that Dana mentioned would keep your roof dry, but would undermine its thermal performance.

  137. Brandon Francom | | #137

    Hi. I am somewhat new to
    Hi. I am somewhat new to wood frame construction, and am trying to understand how to properly seal and insulate a shed roof. I have a low-slope roof of 2 to 12, with a standing seam metal roof on 3" rigid insulation on a p.t. plywood deck. We have a cavity space with 9-1/4" TJI rafters, but we would like to eliminate batt insulation and the requirement of gyp. board underneath for 2" of closed-cell spray foam insulation to the underside of the deck. We were also hoping to use recessed light in the cavity if we were able to go this route. So, if anybody is still checking this, please reply. Basically, what I am asking is can you use a combination of rigid insulation on top with closed-cell spray foam underneath? Or in other words, can you use two impermeable insulation types? (Please click on the images below. One of them is a partial building section.)

    Another question is, should I continue the same insulation assembly outside? The last image shows a large 10' overhang. Should I vent this exterior portion or seal the underside of the p.t. plywood deck with the closed-cell insulation?

  138. Jeff Newcomer | | #138

    Perfect Storm - vaulted ceilings, low-slope & recessed lighting
    Hi Everyone.

    I see I am a little late to the party on this thread but I hope someone can still help me. From my subject line you can see I have quite the house. I am the home owner and frustrated with my leaky roof. I live in WI and my roof has asphalt shingles.

    With my house the middle section (20ft) is vaulted, a 3x12 slope and has 8 recessed lights throughout. I have had leaks in numerous locations in this section of the house, the leaks happen with the thaw and refreezing. It got the point last year where I had to shovel the entire roof after each snow fall. I did notice several areas of ice damning, so I am going to assume that is the source, getting under my asphalt shingles, i am also going to assume that their is not proper ventilation under the sheathing. The other sections of the house have the same slope, but no recessed lighting and no vaulted ceilings, no ice damning. I also have seen a few leaks when we get driving rain.

    This article was very informative and seemed to give a lot of advice on cathedral ceilings. But what what my perfect storm? The house is 30 years old (80's) and im sure the ceiling is not air tight. I want to do something this summer/fall to try to fix these leaks but I am worried that whatever I do wont work and will be a waste of money, especially since I havent heard of a solid solution.

    What I want to so is rebuild my fire place chimney (wood structure) and reflash, currently it is step flashed. As well as remove the ridge vent and sky light (original with house). After reading this article it seems their is more to it and that I should consider doing a above sheathing insulation/ventilation system (possibly the cool vent Hunter Panels). Would you guys recommend this solution, or would you go a different route? There is soffit vent and ridge vent already in place.

    Lastly, I kow the recessed lighting is horrible, but what can I do if it is already installed? With one of the lights I changed it over to a pendant light which reduced the heat near the roof but the air barrier is still a issue. Can anyone recommend a solution addressing already in place recessed lights in a vaulted ceiling?

    I have been racking my brain on this for some time now, I hope someone can give some guidance. I can include pics if needed.

    Thanks, Jeff

  139. User avater GBA Editor
    Martin Holladay | | #139

    Response to Jeff Newcomer
    Jeff,
    If you want to do it right, you need to address air leaks, address your recessed can problem, and add enough R-value to at least meet minimum code requirements. It's a lot of work, but it's not rocket science. Fixing these problems may be expensive, however.

    The best solution is to address the situation from above, by adding a significant amount of rigid foam above your existing roof sheathing. You can build up the layers yourself, of you can buy SIPs or nailbase panels.

    If you are intimidated by the details, it may be time to hire a contractor.

  140. Derek Dykstra | | #140

    Above sheathing ventilation detail
    "If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels."
    Could you please provide a detail of above sheathing ventilation using a continuous Ice&Water membrane, and metal roofing for a mixed climate? Thank you

  141. User avater GBA Editor
    Martin Holladay | | #141

    Response to Derek Dykstra
    Derek,
    The GBA website does not provide custom details. That's what an architect or an engineer is for; I suggest that you hire one.

    If you are a GBA member, you already know that GBA has hundreds of architectural details in its detail library for you to study and download.

    Here is a link to a GBA detail for a vented insulated roof assembly with metal roofing and Ice & Water Shield:

    http://www.greenbuildingadvisor.com/cad/detail/vented-roof-siding-cold-climate-metal-roofing-fiber-cement-siding-over-rigid-foam-furring

    Here is a link to a GBA detail for an insulated roof assembly with vent channels above the roof sheathing and metal roofing:

    http://www.greenbuildingadvisor.com/cad/detail/sip-backventing-roof-cladding

    I'm guessing that neither of these details exactly fits your needs. But as I noted earlier, if you want a custom detail, you probably need to hire an architect or an engineer. Good luck.

  142. Chris Heard | | #142

    3" Closed Cell Foam + Roxul + Rigid Foam
    In the final summary it reads: "If you are using fluffy insulation, you need two air barriers: one below the insulation, and one above the insulation." I would like to use 3" of closed cell foam against the inside of the sheathing + 5.5" of Roxul and cover the whole thing with 1" rigid foam to reduce thermal bridging. Is this an acceptable strategy?

  143. User avater GBA Editor
    Martin Holladay | | #143

    Response to Chris Heard
    Chris,
    Unless you tell us your climate zone, it's impossible to answer your question.

    Depending on your brand of spray foam, 3 inches of closed-cell spray foam will give you about R-19.5 -- which is almost enough for Climate Zone 5, but definitely not enough for Climate Zones 6, 7, or 8.

  144. Jen Gah | | #144

    Best Option Recommendation
    I purchased a disaster of a house that was partially remodeled, it's in Zone 16 (a California designation similar to IRC zones). It has a 6/12 vaulted ceiling over the living/kitchen area. The roof is held up by two long beams that run 4 feet out parallel to the ridge , thus there is no straight air path a the way up to the peak ridge beam. The other half of the house has flat ceilings with attic space above them. The front half (vaulted) part of the house is unfinished raw, 2X6 rafters. It is separated from the back half of the house with sheetrock going up to the underside of the sheathing. So in short, the back half of the house is sealed off from the front and has plenty of insulation above the ceilings.

    The roof appears to have a layer of asphalt shingles over the sheathing with a tin roof over that. The soffits are closed off. The house is way off the beaten path and it's difficult to get any contractors out there to work, let alone provide bids so I'm pretty much on my own. Can you provide some ideas of the best, worst case scenario. I realize it may not be to code, but what do you think of the following approach:

    1.) try to make sure soffits are sealed off from the inside as best as possible.
    2.) fill 2X6 bays with R-21 fiberglass bats.
    3.) install ceiling of taped sheetrock
    4.) remove tin roof and shingles eventually and install a layer of rigid foam and replace tin roof.

    The house is at about 4,500 ft. We only get about 3' of snow a year and it rarely stays on the roof for more than week a couple times a year. I'm on a very tight budget, just trying to make the place livable. Any suggestions are welcome.

  145. User avater GBA Editor
    Martin Holladay | | #145

    Response to Jen Gah
    Jen,
    The longer that you wait to install rigid foam above your roof sheathing after your rafter bays are filled with fluffy insulation, the greater the risk that your roof sheathing will accumulate moisture. One or two years should be no problem, but I wouldn't stretch the work out much farther than that.

    The best approach is to do all the work at once: rigid foam above the roof sheathing (as recommended in the article), and fluffy insulation underneath.

  146. Edward Skakie | | #146

    Unvented attic (new construction)
    I've gone through this section, looking for guidance for my particular design problem: I want to install my HRV centrally in the attic, and building code requires an unvented attic, with R-50 under the roof, so I'd like to use either 9"+ of XPS, or, preferably, EPS of a thickness of 10", or more. I will cut 1.5" wide channels, 4 or 5 inches deep, at 16" o/c, and drop these 8' wide x 4' high foam blocks over of the glue-on-top roof trusses, then screw them down with the proper long screws & washers. My stucco guy says he can immediately lay down an absolutely waterproof layer on the top surface, so no joint taping, as such, required, and also says he can do a stucco roof. I can foam in cavities around the EPS & truss members, and use the same procedure on the outside of the kneewalls above the main ICF shell, but I will have to drywall the whole attic.

    Your thoughts?

  147. User avater GBA Editor
    Martin Holladay | | #147

    Response to Edward Skakie
    Edward,
    I think that you need to post a sketch. I can't visualize what you are talking about from your description. If you want to install an HRV in your attic, why do you need to install a waterproof stucco roof over some type of EPS box? Isn't your attic already dry? Most attics have roofs.

    If you want to create a conditioned attic, this article tells you what you need to know. For more information on the topic, see Creating a Conditioned Attic.

    Two other points:

    1. It's usually a bad idea to install an HRV in an attic. HRVs need to be accessible enough for the homeowners to routinely change the filter.

    2. Stucco isn't roofing. There is no such thing as a stucco roof.

  148. Edward Skakie | | #148

    Unvented attic
    Sorry, Martin, I now realize that "new construction" does not necessarily mean "new home to be built", and "an unvented attic, with R-50 under the roof" should have been "an unvented attic, with an R-50 roof'. Just because it's clear to me doesn't mean it's clear to everyone else.

    I now understand that stucco cannot be applied to a roof, so I will revert to the a steel roof, but still want to have an unvented attic. I've read through "Creating a conditioned attic", and am still not sure as to the best way to proceed.

    My question is on the roof structure.: in the attic, we will have manufactured trusses 16" o/c, covered with blocks/buns of EPS, 10" thick, 8' wide x 4' high, or x 4'W x 8'H, with channels on the bottom to allow a snug fit over the trusses; screws, with plastic washers, will be screwed through the EPS down into the trusses, & the EPS then covered with 5/8" sheathing. We will then waterproof the whole roof, then install vertical strapping under horizontal strapping to create a ventilation space, and, lastly, the steel roof on top. Sprayfoam will be applied in the attic, on the underside of the EPS, to ensure there are no leaks of any kind. The gable end ICF walls will run right up to the roof underside. Our building code requires the attic interior be drywalled, and I want to ask your opinion as which sprayfoam should be used, and if we will need to install a vapour barrier of any kind.

    Thank you.

  149. User avater GBA Editor
    Martin Holladay | | #149

    Response to Edward Skakie
    Edward,
    You are proposing an unusual method of insulating your roof. If I understand you correctly, you intend to install EPS between the tops of your roof trusses and your roof sheathing.

    Before proceeding, you need to talk to two people: (a) an engineer, and (b) your local code official. There are important structural reasons why trussed roofs usually require a layer of OSB or plywood (roof sheathing) to be fastened directly to the top of the trusses.

    EPS is somewhat squishy, and I can imagine problems from your plan.

    The usual ways to use EPS foam to insulate this type of roof include:
    1. Install a thick layer (or layers) of rigid foam above the roof sheathing, followed by a second layer of plywood or OSB.

    2. Install SIPs, following the SIP manufacturer's instructions.

    3. Install nailbase above the roof sheathing. (Nailbase panels are like SIPs, but with OSB on only one side.)

  150. Jim Boyd | | #150

    Cut and cobble
    Martin: I have combed this thread and hope my question is not already answered. I am building a roof with a 6/12 pitch using a 24" parallel cord truss with an energy heel for a 24'x26' apartment over a garage. I want to use 2" of XPS foam attached to 1-3/4" furring strips down from the bottom of the 5/8" ply between the trusses creating an air channel between bottom of ply and top of foam. I was also considering spraying a thin layer of closed cell foam in all corners and in truss gaps. Then filling cavity with blown fiberglass and adding 1-1/2" rigid taped insulation at bottom of truss, then drywall. According to code apparently my rigid insulation must be placed against the bottom of the plywood so I assume my method will not meet code. I am avoid an unventilated roof but want to maximize my insulation. Should I abandon this plan, use OSB (or Solitex Mento 1000 WRB) to form the air channel and add all of the rigid insulation to the bottom of the truss with battens securing it. Is there a more vapor permeable insulation board that I could use on top (to make the air vent cavity). My goal is an R-80 roof in a zone 7 climate. My walls will be R35 (5-1/2" blown fiberglass plus 3" xps on exterior) and I will be using Alpen windows U.16 (ish). My assumption is if I use board, fluff, board method that I am creating a insulated cavity that will have no way to dry. But what if that cavity cannot get wet as the rigid insulation below will be sealed and taped?? Thanks and sorry for any redundancy.

    [Editor's note: Click the box for page 4, below, to read the response to this post.]

  151. User avater GBA Editor
    Martin Holladay | | #151

    Response to Jim Boyd
    Jim,
    You are proposing a vented roof assembly. For a vented roof assembly, the code does not require that any foam insulation be placed in direct contact with your roof sheathing -- on the contrary. (That requirement only applies to unvented roof assemblies.) For a vented roof assembly, the code requires an air gap under the roof sheathing, as you are proposing.

    Your plan to use 2-inch-thick EPS as your ventilation baffle will work. The EPS is somewhat vapor-permeable, so it will allow a little bit of outward drying. (In any case, if you build a fairly airtight ceiling, there won't be any moisture in this assembly that needs to escape.)

    If you want to choose a material that is a little bit more vapor-permeable for your ventilation baffle, you can -- for example, some people use thin plywood or fiberboard in this location -- but you really don't have to. The EPS will work

  152. Seth K | | #152

    Mansard roof question
    I have a mansard roof with steep sides with gables, and a nearly flat top with a rubber roof. There are no vents, it has never been insulated before I started working on it and have 2 questions:

    I have insulated the sides with R-19 fiberglass and now I'm wondering: was that a mistake? It has craft paper and I was planning on a plastic sheet as well, should I skip that?

    Should I treat the flat roof as a cathedral ceiling and go with spray foam (or board), fiberglass, and no moisture barrier?

    Thanks, for any help, I've been getting lots of conflicting advice.

  153. User avater GBA Editor
    Martin Holladay | | #153

    Response to Seth K
    Seth,
    Like most mansard roofs, your roof is unvented. Your mansard roof has different sections -- the steep sections at the perimeter, and the low-slope section in the center. All of these sections need to be insulated in a similar way, because all of these roof assemblies are unvented.

    If you want to create an unvented insulated roof assembly, you have two choices: (a) You can add rigid foam to the exterior side of your roof sheathing, followed by a second layer of roof sheathing and new roofing, or (b) You can insulate the underside of your roof sheathing with spray polyurethane foam.

    It is a code violation to install an air-permeable insulation like fiberglass batts between unvented rafters. It's also a bad idea, because moisture can accumulate on your roof sheathing if you try this method, and you'll end up with sheathing rot.

    More information on how to insulate an unvented roof assembly is provided in the article on this page.

  154. Seth K | | #154

    Mansard
    I have already insulated the steep sides of my unvented mansard roof with 6" fiberglass (and I had it inspected), do you recommend I rip that out and use spray or board foam? I've gotten conflicting info on whether to treat it as a wall or a roof.

    Thanks again, Seth

  155. User avater GBA Editor
    Martin Holladay | | #155

    Response to Seth K
    Seth,
    My opinion is that the steep side of a mansard roof is a roof. This is especially true if the exterior of the sloped assembly is finished with roofing.

    Most types of roofing are vapor-impermeable. (There are exceptions, of course, including cedar shingle roofing, slate roofing, concrete tile roofing, and clay tile roofing.) If you have vapor-impermeable roofing on the exterior side of your steep mansard roofs, you definitely can't insulate these roofs as if they were walls -- because these assemblies can't dry to the exterior.

  156. Seth K | | #156

    Mansard
    The sides of the roof are covered with asphalt shingled (GAF Slateline to be exact). It sounds like I should pull down the fiberglass and have it spray-foamed before I drywall. This room is a bathroom, so moisture is even more of a risk I imagine. I'm in Boston so it sounds like I need R-20, which is about 3 inches of spray foam, right?

    However, 2 other areas of side-roof have already been dry walled over with fiberglass and a plastic sheet -- the bathroom closet and the hallway. I'm afraid to ask but should I open those up again? The closet is easy but the hall would be a pretty big job. Also, a bedroom currently has no insulation and my plan was to have cellulose blown into the steep part of the mansard. Is that a bad idea as well?

    Thanks again

  157. User avater GBA Editor
    Martin Holladay | | #157

    Response to Seth K
    Seth,
    I'm not sure if you are interested in a building science answer or a code answer.

    From a code perspective, this is a roof that needs to be insulated to R-49. However, you can either (a) ignore the building code, which may not apply for the type of work you are doing, or (b) contact your local code official to find out whether your local code differs from the 2012 IRC.

    From a building science perspective, R-20 of closed-cell spray foam, properly installed, would protect the sheathing from moisture damage.

    I'm sorry to be the bearer of bad news, but the sections of roof that are insulated with 6 inches of fiberglass and interior polyethylene were not insulated correctly. In those sections, the roof sheathing is at risk of moisture accumulation and rot.

  158. Seth K | | #158

    Mansard
    Code doesn't seem to apply since it's grandfathered in. It sounds like I need to redo the hall ceiling.

    As for my energy auditor recommending blowing cellulose into the mansard sides, you also recommend against that? I was under the impression blown cellulose prevents moisture migration.

    Thanks again.

  159. User avater GBA Editor
    Martin Holladay | | #159

    Response to Seth K
    Seth,
    You wrote, "I was under the impression blown cellulose prevents moisture migration."

    You are mistaken. Cellulose is vapor-permeable. It is also air-permeable, although not quite as air-permeable as fiberglass.

    Cellulose insulation can store a lot of moisture. This "moisture buffering" ability is cited by some cellulose fans as a major advantage, and indeed this ability can sometimes prevent a small moisture problem from causing a lot of rot. However, the ability of cellulose to store moisture is also evidence that cellulose is not capable of (as you put it) "preventing moisture migration."

  160. Peter Matt | | #160

    Closed Cell + Fiberglass + EPS?
    Great article!!!

    Does it make sense to spray and cut and cobble 4 inch closed cell foam/board on underside of sheathing followed by fiberglass inside the bays and 1 inch of eps/iso under the rafter (to stop thermal bridging) before finishing with drywall? Will I create a vapor sandwich and better leave the final EPS and accept the thermal bridging?

    I already sprayed and cut cobbled 4 inch of closed cell. I will redo the roof next year, but I guess now it is too late to think about eps/iso on top of roof since i already unvented the attic.

    I am in Zone 5 in a 100 year old house.

  161. D Dorsett | | #161

    It's a bit risky doing it with foam board.
    As the house flexes with changes of temperature it's possible, even likely that some of the foamed-in air sealing will leak. It might be worth doing 2" of open cell foam on the interior side of which would be a more flexible and reliable long term air seal.

    It's fine to add foam board above the roof deck even with the closed cell foam-board (what type?) on the interior. Roof decks can't dry to the exterior through 0.1 per shingles anyway, particularly since in zone 5 it will spend a significant amount of time covered in snow/dew/rain with the moisture drive heading inward, not outward. If the moisture content of the roof deck is low when re-roofing (measure it- under 20% is good, under 15% even better), put a self sealing peel'n'stick membrane (eg Grace Ice & Water Shield) over the roof deck, and put the rigid foam above that.

    An inch of EPS on the interior isn't much of a thermal break, but better than nothing. Better yet would be to cut some rafter-edge strips of 1-1.5" rigid polyisocyanurate. 1.5" polyiso on the edge strips with a complete fiberglass fill does more for the whole-assembly R than a full layer of 2" EPS.

    The R value of your cut'n'cobbled foam + exterior foam has to be at least 40% of the center-cavity R value for dew point control at the foam/fiber boundary in zone 5. If it's coming up a bit shy of that before adding the exterior foam it's not a big deal in the short term, but if it's going to be 5-10 years before re-roofing to raise the foam-R to above 40% of the total it's worth using vapor barrier latex primer on the gypsum board, or a smart vapor retarder between the gypsum board & fiberglass.

  162. John Poulin | | #162

    Retrofit
    First, thanks Martin for the wealth of knowledge this website has. I've been visiting it regularly for the last few years, ever since purchasing my first home.

    I am also located in northern VT, zone 6 but I think we may be pushing 7 due to localized conditions. I've recently had an energy audit performed and wanted to verify one of the recommendations. We have an area with a cathedral ceiling, and also a shed dormer built into the cathedral. The cathedral ceiling is insulated with r38, and the shed dormer with r19. Both are kraft faced fiberglass. The roof is 12/12 on the cathedral, not sure but less on the dormer, and has snow coverage (often over 8") from the beginning of January until mid March. There are soffit and ridge vents.

    I was told that hanging polyiso sheets under the rafters and sealing the seams with caulk or tape would be the most efficient way of reducing air leaks and adding insulation. This seems to be contrary to most of the advice , which reverses this assembly. Will this assembly cause any issues with dew points / condensation? I am looking for a cost effective improvement, but want to make sure I'm not causing problems along the way.

    Last detail, it is a newer construction (2007) but the rafters are still exposed, so I have access.

    Thanks for your time,

  163. User avater GBA Editor
    Martin Holladay | | #163

    Response to John Poulin
    John,
    You wrote, "the rafters are still exposed," but I'm not sure what you mean by that. Do you have kraft-faced batts between the rafters, with the kraft facing still visible? Or do you have decorative rafters with nice T&G boards above the rafters, and then a second insulated roof assembly above the T&G boards?

    We really don't know if these roofs have proper vent channels, or what type of vent baffles (if any) were used to maintain the ventilation gap between the top of the fiberglass insulation and the underside of the roof sheathing.

    If this were my house, and if the fiberglass batts are visible, I would pull all of the insulation out of a few rafter bays to see what's up there.

    Once we know what you have for an existing assembly, it will be easier to give advice.

  164. Debra R | | #164

    urgently need advice re: cathedral ceiling insulation
    I have an urgent problem and would be very grateful for advice on insulating our house’s family room cathedral ceiling, built in the 1970’s and currently being re-roofed. (The old roof has been torn off, and they are about to start installing the new shingles.) The house is in San Jose, CA, climate zone 3. The contractor does not want to put insulation above the sheathing, which is 2x8 pine tongue and groove, which up til now served as the finish ceiling. The new roof over the cathedral ceiling will consist simply of the pine T&G, then felt, then asphalt shingles. This is almost the same as the previous roof, except that the previous roof included ½” of sound insulation board, which provided almost no thermal insulation.

    Below the 2x8 pine T&G, there is 9" of beam exposed, but we only have 4” or at the very maximum 5” to play with, because at the top of the stairs which ascend the side of the room, the ceiling is barely above a tall person’s head as it is. In addition, we want to maintain an exposed beam profile for the aesthetics. I hope the attached photos illustrate the situation.

    I believe an unvented roof assembly is the only possibility here, as the highest point of the cathedral ceiling is not the ridge of the roof.

    I am envisioning some combination of XPS board, Roxul Comfortbatt, and/or Roxul Comfortboard IS underneath and directly adjoining the pine T&G. We intend the new finish ceiling to be a new layer of T&G, either pine or cedar, 1x6 or 1x8.

    I would like the insulation stackup to include at least 1” of some Roxul product because I am concerned about sound insulation - as noted above the previous roof included 1/2" of sound insulation board.

    We want to add track lighting to the new ceiling, with its electrical supply running through the new cavity created for the insulation. The existing fan also needs its electrical supply to run through that space.

    A new standalone direct vent gas stove is going in at the far end of the room under the metal box. I figure if we use XPS board, it would have to be pulled back several inches from around that area, but I didn't think it would prevent me from using XPS entirely. Seems like Roxul is less of an issue in terms of heat/fire.

    I don’t think I can meet all of the guidelines that I’ve read on this website, given existing construction constraints and reluctance of my contractor to implement insulation above the sheathing and foam for the new cavity. Given these limitations, what would be the least incorrect way to implement this? What about an unvented assembly consisting of:
    Shingles
    Felt
    Pine T&G
    XPS board
    Roxul board or batts
    Gypsum drywall
    New 1x T&G

    Can I get away without some of those layers given my climate zone? For example, can I use all Roxul, without XPS? Do I absolutely need the drywall?

    Thanks very much for any advice.

  165. User avater GBA Editor
    Martin Holladay | | #165

    Response to Debra R
    Debra,
    Don't make any hasty decisions that you will regret. If necessary, tell your roofer to stop all work on this project until this issue is resolved (assuming, of course, that your roof sheathing is protected by asphalt felt or a similar underlayment.)

    The right way to proceed is to install rigid foam above the roof sheathing. There really aren't two answers to this question. The time to do the work is now. If your contractor won't perform the necessary work, you'll need to hire a different contractor.

    For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

  166. User avater
    Stephen Sheehy | | #166

    Is your contractor doing it for free?
    If not, he doesn't get to refuse to put foam above the sheathing, which would be better and cheaper than fitting it between the beams.

  167. Debra R | | #167

    thanks, we will reconsider
    Thank you, Martin and Stephen. Fortunately we are in a situation where we can pause and reconsider, as the new felt has been placed, but they haven't started installing shingles yet. I will discuss the situation with the contractor again. I may be able to get him to reconsider.

    A few follow-ups:

    I see the recommendation is that when starting with board sheathing, one should place a watertight membrane over the sheathing before installing the lowest layer of foam. However, in my case the roofer has already completed placing new felt over the T&G board ceiling/roof deck. Can that felt substitute for the prescribed watertight membrane, here in San Jose/Climate Zone 3? Is a watertight lower membrane something that is more important in colder climates? Alternatively, if I have them pull off the already installed felt, and put down a membrane instead, will the felt be ruined, or will they be able to reuse it on top of the final topmost layer of plywood, just below the shingles? I'm guessing it will be ruined and we'd have to throw it out and start over.

    If we're going to put some insulation above the ceiling, then I want to put ALL the insulation above the ceiling, and avoid the cost and hassle of a new lowered ceiling. That means a build-up of 8", using XPS. If we build up the roof line above the cathedral ceiling of the family room by 8", we presumably need to build up the rest of the adjoining gable roof as well. However, the rest of that gable roof, other than the part over the family room, goes over the second story bedrooms, which have a conventional 8' drywall ceiling and a vented attic with blown-in cellulose to R-30. Is there any problem mixing these two roof systems, i.e. putting 8" of foam over the roof of the already insulated vented attic?

    I'm not sure what to do about the electrical for the existing fan, and for the not-yet-installed but much desired track lighting. In the old roof, the fan's electrical supply ran on top of the T&G ceiling, and was covered by what looked like electrical tape, then by a 1/2" layer of sound insulation board, and then by the felt and shingles. We had disconnected that electrical supply in preparation for running it through the new interior cavity, but if we no longer need an interior cavity for insulation, it will have to go back on top of the ceiling somehow.

    Thanks again. Appreciate your valuable input.

  168. User avater GBA Editor
    Martin Holladay | | #168

    Response to Debra R.
    Debra,
    Directly above the ceiling boards, you need an air barrier. This is necessary in all climates.

    The existing asphalt felt can stay, even though it is not an air barrier, if you put a new airtight membrane (or new sheathing, like OSB, with taped seams) above it. Or it can be removed, to facilitate the installation of an airtight membrane. There is no way to make asphalt felt airtight. Once it is removed, it can't be re-used.

    Your plan to install 8 inches of rigid foam is a good one, but you may be able to install thinner foam if there is a compelling reason to do so. Of course, 8 inches of rigid foam has a higher R-value, and will perform better, than a thinner layer. Check with your local building department to find out whether you have to comply with minimum code requirements for insulation thickness on this type of job.

    If you decide not to extend the rigid foam over the parts of your roof that have a conventional vented attic, your builder (or an architect) can come up with a trim detail that you may be able to live with. However, if the aesthetics of this change in roof plane bother you, you can continue the rigid foam over the entire roof. That way, you will end up with a conditioned attic. If you do that, you will probably want to seal the soffit vents for that attic.

    You should consult an electrician about the best way to serve your ceiling electrical boxes. There are several possible solutions -- some visible from the interior, and some hidden by the new foam.

  169. Debra R | | #169

    attic questions
    Martin, re: changing the attic over the second story bedrooms to a conditioned attic and sealing the soffit vents.... Prior to this latest idea about insulating with rigid foam on top of the cathedral ceiling, we had been planning to cut a ridge vent into the gable ridge, in order to meet ventilation code requirements. Currently we have eave vents plus two fairly useless gable vents, one on each side. We had been planning to block off the gable vents and install 15' of ridge vent instead at the center of the ridge instead. Are you saying that increasing ventilation to that second story attic would actually be a bad idea, if we continue the foam over the entire second story gable roof, and instead we should make it completely unvented?

    Perhaps the way to go is to settle for 4" of foam over the cathedral ceiling, rather than 8" of foam, and NOT continue the foam over the vented attic. There will be a weird jog in the roof line where the foam starts, but it will mostly only be visible from the back of the house, not from the street. Yes, 4" won't get us to R-38, but compared to the basically zero insulation we've been living with for the last 20 years, the R-20 we would achieve would be heavenly.

    Thanks again.

  170. User avater GBA Editor
    Martin Holladay | | #170

    Response to Debra R
    Debra,
    Q. "Are you saying that increasing ventilation to that second story attic would actually be a bad idea, if we continue the foam over the entire second story gable roof, and instead we should make it completely unvented?"

    A. To take advantage of the insulating value of the rigid foam over the attic, you definitely want to seal up any attic vents, and you certainly don't want to create a ridge vent.

    Of course, if you don't care about the cost (and usefulness) of the new rigid foam, nothing would stop you from keeping a vented attic. But that seems a shame. The foam is expensive and useful -- so why not take advantage of its insulating value? (Obviously, inviting outdoor air to ventilate the space under the insulation makes the insulation basically worthless.)

    For more information on this issue, see Creating a Conditioned Attic.

  171. Debra R | | #171

    attic, etc.
    Martin, thanks. I just talked to the roofer and he rejected the idea of only putting the foam over the family room cathedral ceiling. That weird "jog" in the roofline, having it suddenly bump up 4" or 8" where the family room starts, in the middle of the back side of the roof, would defeat the idea of a continuous slope for rain to come down when it hits the upper part of that back roof. So the 4" or 8" height addition needs to continue across the entire second story roof line.

    So now I'm thinking either (a) 4" of foam added across the entire second story roof, or (b) 4" of foam over the family room cathedral ceiling, and 4" of non-foam height (some other cheaper material, or just build up the framing by 4"?) over the bedrooms' vented attic, and keep the existing attic ventilation/add the ridge vent. I have a feeling the (b) option is not a good one as far as the roof/shingle integrity, though...??

    If it seems like I'm out of my depth here... yeah, you bet, I am. I'm just a homeowner, with only a few days of internet research under my belt. But the contractor is not familiar with this roofing technique, so we're having to figure it out together. Appreciate your help immensely.

  172. User avater GBA Editor
    Martin Holladay | | #172

    Response to Debra R
    Debra,
    Either (a) or (b) is possible.

    You could add 2x4s (which are 3.5 inches high), installed 16" or 24" o.c., and 1/2" plywood or OSB, over the attic. That would provide you with your 4 inches.

  173. Debra R | | #173

    roof assembly stack-up
    Thanks again. The roofer agrees that 2x4 framing to raise the height over the vented attic would be a workable solution to match up the roof lines.

    For the roof assembly over the cathedral ceiling, does the stack-up below capture all the elements I would need?

    2x8 pine T&G (already in place, remains as the finish ceiling)
    asphalt felt (already applied)
    airtight membrane (is this the same as an ice & water guard?)
    2 layers of XPS foam, each layer 2" thick, seams staggered between layers, and taped within each layer
    plywood
    asphalt felt
    asphalt shingles

    BTW, for the foam type, I am leaning toward XPS because 1) I have the impression that ISO is somewhat fragile, such as if it gets at all wet, and 2) the possibility of long term outgassing with ISO. Hopefully any such outgassing would go upwards and outwards rather than downwards into the house interior, but the chance that it would outgas into the interior really worries me. If I'm missing some reason why XPS is a bad idea, please let me know. I know XPS is unfortunately less green of a material than the other rigid foams, but that is a secondary concern at this point.

    Thank you for the generosity of your time and advice!

  174. User avater GBA Editor
    Martin Holladay | | #174

    Response to Debra R
    Debra,
    You can use Ice & Water Shield as an air barrier if you want. But if you choose to use that product, you'll have to remove the asphalt felt so that the Ice & Water Shield can be adhered to the roof boards.

    Other options: a synthetic roofing underlayment that is also advertised as meeting air barrier requirements. Options include Solitex Mento 1000, Delta-Vent-S, or Titanium UDL 30.

    Polyiso is used all the time for this type of roof assembly. Polyiso does not outgas. It is more environmentally friendly than XPS -- and it has a higher R-value per inch.

  175. Debra R | | #175

    air barrier, vapor barrier, etc.
    Martin, thanks to your excellent and timely advice, we are now headed toward a solution of rigid foam over the exterior of the ceiling. Thank you for pointing me in this new and improved direction. We are still working out some details, such as...

    Re: the air barrier to be placed above the pine T&G ceiling and below the foam, would you recommend that we use something which is both an air barrier and a vapor barrier? Or is it important to make sure that the product we select is NOT a vapor barrier, so that any moisture that gets into the roof assembly can dry to the inside? As a case in point, the Grace Ice & Water Shield is both an air barrier and a vapor barrier (0.08 perm/Class I), so I'm wondering if that would be a bad product to use. Of the other underlayments that you mentioned, I see that the Titanium is also a vapor barrier, but the Solitex and Delta products are vapor permeable. Does that make Solitex or Delta the way to go?

    Another dilemma I'm having re: Ice & Water Shield is that I was looking forward to removing the newly placed asphalt felt that is currently over the pine T&G and replacing it with a different material, because the asphalt smell of the felt is coming through the T&G ceiling and bothering me (just in case I needed any evidence that that ceiling is horribly air permeable...). But then I read in the Grace Ice & Water Shield installation instructions that "Due to its slight asphaltic odor, do not apply Grace... Ice & Water Shield... where the membrane is exposed to interior living areas." So if I replace the felt with Ice & Water Shield or a similar asphalt-based product, I'd probably just be jumping from the fire into the frying pan, odorwise. This is leading me to think that the Solitex or Delta products would be better. I haven't priced these but I understand they will be expensive. I guess my other alternative for an air barrier that is not asphalt-based would be a layer of plywood with seams taped, but that would not only be expensive, but also add extra weight to the roof. Am I on the right track with evaluating the options, and would you recommend one direction over the others? I guess fundamentally before I can settle on an air barrier solution, I need to understand whether it should be a vapor barrier or not. I'm guessing it should not but would really appreciate your input.

    Thanks very much again.

  176. User avater GBA Editor
    Martin Holladay | | #176

    Response to Debra R
    Debra,
    Q. "The air barrier is to be placed above the pine T&G ceiling and below the foam. Would you recommend that we use something which is both an air barrier and a vapor barrier? Or is it important to make sure that the product we select is NOT a vapor barrier, so that any moisture that gets into the roof assembly can dry to the inside?"

    A. The air barrier can be a vapor barrier, because the rigid foam is also likely to be a vapor barrier (or close to it) in any case.

    Q. "If I replace the felt with Ice & Water Shield or a similar asphalt-based product, I'd probably just be jumping from the fire into the frying pan, odorwise. This is leading me to think that the Solitex or Delta products would be better."

    A. I agree. In light of the odor issue, I don't think that my suggestion to use Ice & Water Shield was a good one -- so I withdraw it. Either plywood with taped seams, OSB with taped seams, or one of the European membranes (or one of the U.S.-made synthetic roofing underlayments -- especially those advertised as air barriers) would be a better choice.

    I don't think that the weight of plywood or OSB is going to be an issue, but if you are worried about the additional weight, you can always consult an engineer.

  177. Debra R | | #177

    are R-values from separate compartments of insulation additive?
    Martin, thanks again. Another question: we currently have blown-in cellulose to level R-38 on the floor of the attic above the second story bedrooms. (Going from memory I had said R-30 earlier, but I checked and it was actually specified at R-38). The cellulose, of course, does not reach the top of the attic, so there is a substantial air gap between the cellulose and the roof sheathing. If we put R-20 of rigid foam above the roof sheathing and close off all the vents to that second story attic, does that give the interior space R-20 + R-38 = R-58 insulation level? Or is insulation not additive when it is in two non-contiguous compartments, so we'd actually be downgrading from R-38 to R-20 by doing that?

    Thank you.

  178. User avater GBA Editor
    Martin Holladay | | #178

    Response to Debra R
    Debra,
    If you do as you propose, you would gain an addition benefit from the R-20 insulation above the roof sheathing. That said, the approach you describe wouldn't meet code requirements for new construction, because ideally any insulation used to supplement the rigid foam above the roof sheathing needs to be in direct contact with the underside of the roof sheathing.

    This is a complicated topic, and if you are worried about code requirements in your town, then by all means you should ask this question at your local building office. In most cases, renovation work can be performed without bringing all aspects of your house up to current code requirements.

    In general, though, there is no harm, and there will be a thermal performance benefit, from proceeding as you describe.

  179. Charlie Sullivan | | #179

    Response to Debra R
    The reason to put the additional insulation in contact with the underside of the roof is that it's hard to verify that you have successfully closed off all vents to the attic. But if you really do succeed in doing that, there should be no problem. With a blower door is possible, though tricky.

  180. George Baum | | #180

    Cathedral roof vent
    Have room over garage center with closets on both sides between the closet kneewalls and the eaves. Insulation over the room section is 6 inches of foam slabs sealed by "Great Stuff" spray-in foam. Above this ceiling foam and under the roof plywood, there are 1 inch x 20. inch unobstructed foam encased ducts between the rafters for venting to the roof peak vent. The side sections outside the knee walls are insulated with rolled fiberglass fiber with durovent ducts between the insulation and the roof plywood. There are substantial soffit vents at the base of the durovent ducts. After a new roof was installed, the fiberglass started to absorb substantial moisture and mold appeared on the plywood as shown in the picture. This only happened on the north side of the garage, none on the south side. Since the plywood must now be removed, I am thinking of increasing the duct heights by raising the roof to give 2 " ducts rather than the 1". Also I would add a mid-roof SmartVent to increase airflow. In addition I would increase the peak vent from Coravent 300 to CoraVent 600 to double the outlet area. Does this look like a reasonable fix?

  181. Alan B | | #181

    Sorry for beating a dead horse
    http://buildingscience.com/documents/insights/bsi-081-zeroing-in
    This roof assembly looks pretty cathedral but if i understand correctly its not vented at all, but has generous above sheathing foam. Is this considered a vapour safe assembly considering this article says

    "Install rigid foam insulation above the roof sheathing and air-permeable insulation between the rafters. If you choose this method, it's possible to install vent channels between the top of the rigid foam and the top layer of roof sheathing by installing a series of parallel 2x4s — one above each rafter — extending from soffit to ridge."

  182. User avater GBA Editor
    Martin Holladay | | #182

    Response to George Baum (Comment #180)
    George,
    It looks like you posted your questions on two different pages. I have provided an answer on the other page where you posted. Here is the link to my answer:
    https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/57976/need-correct-moisturemold-problem-cathedral-roof#comment-150916

  183. User avater GBA Editor
    Martin Holladay | | #183

    Response to Alan B (Comment #181)
    Alan,
    You provided a link, but you never explained which illustration you were referring to in the article you linked to. I'm guessing that you were talking about the illustration that I have reproduced below.

    Yes, it is safe (although before you ask a question about a particular roof assembly, it would be good to know your climate zone).

    You seem uncertain. For example, you wrote, "it's not vented at all." But in my article -- the article on this page, where you posted your comment -- I wrote, "there are three ways to build an unvented roof assembly." I then proceeded to explain how to build this type of roof.

    I said that it is possible to install vent channels between the top of the rigid foam and the top layer of roof sheathing. I never said that it was necessary. Unvented roof assemblies are safe. The main reason that some people install ventilation channels above the rigid foam is because they live in areas with lots of snowfall, and they prefer a "cold roof" to minimize ice damming problems.

    Thanks for your comment. I will edit my article to try to clarify any ambiguity on this issue.

    .

  184. Debra R | | #184

    roof assembly stack-up, again
    Martin, following up on our earlier discussion:
    We've decided to insulate on the exterior of the roof sheathing all the way across both sides of the gable roof, in order to match up the roof lines. We will eat the cost of the foam in the areas where there will be a vented attic underneath the foam -- basically the foam will simply be an expensive spacer on that part of the roof.

    How does this look for a plan? Anything you would change/add/reorder? Do you agree with taping the seams in both layers of plywood?

    Existing roof deck (2x8 T&G boards in back of house/cathedral ceiling, 2 x 4's in front of house over attic)
    1/2" CDX plywood, seams taped -- air barrier (the felt which is currently in place will be removed)
    2 layers of rigid foam, each 2" thick, staggered horizontally and vertically, seams taped
    1/2" CDX plywood, seams taped
    30# asphalt felt
    asphalt shingles

    Thanks much.

  185. User avater GBA Editor
    Martin Holladay | | #185

    Response to Debra R
    Debra,
    It sounds like an excellent plan. Depending on what type of rigid foam you select, you will end up with insulation in the range of R-16 to R-24. While that's less than code requirements, it's much, much better than what you have now.

    You could skip the tape on the top layer of plywood to same some time and money. That's probably what I would do if I were you. That said, there is no harm in taping the seams of the top layer of plywood.

  186. User avater GBA Editor
    Martin Holladay | | #186

    Response to Ryan Nelson
    Ryan,
    The best insulated sloped roof, in my opinion, is described in these two articles:

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation.

    In your climate zone (Zone 4C), here's what that roof would look like, from the top down:

    1. Roofing.

    2. Roofing underlayment.

    3. Roof sheathing.

    4. If desired, a 1.5-inch-deep ventilation channel created by flatways 2x4s.

    5. R-10 of rigid foam (2.5 or 3 inches of EPS).

    6. An air barrier.

    7. Roof sheathing.

    8. R-39 or R-40 of fibrous insulation (cellulose or mineral wool) under and in direct contact with the roof sheathing.

    I've heard too many reports of homeowners with problematic SIP roofs installed in the 1980s -- some of these roofs are now rotting at the seams due to condensation of moisture from exfiltrating interior air -- to feel comfortable recommending a SIP roof.

  187. Ryan Nelson | | #187

    New build on '47 foundation
    Martin,

    I just joined GBA, and I'm looking for some wisdom.

    I'm in zone 4 (Portland, OR), and want to rebuild the house on an existing foundation. The foundation is sound, but what's above it is horrible.

    Having visited Premier SIPs up in WA state, I'm 90% sure SIPs are a great way to go. More importantly, I've convinced my wife (the finance manager for this project) as well! That is, for the walls at least...

    I would like a cathedral ceiling for this build, and the roofline will be moderately "tortured", as you say. After reading your blog and some of the comments, i think a non-vented, insulated ceiling is best. I'm down to, 1. SIPs all the way, or 2. stick framed with rigid foam inside, as well as outside roof sheathing.

    By the time i add several layers of rigid foam, and two layers of sheathing, i think i will be wishing i went SIPs... my question to you is, if you could start from scratch on a new roof in zone 4, would you recommend my options 1 or 2, or something different altogether? And also, do you have any quantitative or even anecdotal cost comparisons between my options 1 and 2?

    Martin, I'm grateful for your time, and after reading many of your replies, i can see you are passionate about building things right. Right for people, and the planet. Thank you for what you do!!

    Ryan Nelson

  188. Ryan Nelson | | #188

    New build on '47 foundation
    Martin,

    Thank you for your reply. Those articles are both very informative. I'm getting the sense that SIPs can be done well, but can be beat in price with exterior rigid foam systems as you've recommended, with equal or better performance (with emphasis on air-tightness). I'm happy to rethink my stance as I'm still in the planning stages.

    For the air gap in layer 6, is 1.5" a good thickness? Flatways 2x4 parallel and aligned to the rafters for that as well? Sounds like I should expect to be using 10" screws for the full assembly, is that about right?

    Should i fully support sheathing seams parallel to the ridge line, for sheathing above an air gap? Wouldn't that isolate the air under each sheet of sheathing, and is that okay?

    Thanks again,
    Ryan

  189. Ryan Nelson | | #189

    I mis-read you,
    I misread you, i see layer 6 is an air barrier, not an air gap! My mistake!

  190. User avater GBA Editor
    Martin Holladay | | #190

    Response to Dylan Kinsey
    Dylan,
    There are a couple of potential problems with your approach. The first problem is that plywood roof sheathing isn't very vapor-permeable; when dry, plywood has a vapor permeance of 0.5 to 0.7 perm, so it meets the old definition for a vapor retarder.

    Its saving grace (in your application) is that it has variable vapor permeance; as it gets wet, plywood becomes more permeable.

    The second problem is that not all building inspectors will accept the "above-the-roof-sheathing vent channel" approach, and may deem that your proposed roof assembly is non-compliant.

    The best way to proceed would be to choose a more vapor-permeable sheathing material -- something like fiberboard.

    If you go ahead with your plan, you should include a smart vapor retarder on the interior side of the assembly, to minimize the transport of moisture from the interior to the sheathing.

  191. Dylan Kinsey | | #191

    Venting over roof sheathing
    Hi Martin,

    I'm trying to decide on a roof venting strategy for my next project; a story and half cape with 12" double 2x4 walls with dense pack cellulose for R-40, and 2x10 rafters built down from inside like a Larsen Truss to 18" depth to achieve R-60, also dense packed cellulose. On the interior, I'd like to use Intello for vertical walls and taped cdx on sloped ceilings.
    I'd like to use the exterior wall and roof sheathing as my primary air barrier; cdx plywood with all seams taped, which sort of forces me to keep the roof venting on top of the roof sheathing, as you describe in paragraph 7 of this article. I'd like to use Solitex Mento as a permeable roofing underlayment, as well as a WRB on vertical walls, and 2 layers of cross hatched 1x4 strapping over roof ply and Mento; one running down the roof slope, nailed to the rafters, and the second run horizontally over them to support corrugated steel roofing. (rake and eave overhangs are "applied" after Mento is installed, so as not to interrupt the air barrier).
    Does this strategy sound as effective as vent channels constructed on the inside of the roof sheathing? I'm struggling to see why this wouldn't work as well or in practically the same manner as an in-board site-built vent channel. It's essentially like a rain screen or vented cavity, same as on the walls. Any thoughts on this?

    Thanks!

  192. Dylan Kinsey | | #192

    Martin,
    I see your point. I

    Martin,

    I see your point. I haven't worked with fiberboard. Can it be taped and considered a reliable primary air barrier?

    Not much worry about building inspectors here in South Albany, VT, but I'll keep my eyes open. : )

    If I changed my venting detail to a site built style, between the rafters, as in your many blogs on this topic, my air barrier would need to transition back inside before it got to the sloped roof; very cumbersome at the gable walls. Not really sure how I'd do that.
    Also, wouldn't the site-built plywood baffles between the rafters have the same issue; slow drying via diffusion? ( I suppose I could use fiberboard for baffles)

    What got me started on the "air tight walls and roof sheathing" was your article of the same name; specifically South Mountain Company's great article on the Martha's Vineyard housing development. I saw Marc's presentation on those houses at BBD as well.
    The wall and roof section of those houses indicates that they're unvented assemblies. (Still unclear to me how that is safe). I thought if I did basically the same thing, but added over-sheathing vent strips for steel roofing, I'd be golden, but I think I'm missing something here.

  193. User avater GBA Editor
    Martin Holladay | | #193

    Response to Dylan Kinsey
    Dylan,
    Most people building vented cathedral ceilings use the interior drywall as their ceiling air barrier. The standard way to make the transition between the exterior wall air barrier and the interior ceiling air barrier is to install a wide piece of plywood on top of the wall's top plate. If the top plate is 5.5 inches wide, the plywood is 7 or more inches wide. That gives you an interior lip -- you can seal the seam between the plywood and the drywall on the interior, and you can use tape on the exterior to seal the seam between the wall sheathing and the plywood that is installed above the top plate of the wall.

    The South Mountain Company built unvented cathedral ceilings packed with cellulose -- probably a code violation, and a method that the South Mountain Company no longer uses.

    Concerning questions around whether ventilation baffles need to be vapor-permeable: those questions are addressed in my article, Site-Built Ventilation Baffles for Roofs.

  194. Dylan Kinsey | | #194

    Response to Martin
    Martin,

    I'm familiar with that approach and have used it on previous double walls with flat ceiling/ truss roof assemblies. Worked beautifully.
    Thinking it through on my Cape, I might prefer to go back to the method you describe; make the air barrier jump back inside at the wall top plate, vent the sloped ceilings with a site-built vent channel between the rafters, and go back to a flat ceiling on the second floor, using gable wall vents to vent an unconditioned attic. The only stumbling block I see is transitioning the gable wall exterior air barrier to the interior flat ceiling air barrier; one of the reasons I was seduced by the simplicity of the "air-tight wall and roof sheathing" approach. Looks so tidy and straight forward, but thinking now that it's too difficult to incorporate a robust proven venting strategy for that assembly and the poor drying-potential risk factor is too high.
    Your blogs on this topic have been extremely helpful and I appreciate your advice. ( I do study before I submit questions : )

    Thanks!

  195. Chuck Collins | | #195

    wood cathedral ceiling?
    Hi Martin,

    Before reading all this great info I replaced some beams and completely re-boarded the roof deck with unfinished 3/4" beaded pine to be the ceiling.

    Project is:
    1 room on 2nd floor of 1-3/4 sty ell
    200 yr old timber frame
    400 total roof area 5:12 pitch
    zone 5 in MA

    Was planning to do the following:
    1. cover boards with IKO synthetic underlayment ("roof guard cool grey")
    2. 4-1/2" polyiso
    3. 4-1/2 EPS
    going to lay down 2x4's 16 o.c mid way in foam. to increase fastening spots for top plywood (have horiz. purlins 30" apart)
    4. 1/2" plywood (unvented top deck)
    5. either ice and water full coverage or more IKO synthetic underlayment
    6. IKO architectural shingles
    Having tight, staggered, taped, panels
    and at least at the ridge, where joints are close together, to can foam with low expansion flex foam

    Questions I have are
    1.Can this work to have an unfinished wood ceiling?
    2.Having trouble finding 2" 4x8 EPS foam. Home Depot and Lowes have 1" thk or 2" 2x8.
    Do you know of a retail seller in east MA or south NH
    3. Are the 3/4 boards and synthetic underlayment a sufficient thermal barrier?
    4. Would there be a benefit to foam and tap all joints.

    Any suggestions will be greatly appreciated
    Thanks
    Chuck

  196. User avater
    Stephen Sheehy | | #196

    Reclaimed foam
    Chuck- Google "reclaimed foam." There are a number of sellers of reclaimed foam in MA. You'll save a bundle and help the planet to boot.

  197. Brad Hardie | | #197

    Reclaimed foam and exposed pine board ceiling
    Chuck,

    I'm doing the same detail on my new construction. I bought the insulation from David Volpe @ NationwideFoam.com. There is a location in Framingham, MA. I bought several tractor trailer loads to insulate my house and barn for a very, very reasonable price. Reclaimed insulation can be hard to source (most suppliers don't/won't store it for long because it isn't as valuable as Iso), so they will usually source it from an active job site.

    It may be hard to find insulation that is the exact measurement you want, so it can be helpful to source the insulation, and then design around that. That is what I did, and it's working out great.

    There is also a source in Barre, VT for Iso.

    I would recommend a cold roof on top of the insulation.

    If you need long screws look to Tru-fast screws from Cor-ten.com When it comes to the screws, make sure to use a slight upwards slope of about 5 degrees to create a truss out of the screw.

    If you don't have any luck sourcing it let me know and I'll help you out.

  198. User avater GBA Editor
    Martin Holladay | | #198

    Response to Chuck Collins (Comment #195)
    Chuck,
    Q. "Can this work to have an unfinished wood ceiling?"

    A. Yes, you can have a board ceiling on the underside (interior side) of this roof assembly, as long as you have a good air barrier (either synthetic roofing underlayment with taped seams, or rigid foam with taped seams) directly above the board layer.

    Q. "Are the 3/4-inch boards and synthetic underlayment a sufficient thermal barrier?"

    A. Whether of not 3/4-inch boards are acceptable as a thermal barrier or ignition barrier depends entirely on your local building inspector, since code interpretation on this point varies widely from jurisdiction to jurisdiction. Call up your local building department for more information on this issue. You might also want to read this article: Thermal Barriers and Ignition Barriers for Spray Foam. (Although the article talks about spray foam, rigid foam is treated similarly by building codes.)

    Q. "Would there be a benefit to foam and tape all joints?"

    A. Yes, because taping the joints of the rigid foam reduces air leakage. For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

  199. User avater GBA Editor
    Martin Holladay | | #199

    Response to Peter Anderson
    Peter,
    Q. "Can I use rigid insulation below interior plaster finish to increase R-values?"

    A. Yes.

    Q. "I was concerned with duplicating vapor retarder. No problem sandwiching existing plaster?"

    A. No -- no problem.

    Q. "Is adding as little as an inch of extruded or polyiso worth it?"

    A. Only you can answer that question. In my mind, it's worth it. Two inches is even better, of course.

  200. Peter Anderson | | #200

    Adding insulation
    Zone 4a. Have cape with 2x6 rafters, filled with r13 3.5 inch insulation with baffles above. Insulation has vapor retarder facing. Can I use rigid insulation below interior plaster finish to increase R-values?
    Martin, yes I would sheet rock below rigid insulation. I was more concerned with duplicating vapor retarder. No problem sandwiching existing plaster? Obviously am never getting to code minimums, but is adding as little as an inch of extruded or polyiso worth it?

  201. Chuck Collins | | #201

    Re: Replies 196,197,198 Stephen,Brad,Martin
    Thank you guys for your input / answers.
    Did find the reclaimed foam suppliers in the Worcester and Framingham areas.
    Nationwide, who you bought from Brad, only sells tractor trailer loads.
    Green Insulation Group has some 3-1/4", not ideal but may have to work with it as you suggest.

    As a DIYer this site has been a great help thanks again
    Chuck

  202. Brian Lengel | | #202

    If simply blocked the soffit/ridge vents, would I be better off?
    New to the site and really like it. I have such a 1975 house (Austin, TX) with cathedral ceiling with under size batts albeit rockwool. When we had a Galvalume roof, we got R-9 polyiso on top of the roof deck, underneath the Galalume. Now we have "Remember: if you choose to install rigid foam on top of your roof sheathing, don't install ventilation channels under the roof sheathing; these two practices are incompatible." If blocked the soffit vents and the ridge vent in an airtight manner, our R value sounds like it improves right away because then we aren't "wasting" the R-9 on top of the deck. Right?

  203. User avater GBA Editor
    Martin Holladay | | #203

    Response to Brian Lengel
    Brian,
    Yes, you guessed right. If you have rigid foam on the exterior side of your roof sheathing, you don't want to have any ventilation air on the interior side of the roof sheathing. So you need to seal the vent openings near the soffits and at the ridge.

  204. Tyler Keniston | | #204

    Old capes?
    "Insulated cathedral ceilings are a relatively recent phenomenon. The craze for insulated cathedral ceilings (and great rooms) really took off in the 1970s and 1980s"

    This question may show my ignorance concerning... well anything really... but, are not 'Cape' style houses (1 1/2 story) fairly old and traditional (in New England), and did not these include insulated 'cathedral ceilings'?

    As a follow-up; If one were building a story and a half, is there any advantage/disadvantage to insulating the slope up from the knee wall (if high posted) and then insulating horizontally over the interior ceiling vs insulating a straight shot up the slope to the ridge?

  205. User avater GBA Editor
    Martin Holladay | | #205

    Response to Tyler Keniston
    Tyler,
    Most old Cape Cod homes had no insulation in the sloped ceiling sections (even at the Levittown houses built after World War 2). That said, you are correct that sloped ceilings have existed for thousands of years. (Even tipis have sloped ceilings.)

    For more information on this issue, see Insulating a Cape Cod House.

    Q. "If one were building a story and a half, is there any advantage/disadvantage to insulating the slope up from the knee wall (if high posted) and then insulating horizontally over the interior ceiling vs insulating a straight shot up the slope to the ridge?"

    A. For a thorough discussion of this question, see “Two Ways to Insulate Attic Kneewalls.”

    -- Martin Holladay

  206. Tyler Keniston | | #206

    Response to Martin Holladay
    "Most old Cape Cod homes had no insulation in the sloped ceiling sections"
    Ah ha. I didn't realize that. Thanks for the info and links.

    Tyler

  207. Steve Harris | | #207

    T&G under closed cell spray foam
    I am in zone 6 and plan to use closed cell spray foam between the rafters in a cathedral ceiling, no venting. I was hoping to install t&g cedar on the rafters for the finished ceiling, but I'm confused by the statement in the article 'If you are building a cathedral ceiling, the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall. A board ceiling is notoriously leaky, and this type of ceiling is often associated with roof sheathing rot.' Does this statement apply in my case?

  208. User avater
    Dana Dorsett | | #208

    Closed cell foam at code-min R is an air barrier.
    If you're installing R49 (7-9") of closed cell foam you won't have moisture accumulation on the surface of the foam even if the t & g leaks as much air a tennis racquet. The temperature of the surface of the foam will be very close to room temperature, and well above the dew point of the room air. Closed cell foam that thick is also an air barrier, and a class-II (almost a class-I) vapor retarder, which keeps moisture from reaching the roof deck as well. The t & g has to be thick enough (1x or thicker) to qualify as a thermal barrier against ignition of the foam, but half-inch gypsum board qualifies on it's own.

    That much closed cell foam is the opposite of "green", especially closed cell foam blown with HFC245fa (the vast majority of the market). In zone 6 only half the total R needs to be closed cell foam to provide dew point control on an equal amount of (much greener) fiber insulation, so dropping back to 4" of foam (R24-R28) and R25 of fiber snugged up to the interior side of the foam would be fine. HFO-blown foam has ~1/1000th the amount of damage from the blowing agent, but it's slightly more expensive, with only a few vendors. It has the advantage of being able to install 4"-7" at a time without quality or fire-hazard issues. (HFC blown closed cell foam has both problems if installed in lifts thicker than 2", with a substantial cooling period between lifts.)

    But closed cell foam still has a very high polymer/R ratio making it still one of the least-green insulation options, even when blown with low-impact blowing agents. Using the minimum necessary for dew point control would be kinder to the planet, and kinder to your wallet.

    With the air leaky t & g it's still useful to have some sort of air barrier to keep moisture from accumulating in the fiber during cold-snaps, but a membrane type of vapor retarder such as Intello Plus or Certainteed MemBrain detailed as an air barrier should be enough, either of which is far cheaper than another inch of closed cell foam. Don't substitute (much cheaper) 4-6 mil polyethylene here, since it has the potential of creating a moisture trap.

  209. User avater GBA Editor
    Martin Holladay | | #209

    Response to Steve Harris (Comment #207)
    Steve,
    Dana Dorsett gave you good advice.

    You quoted my article, but you left out important information from the section you quoted from. If you had read the entire section, your question would have been answered. Here's what my article states:

    "Do I need to install an air barrier under the insulation?
    "Yes, of course — especially if you are using fluffy insulation like fiberglass batts, blown-in fiberglass, or dense-packed cellulose. (If you insulate your ceiling with spray foam, the spray foam should create an air barrier, as long as the installer does a good job.)

    "If you are building a cathedral ceiling, the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall. A board ceiling is notoriously leaky, and this type of ceiling is often associated with roof sheathing rot."

  210. user-6815827 | | #210

    I'm in zone 6. New structure
    I'm in zone 6. New structure build. I'll be installing a beam/purlin system and overlaying 1x6 tongue and groove pine boards. Then I plan to build up a roof which I want to make unvented and at least R40. I plan to overlay a 5/8" sheathing layer (am assuming this should *not* be zip system to avoid a vapor barrier here, right?) on top of the tongue and groove, and tape it, in order to improve the fire resistance to internal (to the structure) fires, as well as to address the air barrier issue. Am intrigued by the detail at the gable edges to break continuity of the T&G into the overhang, still figuring that out. Here's my main question... I plan on at least 8" of insulation above the sheathing, then another sheathing layer (zip system?) and then standing seam metal roof. An obvious approach would be 2 sheets of 4" thick XPS, with joins offset. However... I would much prefer to use mineral wool than XPS. Say, 3 sheets of 3" thick Roxul Comfortboard... in which case I might even forgo the bottom sheathing layer and just use Membrain over the tongue and groove. But I've seen commentary previously to avoid mineral wool on roofs. Not entirely clear why. Would be grateful for comments either way as to why not to use mineral wool there, or alternatively how to use it properly to avoid whatever the issues are. Thanks!

  211. User avater GBA Editor
    Martin Holladay | | #211

    Response to N/A N/A
    (It would be nice if you told us your name.)

    Semi-rigid panels of mineral wool are almost never installed on the exterior side of the roof sheathing on a sloped roof. I'm not saying you can't do it -- just that it is very rarely done, so you will be one of the guinea pigs learning from the experience. This information will either spur you on or turn you off, depending on your personality.

    If you want to hire a builder or a roofer to do the work, the builder or roofer will almost certainly balk, and tell you that you are crazy.

    Whether you use mineral wool or rigid foam, you need a very robust air barrier above the tongue-and-groove boards. (Moreover, the tongue-and-groove boards should not cantilever beyond the exterior walls if you want to avoid air leaks.) Taped plywood or OSB is fine. If you are using rigid foam above the roof sheathing, the vapor permeance of the air barrier is irrelevant. (After all, the rigid foam is already a vapor barrier.)

    Semi-rigid mineral wool boards are a little squishy, which makes some builders nervous. John Straube directed a group of builders to use mineral wool as you propose on a project on his own property, and Straube told me it worked. That said, he wasn't the installer.

    By the way, most green builders prefer EPS or polyiso to XPS, because XPS is manufactured with a blowing agent with a very high global warming potential.

  212. user-6874083 | | #212

    Attic Roof Insulation - Thermal Bridging, Vapor Permeability,...
    I plan to convert my attic into conditioned space for housing a heat-pump air handler and associated ducts. It's a 1950’s L-shaped ranch house with a 4/12 roof pitch. The main section of the house has 2"x8" rafters (actual size 1-1/2" x 7-1/4"). The short extension (other side of the L shape) uses 2"x6" rafters (actual dimensions 1-1/2” x 5-1/2”). The house is in Westchester County, NY. I believe it's in climate zone 4 (R38 roof insulation required).

    I am thinking of installing site-built ventilation baffles using 1" foil-faced polyiso (with 1" air gap) as shown here and closed cell spray foam insulation under the baffles.

    With the 5-1/4" left after accounting for 1" air gap and 1" polyiso, should I fill the remaining rafter cavity with closed cell spray foam? Or should I use 2" of closed cell and 3-1/4" of mineral wool batts?
    With a vapor impermeable layers of polyiso (ventilation baffles) and closed-cell spray foam, possibly followed by a vapor permeable layer of mineral wool, what are my options for board insulation under the rafters for addressing thermal bridging? Board insulation under the rafters would make it like a cathedral ceiling. Would it be OK to use another layer of foil faced polyiso (vapor impermeable)? Should I use something like Roxul ComfortBoard (vapor permeable)? Any other options?
    If there was no board insulation under the rafters, would mineral wool under closed-cell spray foam suffice as a thermal barrier and/or ignition barrier?
    If there was no mineral wool, would foil-faced polyiso and/or Roxul ComfortBoard qualify as thermal or ignition barriers?

    I don’t have the option to insulate above the roof deck. Therefore, I’ve been considering all options for achieving adequate insulation and venting under the roof deck. In the section of the roof with 2x6 rafters, I believe I’ll have to fill the entire rafter cavity (after 1” air gap and 1” polyiso) with closed-cell spray foam. That would still leave me short of R38; I’d have to add 2 inches of board insulation to achieve the R38 target. Any other suggestions? Thanks!

  213. User avater GBA Editor
    Martin Holladay | | #213

    Response to User-6874083
    User [It would be nice if you would share your name],
    Q. "With the 5-1/4 inches left after accounting for 1" air gap and 1" polyiso, should I fill the remaining rafter cavity with closed cell spray foam? Or should I use 2" of closed cell and 3-1/4" of mineral wool batts?"

    A. In the section with 2x8 joists, you have room for about R-34 of spray foam. (Of course, the installer will probably have to install this foam in at least two lifts.) You should know, however, that it can be difficult for spray foam contractors to fill a framing bay completely with closed-cell spray foam, which is very difficult to trim. It will probably be less than 5.25 inches thick.

    But if we assume R-34 for the spray foam, you'll also have about R-6 for the ventilation baffle, for a total of R-40.

    If you instead use the flash-and-batt technique, you'll have R-6 [baffle] + R-13 [closed-cell spray foam] + R-13 [mineral wool] = R-32.

    Either method would work, although the use of closed-cell spray foam is often avoided by green builders (unless you specify one of the new types of closed-cell spray foam that uses a blowing agent with a low global warming potential; for more information on this topic, see Next Generation Spray Foams Trickle into the Market).

    Q. "With a vapor-impermeable layer of polyiso (ventilation baffles) and closed-cell spray foam, possibly followed by a vapor permeable layer of mineral wool, what are my options for board insulation under the rafters for addressing thermal bridging?"

    A. I would say that any type of rigid foam would work. This type of assembly is not at risk for moisture accumulation, because the ventilation channel provides safety.

    Q. "Would it be OK to use another layer of foil faced polyiso (vapor-impermeable)?"

    A. Yes.

    Q. "Should I use something like Roxul ComfortBoard (vapor-permeable)?"

    A. That would be unusual (and possible hard to deal with, because it's squishy), but it might work.

    Q. "If there was no board insulation under the rafters, would mineral wool under closed-cell spray foam suffice as a thermal barrier and/or ignition barrier?"

    A. Yes.

    Q. "If there was no mineral wool, would foil-faced polyiso and/or Roxul ComfortBoard qualify as thermal or ignition barriers?"

    A. Foil-faced polyiso is neither a thermal barrier or an ignition barrier. If you have a layer of rigid foam, you would need to cover it with drywall. Roxul ComfortBoard, on the other hand, could probably be left exposed (although I would check with your local building office to make sure that your inspector agrees with my interpretation).

  214. Andrew Southam | | #214

    Spray Foam Roof
    Hi everyone,
    I am working to build a small 100 square foot loft cabin, much like the "Cottage Life Bunkie" by Wayne Lennox. Unlike in that design, which has no insulation, I am looking to use this cabin year-round. I've been reading through as many of the articles on this site, which have been an excellent resource for me, but I've got a few questions I've not explicitly been able to find answers to. I’ve limited building experience, so bear with me on some of the more basic questions.

    1. For the life of me I can't confirm my climate zone. I'm assuming a Zone 7 equivalent for the Huntsville Ontario Canada area. Am I correct in that?

    2. I was surprised to read the article on warm sheathing in “Calculating the Minimum Thickness of Rigid Foam Sheathing”, and the risks of making walls too thick, and it got me thinking about my roof. If I were to spray a thinner layer of foam (say R-20 or so) on the underside of the sheathing, but ventilate the roof, would this mitigate rot concerns? Would this low an R value end up producing condensation on the interior face of the spray foam? My biggest concern with the cabin is rot/mold, not heating costs.

    3. I haven’t been able to find any information on insulated floors. The floor of my cabin will essentially act like an exterior wall. In planning insulation for this floor, should I approach it in the same way as I would an exterior wall?

    Thanks in advance for feedback. I’ve only just completed the foundation this summer so I’ve got lots of time to stew about my approach to the actual building itself this coming spring…

    Andrew

  215. User avater
    Dana Dorsett | | #215

    It looks like about 4800HDD-C
    It looks like about 4800HDD-C (base 18C) for the 25 year average in nearby Muskoka:

    https://muskoka.weatherstats.ca/charts/hdd-yearly.html

    That would make it the cool edge of zone 6, not zone 7:

    http://www.greenbuildingadvisor.com/sites/default/files/images/Table%20301.3%282%29_0.preview.jpg

    Condensation only occurs on the surfaced colder than the proximate air mass. What you're really concerned about is whether the interior moisture drives will be taken on as adsorbed water in the moisture susceptible roof decking. With R20 of closed cell foam (about 3") the vapor diffusion rate is quite low even when the roof deck is quite a bit colder than the dew point of the conditioned space, and it really wouldn't matter if it's ventilated above the roof deck or not. But any fiber insulation below the foam would have to be no more than R20 to keep the fiber insulation dry. How deep are the rafters?

    But if it's R20 of open cell foam on the underside of the roof deck it could be an issue, unless there there is slightly more R than the total cavity insulation R mounted ABOVE the roof deck.

    It's really the ratio of the R value above the deck to the total R that determines the average winter temp of the roof deck. The roof deck has to be reliably warmer than the dew point of the interior air to be fully protected from interior moisture drives. In a 4800HDD climate it would need to be 50% or more of the insulation value on the exterior for roofs (which suffer radiational nighttime cooling to below the outdoor air temperature, unlike walls.) If re-roofing is an option, installing 4" of polyisocyanurate above the roof deck would have an average wintertime performance of R20 or greater with R20 of fiber insulation under the roof deck.

    What's under the floor? A vented crawlspace with a dirt floor? An open pier foundation? Elves seated on mushrooms holding up the floor with tiny hands? Is there any plumbing under the floor to worry about?

  216. Andrew Southam | | #216

    Response to Dana
    Q: How deep are the rafters?
    A: I'm using 2x8 rafters. Though, I was hoping to avoid any fiber insulation below the closed cell spray foam.

    This will be a completely new build, so I have all options open. The challenge I foresee is my own ability to install polyiso on the roof exterior effectively. I had thought of doing a nice thick layer of polyiso on the roof deck but have read a lot of horror stories for the challenge of breaking screws and missing rafters.

    Q:What's under the floor?
    A: I actually laughed out loud at the elves on mushrooms idea. It's an open pier foundation with no plumbing.

  217. User avater GBA Editor
    Martin Holladay | | #217

    Response to Andrew Southam
    Andrew,
    Here is a link to an article that discusses ways to insulate floors that are exposed on the underside to outdoor conditions: How to Insulate a Cold Floor.

  218. Andrew Southam | | #218

    Response to Martin
    Thanks Martin, appreciate the guidance.

  219. Trent Willey | | #219

    Membrane or underlayment required
    Awesome article, thank you! I want to enclose my attic by using only closed cell spray foam. What underlayment or membrane is recommended to place on top of the roof deck sheathing in two scenarios: 1) a hot roof without ventilation and 2) a hot roof with a ventilation panel such as Hunter Cool Vent panels? The project is located in Climate zone 6, Dry. Thanks again!

  220. Andre60 | | #220

    unvented cathedral and closed cell spray foam in rafter bays
    First of all...Thank you for this great article.
    I know you said, in no uncertain terms, do not install tongue and groove ceilings nor install recessed lighting BUT if I use closed cell spray foam for the full insulation (2 X12 rafters) which will give great R values and vapor barrier, what do you thing of using slim LED recessed lighting (about 1 inch thick) and tongue and groove ceilings?
    Your comments would be greatly appreciated. Thanks.

  221. User avater GBA Editor
    Martin Holladay | | #221

    Response to Trent Willey (Comment #219)
    Trent,
    Q. "I want to enclose my attic by using only closed-cell spray foam. What underlayment or membrane is recommended to place on top of the roof deck sheathing in two scenarios: 1) a hot roof without ventilation?"

    A. Since most manufacturers of synthetic roofing underlayment forbid the use of their products over an unvented roof assembly, your safest bet is ordinary asphalt felt. If you do choose to use a synthetic roofing underlayment, call the manufacturer to check whether they allow the use of their underlayment over unvented roof assemblies.

    Q. "What about a hot roof with a ventilation panel such as Hunter Cool Vent panels?"

    A. You can use any type of roofing underlayment you want. Asphalt felt is traditional, and still works well. If the roofing underlayment will be exposed to the weather for several weeks or months, you may prefer to use a synthetic roofing underlayment.

  222. User avater GBA Editor
    Martin Holladay | | #222

    Response to Andre60 (Comment #220)
    Andre,
    Q. "If I use closed-cell spray foam for the full insulation (2 X12 rafters) which will give great R-values and vapor barrier, what do you think of using slim LED recessed lighting (about 1 inch thick) and tongue-and-groove ceilings?"

    A. In my article, I addressed the situation you raise. I wrote, "Do I need to install an air barrier under the insulation? Yes, of course — especially if you are using fluffy insulation like fiberglass batts, blown-in fiberglass, or dense-packed cellulose. (If you insulate your ceiling with spray foam, the spray foam should create an air barrier, as long as the installer does a good job.)"

    So, to be absolutely clear: If you have insulated your rafter bays with closed-cell spray foam, you have an air barrier. So in that situation, slim LED recessed lighting and tongue-and-groove boards are OK. The only issue you have to clarify with your local code authority is the need for a thermal barrier (usually interpreted as 1/2-inch drywall) in the interior side of the spray foam for fire protection. Different inspectors interpret this code requirement differently, but many inspectors will require the 1/2-inch drywall between the spray foam and the tongue-and-groove boards. For more information on this issue, see Thermal Barriers and Ignition Barriers for Spray Foam.

  223. User avater GBA Editor
    Martin Holladay | | #223

    Response to Jason
    Jason,
    I don't know the definition of an "architectural" standing-seam roof. I'm assume you are talking about what is usually called a standing-seam metal roof -- probably painted steel. Does "architectural" mean something else?

    As my article indicates, the type of roof you are describing can be detailed as a vented roof (as you propose), but only if you include soffit vents (something you didn't mention), and only if there are no interruptions in any of the rafter bays from the soffits to the ridge. That means no valleys, no hips, no skylights, and no dormers. If you have that type of roof, the unvented approach is fine.

    As my article also indicates, you can also detail your roof as an unvented roof, as your contractors propose.

    You didn't mention the name of your state -- I assume that "coastal Carolina" could include either North Carolina or South Carolina. It sounds like you are in Climate Zone 3, where the minimum R-value for roofs from the prescriptive table in the building code is R-38. Your plan to install R-30 insulation isn't ideal, although there may be legal ways to go with R-30 if you really want to. (For more information on this topic, see Three Code-Approved Tricks for Reducing Insulation Thickness.)

    If you decide to create an unvented roof assembly with spray foam, remember to use closed-cell spray foam, not open-cell spray foam.

    I don't think the existence of a ventilation channel improves the chance that a roof leak will be detected earlier than otherwise.

  224. papajay18 | | #224

    Humid Climate
    Hello all,
    Thank you Martin, for this and many other fantastic articles. Another thanks to all the commenters, this site is such a great forum.

    I'm working on a new build outside of my familiar climate zone and would greatly appreciate some guidance. Research consistently turns up solutions most geared to northern climates. This job is in Coastal Carolina, which is quite humid. I'd previously reviewed this thread and shared the following intended approach with some reputable local contractors:

    -'Architectural' Standing Seam Roof
    -over underlayment
    -over roof sheathing
    -over framing w/ 'accuvent cathedral vent' and continuous 1-2" depth vent vaffle under sheathing. Tape-and-caulk air sealing of vent from wall to ridge vent. R-30 Roxul insulation below baffle.
    -Drywall Ceiling

    My intention was to provide drying capacity to the sheathing without adding the cost of two layers of strapping and a 'Structural' standing seam roof system. The metal roof is already going to stretch the budget.

    The contractors both said to forget the baffle, and instead spray foam directly onto the underside of the sheathing. They advised that any vented/unconditioned space, even the full unfinished attic spaces in the project, will quickly be full of mold. I see no reason to doubt their knowledge about the climate.

    However my major concern with their approach is that any leak through the metal roof/underlayment will be undetectable until massive rot has occurred. The sheathing will just have no capacity to breath. Another concern is heat gain as the current design calls for a dark grey metal roof.

    Thank you in advance, looking forward to your thoughts on this!

    Jason

  225. papajay18 | | #225

    Response to Martin
    Martin,
    Many thanks for the quick reply! The project is Zone3 near Wilmington, North Carolina, where I believe R-30 is the required insulation value. R-38 would be accommodated if needed.

    'Architectural' seems to be an industry term that indicates direct installation over sheathing is required, vs 'structural' which can go directly over strapping.

    Soffit vents would be used, but as you've said, my proposed approach would only add some resilience, it would not allow significant leaks to be detected.

    The roof has a 9/12 slope and is as simple can be. I also have the ability to eliminate all roof penetrations, so perhaps I'll go that route with a high end roofer and dig deep to find some faith in the unvented assembly.

    Even in this location (which has about 2000 annual heating degree days & 2000 cooling) you do see the closed cell foam as a prudent investment?

    Thanks again, and have a great weekend!

  226. User avater
    Dana Dorsett | | #226

    That's US climate zone 3A
    Wilmington is zone 3A. The IRC 2015 calls out R38, but local code may be based on earlier versions.

    At 3" or more closed cell foam becomes structural, and at R30 (4.5" of HFO blown 2lb foam, or 5" of the more environmentally damaging HFC blown goods) would be enough that even if the sheathing rotted away completely the roof still wouldn't collapse.

  227. User avater GBA Editor
    Martin Holladay | | #227

    Response to Jason (Comment #225)
    Jason,
    Q. "Even in this location (which has about 2000 annual heating degree days & 2000 cooling) you do see the closed-cell foam as a prudent investment?"

    A. I wasn't implying that closed-cell spray foam is preferable to mineral wool. Indeed, it has disadvantages from an environmental standpoint.

    Assuming that you or your contractor understand air barriers and airtight construction techniques, R-value is R-value. You'll get the same performance from R-38 of mineral wool as you will from R-38 of spray foam -- assuming you know how to prevent air leaks.

  228. User avater
    Dana Dorsett | | #228

    That's not exactly true @ Martin & papajay18
    "You'll get the same performance from R-38 of mineral wool as you will from R-38 of spray foam -- assuming you know how to prevent air leaks."

    Sort of, maybe, but not really. That's only true if it is NOT being thermally bridged by framing, which is never the case if applied to the under side of the roof deck.

    The ~50% higher R/inch of closed cell foam compared to mineral wool means a shorter, and thus lower-R framing fraction. The overall performance of the assembly is LOWER with R38 of closed cell foam than it is with R38 of mineral wool due to more heat being transferred through the framing.

    With very small framing fractions such as I-joist rafters 24" o.c. the performance hit is small almost negligible, but at typical ~7-8% framing fractions of milled 2x lumber raftered roofs it's a real hit (albeit a smaller hit than with the 20%+ framing fractions typical of walls.) But in this assembly R38 rock will will outperform R38 closed cell foam on a thermal basis.

    In climate zone 3A it only takes an inch of closed cell foam on the underside of the roof deck for dew point control, and at 1" most closed cell foam is still over 1-perm, a class-III vapor retarder. A flash-inch of closed cell (or even 2 inches) filling the rest of the rafter bay with rock wool would offer a LOT more drying capacity than R30-R38 of closed cell alone. If it's 2x10 rafters 2" of closed cell (R13-ish) would leave just enough space for an R30 rock wool batt, for R43 total. With 2x12 rafters just ONE inch of closed cell (R6-ish) and doubled up R23s (R46), or R30 + R15 batt stackup would fill the space for ~R52 total.

    At 2" closed cell foam would run about 0.5-0.7 perms (depending on vendor- look up the specs), which is about the same vapor retardency of the deck itself when the deck is dry, and still a sufficient drying path. At just 1" the closed cell foam is somewhat more vapor permeable than a dry roof deck, but not so vapor permeable that the roof deck will accumulate significant amounts wintertime moisture via diffusion from the interior in climate zone 3A.

  229. User avater GBA Editor
    Martin Holladay | | #229

    Response to Dana Dorsett
    Dana,
    You're right, of course. While R-38 = R-38, the closed cell spray foam is thinner, so it leaves more rafter sticking out, exposed. The mineral wool covers more of the rafters, and therefore the mineral wool assembly performs a little better.

  230. User avater
    Dana Dorsett | | #230

    Regarding the "prudent investment" @ papajay18
    At a buck or more per square foot per inch (15-18 cents per R per square foot) R38 of closed cell foam is going to cost on the order of $6 per square foot, give or take.

    Holding the foam line at no more than 2" (R14 max) the foam is going to run $2.00-2.50 which leaves you $3.50-$4 to spend on filling the rest of the bay with rock wool. In most areas the cost of R30 rock wool is going to come in well under $3 per square foot, in some places it would be close to $2 per square foot. But say i'ts $3 per square foot, added to the $2-2.50 for 2" of foam it's still quite a bit cheaper than an all-foam solution.

    So a 1-2" foam + R30-38 rock wool solution has:

    * higher center-cavity R

    *better overall performance due to the reduced thermal bridge

    *lower overall cost.

    An additional unaccounted for cost is the time-is-money and management cost of having to schedule two insulation crews rather than just one, but on a per square foot basis that should be pretty small.

    That adds up to a more prudent investment than a rock wool only or closed cell only solution, since it's more protective of the roof deck than a single insulation type solution, and it's cheaper and higher performance than the foam-only solution.

  231. papajay18 | | #231

    Thank you Dana and Martin!
    I hugely appreciate the thoughtful responses. I hope you both had nice holiday weekends!

    -Jason

  232. Andrew Southam | | #232

    Underlayment Question
    Hi everyone,

    In the scenario shown in Image 3 of 3, with an insulated roof assembly with all insulation on the exterior side, where would roofing underlayment be required? Atop the first layer of sheathing below the insulation; atop the second layer of sheathing below the metal roof; or both?

    Thanks!

  233. User avater GBA Editor
    Martin Holladay | | #233

    Response to Andrew Southam
    Andrew,
    Building codes require roofing underlayment under the roofing -- which in the case you are discussing means above the top layer of sheathing.

    It's also necessary to have an air barrier under the bottom layer of rigid foam. This might mean taping the roof sheathing seams (for example, with Zip System tape), or it might mean a layer of Grace Ice & Water Shield.

    For more information on this type of roof, see "How to Install Rigid Foam On Top of Roof Sheathing."

  234. Andrew Southam | | #234

    Underlayment Question
    Thanks Martin! I'm going to be using T&G so it will have to be the Grace Ice and Water Shield. Appreciate the advice.

    Andrew

  235. User avater GBA Editor
    Martin Holladay | | #235

    Response to Andrew Southam
    Andrew,
    Note the other possibilities mentioned in the article I linked to -- for example, a European air barrier membrane like Solitex Mento (a product available from 475 High Performance Building Supply), or a layer of synthetic roofing underlayment that is rated for use as an air barrier (with taped seams, of course).

    Some homeowners worry about possible odors when using Grace Ice & Water Shield above exposed board roof sheathing. I have no opinion on the matter, but I thought I should mention it.

  236. Optimist008 | | #236

    Deep Scissor trusses and blown cellulose
    Hello,
    I'm a newbie here, and wish I had found you a while ago. I am in the throes of building a straw bale house...bales in and scratch coat of hydraulic lime plaster on. My roof is a 6/12 pitch with scissor trusses. My house is 40 x 40, with an 8 foot porch all around, so my trusses are 56 feet wide. None of the interior wall are going to the ceiling, although I will have 2 chases through the ceiling to meet the vents in the roof for plumbing and for a solar tube that goes into the master bath. Only 3 electrical boxes in the ceiling for ceiling fans.
    I have about 3 feet between the roof sheathing and the bottom of the truss. Here in Chewelah WA, about an hour north of Spokane, no insulation company I've talked with will blow cellulose in a 6/12 pitch. So I guess I need to figure out how to do it myself. :)
    Here in Stevens County, I am building without a permit, called the owner/builder ordinance. So inspections aren't an issue. I DO want to do it right though. I am under the impression that code isn't ALWAYS right, it's just the code. So with that in mind, what are your suggestions for this 3 feet of space? I'd like to get to at least R60. I'm using radiant floor heat in slab, with a wood stove for quick heat-ups in fall and spring. I have 2 dormers on the south side for light. The roof ridge is vented, porch ceiling not in yet, but soffit vents will be placed.

    For the dormers, should I just use closed cell spray since that is flat and unvented? Actually, I should just shut-up now and listen to your suggestions!
    Thank you!
    Kathryn

  237. User avater GBA Editor
    Martin Holladay | | #237

    Response to Optimist 008
    Optimist,
    First of all, I'm going to have to provide a message to other GBA readers -- even though the advice I'm about to give won't help you very much. Here's the advice: Plan your insulation details before you begin construction. Once the roof trusses and roofing have been installed, it's too late to follow certain strategies. For more information on this idea, see "Plan Ahead For Insulation."

    So, Optimist: Roof assemblies can be either vented or unvented. If a roof has dormers, a vented strategy is usually impossible, because dormers usually interrupt the path of a ventilation channel from the soffits to the ridge.

    However, it's hard for me to tell what's going on at your house from your photos. Perhaps some rafter bays can be (theoretically) vented, while other rafter bays can't be vented. I'll warn you, however: trying to mix vented strategies with unvented strategies isn't easy, because you can easily end up with ventilation channels that face a dead end instead of reaching the ridge vent, and you don't want that.

    If you think you can vent part of your roof, remember that every single rafter bay needs a clear channel, from the soffit to the ridge, with no impediments.

    Any rafter bay that can't be vented needs an unvented approach.

    To create an unvented insulated roof, your only option is to install closed-cell spray foam on the underside of the roof sheathing. Further details are in my article.

  238. Malcolm Taylor | | #238

    Kathryn
    It's hard to tell from your photos but scissor trusses are usually a different pitch on the bottom than the top. Are yours really 6/12 on the bottom too. If not, installers may be more willing to fill them knowing that.

    I know you aren't subject to inspections, but in an area of high seismic activity a roof like that needs additional bracing and hardware - especially structural blocking between the trusses at the exterior walls. It might be worth someone to the a look at it for you.

  239. Optimist008 | | #239

    Thank you, Martin and Malcom
    The entire north side can be vented, but the south side can't be totally vented due to the dormers. Your articles are awesome. Thank you.

    Malcolm, the porch part of the rafters are a 4/12 pitch, but won't be insulated. I do have special Simpson ties at the exterior walls, but seismic activity here is very low on the scale.

    I truly appreciate your feedback.
    Kathryn
    .

  240. Rick Evans | | #240

    Closed and open cell
    Kathryn,

    Like Martin said, I would used Closed cell spray foam against the under side of the roof decking (probably 6 inches or so? You could then attach netting on the bottom chord of the trusses and then dense pack with fiberglass or cellulose. However, I think an easier approach would be to forgo the fluffy stuff and simply add open cell foam (6-8") after spraying in the closed cell. This will be much cheaper than using closed cell foam for the entire roof and also be better for the environment. Assuming you drywall the ceiling later- you will have a nice, conditioned service cavity for ductwork, can lights, etc.

    Good luck!

  241. Optimist008 | | #241

    Response to Rick
    Thank you for your response. I need to do some calculations on what is needed depth-wise for the dormer areas and whether I will need to increase the depth of the walls on the dormer to get the R-value where I want it.
    Right now, I'm leaning toward using closed cell where it is required (no open air flow) for the dormers and baffles and blown cellulose everywhere else. I have a contractor coming tomorrow for a quote, but he has already stated he won't use blown cellulose on a 6:12 pitch, so we'll see!

    Kathryn

  242. Malcolm Taylor | | #242

    Kathryn
    The contractors aren't being difficult, loose fill cellulose won't stay put on a 6/12 pitch.

  243. Optimist008 | | #243

    Reponse to Malcolm
    Malcolm.
    I understand the loose fill won't stay put, but in a video you referenced several years ago, at about 8:35 or so, the gentleman teaching shows how to use blown in cellulose on a 6/12 roof. Is it just that it isn't common or just not done anymore? Or where you assuming I was talking about loose, not dense blown-in?

    Thank you,
    Kathryn

  244. User avater GBA Editor
    Martin Holladay | | #244

    Response to Kathryn / Optimist
    Kathryn,
    You can't use the dense-pack method when insulating a scissiors truss. The dense-pack method requires a sealed cavity, with the cellulose filling 100% of the volume of the cavity.

    When there is air above the top of the cellulose -- as when cellulose is installed on an attic floor or in a scissors truss -- you end up with blown-in cellulose, but the cellulose can't be dense-packed.

  245. user-7111533 | | #245

    Question about air barriers.
    Hello there,
    It's hard to find good resources for this stuff.
    My question was concerning air barriers the rigid foam + fluffy insulation roof type.
    As I understand the articles:
    I have an underlayment on the top sheathing.
    An air barrier on top of the bottom sheathing
    Do I need another air barrier below thethe fiberglass as well?
    The detail provided doesn't seem to match the text for this roof type: It's missing at least one air barrier.

  246. User avater GBA Editor
    Martin Holladay | | #246

    Response to User 7111533 (Comment #245)
    User 7111533,
    (By the way, it would be good to know your name.)

    1. The roofing underlayment installed above the top layer of roof sheathing doesn't have to be an air barrier. Ordinary asphalt felt will work for this layer. It doesn't have to be airtight. Note that roofing underlayment is required by building codes.

    2. How many air barriers you include in your roof assembly depends on your level of fastidiousness, how worried you are about future problems, your budget, and your airtightness goals. Clearly, you don't want to have a roof assembly that is so leaky that air movement causes moisture problems. At a bare minimum, you need an air barrier at the layer of the bottom roof sheathing. This might be taped Zip sheathing, a peel-and-stick product, or a European air barrier membrane with taped seams from Small Planet Supply or 475 High Performance Building.

    An additional air barrier at the ceiling level -- usually taped drywall suffices, as long as your ceiling has no leaky electrical boxes or recessed can lights -- is good insurance.

  247. Lance1320 | | #247

    Air Barriers
    In the first bullet of your final summary you mention needing two air barriers if using fluffy insulation. I understand below the insulation would be taped drywall, but what would constitute an air barrier above the insulation? Any details you could provide would be helpful. Trying to understand what this looks like between my venting and the start of the batt insulation. Thank you!!

    After posting this I saw the previous question and answer (245 & 246) which seems to be the same question I had. If understand your answer, you are suggesting a barrier attached to the bottom side of my roof sheathing, then the vent channel, then starting my batt insulation. Please confirm.

  248. User avater GBA Editor
    Martin Holladay | | #248

    Response to Lance1320
    Lance,
    If you are using a fluffy insulation like fiberglass batts to insulate a vented cathedral ceiling, the lower air barrier would be (as you correctly noted) the drywall, while the upper air barrier would be ventilation baffles installed in an airtight manner.

    The best insulation baffles are site-built, but most brands of commercial ventilation baffles can also be installed in an airtight manner, if their edges and seams are sealed with caulk or tape.

    For more information on ventilation baffles, see "Site-Built Ventilation Baffles for Roofs."

  249. User avater
  250. Lance1320 | | #250

    Air Baffle Gaps
    One further question.....some manufacturers of air baffle (venting) products recommend a gap between each adjoining piece. Doesn't that defeat the purpose of it being an air barrier itself? What is your recommendation?

    [Click the number "6" below to advance to the next page, where the answer to this comment is posted.]

  251. User avater GBA Editor
    Martin Holladay | | #251

    Response to Lance1320
    Lance,
    Q. "Some manufacturers of air baffle (venting) products recommend a gap between each adjoining piece. Doesn't that defeat the purpose of it being an air barrier itself?"

    A. Yes.

    Q. "What is your recommendation?"

    A. Ventilation baffles should be installed in an airtight manner, with caulk or tape to seal the laps.

  252. Ryan Gillard | | #252

    We have a cathedral ceiling with 2x12 rafters that we are planning to fill with dense-pack cellulose.. Above the rafters is triflex, above the triflex are 2x2s parallel and on top of triflex and rafters creating a 2 inch ventilation space from soffit to ridge. Plywood sheathing is on top of 2x2, then tar paper, and metal roofing. We need a thicker rafter bay to meet our r-value. Someone at Efficiency VT recommended 1.5" taped poly-iso sheet foam on the underside of our rafters before we install sheetrock. Our other option was furring out rafters another 2.75" with 2x material and then dense-packing the whole bay with cellulose. Either way we are finishing with taped sheetrock. Thoughts on which system would be superior?

  253. User avater GBA Editor
    Martin Holladay | | #253

    Ryan,
    There are two kinds of Tri-Flex that I am aware of. (I assume you are talking about Tri-Flex synthetic roofing underlayment.) One is called Tri-Flex Xtreme; it has a vapor permeance of 0.04 perm. Another is called Tri-Flex 15; it has a vapor permeance of 0 perm.

    In other words, these synthetic roofing underlayments are vapor barriers.

    That means that your ventilation channel is mostly useless, because it won't help remove moisture from the bottom layer of roof sheathing -- the layer of roof sheathing that gets wet during the winter. It also means that your roof assembly is illegal, because you need an effective ventilation channel above the top of your cellulose layer to meet code requirements.

    You've got a problem, in other words. Any chance you can rip out the Tri-Flex and substitute a vapor-permeable roofing underlayment?

    1. Ryan Gillard | | #255

      There is a 2" air space between the tri-flex (covering all rafters) and the backside of the roof sheathing. Why won't this air space help remove moisture from the bottom layer of sheathing?

      I understand (now!) that the Triflex XT will not be vapor permeable, but I'm wondering how this will be an issue. Can't the ceiling dry to the inside?

      And I am now am at a place of deciding how to finish off the inside of the ceiling in a way that 1. is air-tight 2. allows inward drying (if outward it seems like is not a possibility) and 3. achieves our desired thickness.

  254. Malcolm Taylor | | #254

    Martin, I think the Tri-flex is being used as ventilation baffles over the rafters, but under the vent space created by 2"x2"s.

  255. User avater GBA Editor
    Martin Holladay | | #256

    Malcolm,
    Whether or not there are two layers of roof sheathing or just one layer of roof sheathing, the Tri-Flex underlayment is a vapor barrier that prevents the ventilation channels from performing any useful drying. So my comments remain valid in either case.

  256. Malcolm Taylor | | #257

    Martin, the problem is probably mine, but as I read his stack up it is:
    - Rafters
    - Synthetic underlayment stretched over roof framing
    - 2"x2"s on top of (parallel to) each rafter
    - Plywood
    - Building paper
    - Metal roofing

    With the cavity created by the 2"x2"s being vented at the eaves and ridge.

    1. Ryan Gillard | | #258

      That's correct Malcom. And Martin, I hear your concern that its permeability is only 0.04 so it won't allow outward drying. But based on your article (https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs) the fact that tri-flex isn't vapor permeable shouldn't be a serious issue if air sealing is done properly on all six sides of the cellulose.

      I still have all of my above questions:

      "There is a 2" air space between the tri-flex (covering all rafters) and the backside of the roof sheathing. Why won't this air space help remove moisture from the bottom layer of sheathing?

      I understand (now!) that the Triflex XT will not be vapor permeable, but I'm wondering how this will be an issue. Can't the ceiling dry to the inside?

      And we are now at a place of deciding how to finish off the inside of the ceiling in a way that 1. is air-tight 2. allows inward drying (if outward it seems like is not a possibility) and 3. achieves our desired thickness."

      Unless this seems like a disastrous plan, we would like to proceed without changing the triflex (it would either require roof removal or cutting from the inside and replacing with something that would be tricky to get as air-tight).

  257. User avater GBA Editor
    Martin Holladay | | #259

    Ryan,
    OK, I get it. And I'm sorry for the confusion.

    I think that it would have been better to choose a vapor-permeable membrane to separate the cellulose from the ventilation channel. As things stand now, moisture that accumulates in the cellulose over the winter can't dry outward (although the ventilation channel does dry out the sheathing layer).

    Will this assembly be problematic? It's hard to predict. As I wrote in one of my articles, "Site-Built Ventilation Baffles for Roofs":

    "What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?

    "The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.

    "That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
    • Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
    • The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
    • Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting."

    In your case, the rafter ends don't extend into the ventilation channel, so this last method of moisture transfer is missing. This reduces the rate of drying compared to a system that includes conventional ventilation baffles between protruding rafters, so that is a potential worry.

    In your case, I would advise (1) paying close attention to airtightness, and (2) crossing my fingers. I think you'll be all right. (That said, other GBA readers who are contemplating this approach should choose a vapor-permeable membrane like housewrap to separate the insulation from the ventilation channel.)

    When it comes to a choice between (a) adding a continuous layer of interior polyiso and (b) thickening the framing cavity to allow for more cellulose, the second choice is somewhat safer, because it allows the cellulose to dry inward during the summer. If you go with option (b), including interior MemBrain will also add a layer of safety, by reducing outward vapor movement during the winter.

    1. Ryan Gillard | | #260

      Thanks Martin. That is what I supposed based on what I read last night while trying not to have a heart attack thinking about tearing our roof off to change the membrane. That being said, I did a lot of research last year on but overlooked this; and I took a friend's advice on the tri-flex assemby, not knowing it was vapor-impermeable.

  258. pvel3 | | #261

    This is an incredible resource. Thank you for this article. I'm designing a house in Laredo, Texas (Zone 2) and am calling for Flash & Batt between 2x8 rafters. From out to in: Shingles, Membrane, Sheathing, Closed-Cell Spray, Batt, Gyp. The R-Value for this zone is R-30. Will using R-15 CCFoam + R15 Batt meet the requirement. Finally, would there be need/room for a radiant barrier in the assembly?

  259. User avater GBA Editor
    Martin Holladay | | #262

    Pvel,
    Q. "In Zone 2, will using R-15 CC Foam + R15 Batt meet the requirement?"

    A. Yes, as long as your local building inspector accepts R-30 as adequate. (Some building codes require a minimum of R-38 in Zone 2.). For more information, see "Flash-and-Batt Insulation."

    Q. "Would there be a need for a radiant barrier in the assembly?"

    A. No. A radiant barrier only works if it is adjacent to an air space. In any case, R-30 insulation is far more effective than a radiant barrier. For more information, see "Radiant Barriers: A Solution in Search of a Problem."

  260. Hugh Knowlton | | #263

    Hugh Knowlton

    I am a home owner living in a house as old as I am (67 years) located in Bergen County, NJ. I replaced the shingle roof in April of 95 with Woodlines 25 year GAF architectural shingles (This was the 3rd roof the house has had and as far as I know there has been no need to replace any plywood sheathing.).

    I have a number of issues (like rotted fascia) not directly related to the state of the shingles that have me leaning on replacing the roof sooner than 25 years. I am trying to educate myself on attic ventilation and hope that GBA can help me.

    I have 2 concerns with ventilation: one with the attic and the other with a cathedral ceiling in the master bedroom which was installed in the 70's or early 80's. I will express my concerns and questions with the attic ventilation in a separate post.

    About half of the roof contractors that have given me estimates for replacing the roof and rotted fascia have suggest installing a ridge vent on the main roof ridge which is 56.5’ long. About 16-17’ of this ridge vent would be over the cathedral ceiling. I have read a number of GBA’s articles on roof/attic ventilation.

    The roof rafters are 2” x 6” with 16” o.c. spacing and from my investigation have fiberglass insulation with no vent baffles; there are 4 small vents (not many) at the soffits. My concern is that installing a ridge vent over this cathedral ceiling might introduce more moisture problems than alleviate them. I seriously doubt much air will flow from the limited vents at the soffit to the ridge. With no vent baffles between the rafters I assume that insulation is right up against the roof sheathing: preventing airflow. From what I have read on your site airflow over fiberglass is not recommended anyway.

    The condition of the drywall on the cathedral ceiling is good; there is some cracking in the drywall on one wall near the peak but is not a major problem. Where the joint is taped at the peak of the ceiling does show with time some bubbling in the tape. We repair this whenever we paint the room. Perhaps, this is due to the lack of ventilation at the peak where there are high temperatures.

    I realize my house is imperfect and the way this cathedral ceiling was built would not be the way it would be done today. Because there is no evidence of rotting plywood sheathing and no leaking inside (and it has been like this for at least 35 years) I am inclined to leave this section of roof “as is” with no ridge vent. I can make the house more perfect by spending money but is it necessary? For now, my thinking is even though this cathedral ceiling is not perfect ”it doesn’t seem to be broken so why fix it”. Would you agree or suggest remedial steps to improve the ventilation? I don't mind spending money but just want to do it wisely.

    Thank you in advance for you advice!

  261. User avater GBA Editor
    Martin Holladay | | #264

    Hugh,
    As my article notes, adding a ridge vent without performing air sealing work at the ceiling level can make the situation worse.

    Your suggested approach -- doing nothing to change the ventilation details -- will probably work (in the sense that your roof assembly will perform no worse than it did before). But there are a lot of unknowns here -- there may be moisture damage near the ridge that you are unaware of.

    In general, it's best to improve the situation by following up-to-date advice and current best practices. But if you can't afford to update the details on your roof assembly, you can leave everything the way it is, and skip the ridge vent.

  262. snmhanson | | #265

    I ran across this earlier today while researching how to construct our ceiling in the house we are building. I didn't get through every post in the thread, but there seems to be a high level of knowledge and experience here so I registered for the site and am hoping I can get some advice.

    The house we are building has a shed roof throughout the upper level with all 16" TJI joists. Roofing is going to be standing seam metal over a layer of waterproofing titanium rolled on top of 5/8" plywood. The current insulation plan is for a few inches of spray foam insulation directly under the sheathing and then fiberglass batt insulation below that to fill the remainder of the cavity. With a minimum of 2" of spray foam I think we will be able to surpass an R-60 rating for the ceiling. However, as I am slowly discovering, there are two potential issues we may still run into.

    First, we are planning on our primary light source throughout the house being can lighting. We are going to go primarily with 4" LED cans and hopefully we can get them in a low profile so they don't steal too much space that would otherwise be insulated. I'll also request that we get airtight fixtures to minimize air infiltration around the fixtures. Hopefully, with the combination of a thick ceiling cavity, spray foam insulation, cooler temperature of LED fixtures, and airtight fixtures we won't have a huge problem with the lights allowing too much air into the ceiling.

    Our bigger problem though is that we really would like to install a plywood ceiling with aluminum T-bar trim between the panels. Before discovering the issue of air infiltration into the ceiling and the resulting condensation and rot, I figured we'd just attach the T-bar trim up to the rafters and fit the plywood inside of it. Now I realize that it might not be that straight forward. I guess the best course of action would be to just do a drywall ceiling and then skin it with 1/4" plywood. However, as good as that could end up looking, I am a purist at heart and don't like the idea of the plywood only being for aesthetics - not to mention the additional costs involved. That leads me back to just using the plywood as the ceiling and wondering if the spray foam insulation and thick and heavily insulated ceiling cavity might be enough to combat air intrusion up to the roof sheathing?

    Any ideas if we are going to be running into issues with our plans. I know the prudent/practical approach would be a regular drywall ceiling with minimal protrusions (can lights) into the ceiling. Putting that aside for now though, if we move forward with things the way I described what are the odds that we will have issues down the road? Any other suggestions how we could accomplish what we are shooting for?

    Thanks for any input or advice.

    Matt

  263. User avater GBA Editor
    Martin Holladay | | #266

    Matt,
    You are planning to follow the flash-and-batt approach. For more information on this insulation method, see "Flash-and-Batt Insulation."

    Your plan to use only 2 inches of spray foam will only work if you live in Climate Zone 1, 2, 3, or 4C. If you live in Climate Zone 4A, 4B, or anywhere colder, then 2 inches of spray foam isn't enough. See the article for a full explanation.

    If your spray foam contractor is skilled, the spray foam layer will provide a fairly good air barrier -- although there are cases where the spray foam shrinks away from the roof framing, leaving cracks and air leakage paths. Omitting the interior air barrier is possible but somewhat risky.

    If this were my house, I would either come up with a better lighting design -- one that doesn't involve recessed cans -- or I would purchase airtight electrical boxes to house the LED fixtures. (Pay attention to air sealing when installing these boxes.) I would also abandon the idea of a plywood ceiling, and instead would install gypsum drywall.

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