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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

Posted on Nov 18 2011 by Martin Holladay

UPDATED on April 8, 2016

Although the GBAGreenBuildingAdvisor.com 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , 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.

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, vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., 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 (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.), “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 EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. or XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. 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(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. 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, 2x10 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 bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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 ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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 three 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.

      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 insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.] 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.”


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101.
Feb 15, 2013 8:51 AM ET

Response to Erich Riesenberg
by Martin Holladay

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.
Feb 15, 2013 12:37 PM ET

cobble is the right word
by Erich Riesenberg

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.
Apr 1, 2013 11:53 AM ET

unvented spray foam + polyiso inside
by Mark Fredericks

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.
Apr 1, 2013 12:49 PM ET

Response to Mark Fredericks
by Martin Holladay

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.
Apr 2, 2013 2:38 PM ET

Engard in the rafters
by Steve Knapp CZ 3A Georgia

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.
Apr 2, 2013 2:50 PM ET

Edited Apr 2, 2013 2:53 PM ET.

Steven Knapp
by Martin Holladay

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.
Apr 3, 2013 7:04 PM ET

Commercial unvented roof question
by Milo Shubat

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.
Apr 4, 2013 4:49 AM ET

Response to Milo Shubat
by Martin Holladay

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.
Apr 4, 2013 12:12 PM ET

Commercial unvented roof question
by Milo Shubat

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.
Apr 4, 2013 12:29 PM ET

Response to Milo Shubat
by Martin Holladay

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


111.
May 19, 2013 5:59 AM ET

Unveiled air space
by Annemarie Belteu

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.
May 19, 2013 9:33 AM ET

Response to Annemarie Belteu
by Martin Holladay

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.
Jun 29, 2013 5:04 PM ET

polyiso facing question
by joell solan

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.
Jul 1, 2013 5:21 AM ET

Response to Joell Solan
by Martin Holladay

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.
Jul 5, 2013 7:32 PM ET

Bringing ducts "inside" and duct condensation
by James Anderson

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-cou...), 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.
Jul 6, 2013 5:17 AM ET

Edited Jul 6, 2013 5:20 AM ET.

Response to James Anderson
by Martin Holladay

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.
Aug 19, 2013 7:06 PM ET

Edited Aug 19, 2013 7:14 PM ET.

Cathedral Ceiling repair work
by S. Chapman

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

2013-02-09 14.34.42.jpg


118.
Aug 20, 2013 7:13 AM ET

Response to S. Chapman
by Martin Holladay

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.
Aug 20, 2013 5:19 PM ET

Thanks for the confirmation
by S. Chapman

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.
Aug 24, 2013 1:29 PM ET

Venting an arched cathedral roof
by Milo Shubat

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.
Aug 24, 2013 1:36 PM ET

Venting an arched cathedral roof
by Milo Shubat

Here's a graphic for the roof framing

arched roof.jpg


122.
Aug 25, 2013 4:59 AM ET

Edited Aug 25, 2013 5:06 AM ET.

Response to Milo Shubat
by Martin Holladay

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.
Aug 25, 2013 5:45 PM ET

Venting an arched cathedral roof
by Milo Shubat

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.
Aug 26, 2013 5:17 AM ET

Reponse to Milo Shubat
by Martin Holladay

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.
Aug 26, 2013 12:06 PM ET

Venting an arched cathedral roof
by Milo Shubat

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.
Aug 26, 2013 1:00 PM ET

Response to Milo Shubat
by Martin Holladay

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.
Oct 23, 2013 8:36 PM ET

Cathedral ceiling question
by Daniel Cabe

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.
Oct 24, 2013 4:45 AM ET

Response to Daniel Cabe
by Martin Holladay

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.
Nov 6, 2013 12:38 PM ET

response to milo
by john c

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.
Jan 19, 2014 11:19 AM ET

What to do with ridge vent in unvented assembly
by Hari Kamboji

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.
Jan 19, 2014 12:02 PM ET

Response to Hari Kamboji
by Martin Holladay

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.
Mar 24, 2014 6:50 PM ET

WSU for Valleys/Eaves/Low-Slope on Unvented Cathedral
by Eric Matsuzawa

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-climat...

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

roof polyiso.jpg roof sheathing.jpg


133.
Mar 25, 2014 5:20 AM ET

Edited Mar 25, 2014 5:22 AM ET.

Response to Eric Matsuzawa
by Martin Holladay

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.
Mar 27, 2014 7:32 AM ET

Response to Martin Holladay
by Eric Matsuzawa

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.
Apr 15, 2014 10:14 PM ET

not a perfect science
by John Vickery

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.
Apr 16, 2014 5:11 AM ET

Response to John Vickery
by Martin Holladay

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.
May 7, 2014 7:46 PM ET

Edited May 7, 2014 8:00 PM ET.

Hi. I am somewhat new to
by Brandon Francom

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?

Capture2.JPG Capture.JPG Capture3.JPG


138.
Jun 9, 2014 9:41 AM ET

Perfect Storm - vaulted ceilings, low-slope & recessed lighting
by jeff newcomer

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.
Jun 9, 2014 11:28 AM ET

Response to Jeff Newcomer
by Martin Holladay

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.
Sep 1, 2014 11:13 PM ET

Above sheathing ventilation detail
by Derek Dykstra

"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.
Sep 2, 2014 8:24 AM ET

Edited Sep 2, 2014 8:25 AM ET.

Response to Derek Dykstra
by Martin Holladay

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-c...

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-clad...

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.
Jan 30, 2015 12:33 AM ET

3" Closed Cell Foam + Roxul + Rigid Foam
by Chris Heard

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.
Jan 30, 2015 9:47 AM ET

Edited Jan 30, 2015 9:49 AM ET.

Response to Chris Heard
by Martin Holladay

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.
Feb 8, 2015 10:22 PM ET

Best Option Recommendation
by Jen Gah

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.
Feb 9, 2015 10:48 AM ET

Response to Jen Gah
by Martin Holladay

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.
Mar 23, 2015 8:43 PM ET

Unvented attic (new construction)
by Edward Skakie

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.
Mar 24, 2015 5:46 AM ET

Response to Edward Skakie
by Martin Holladay

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.
Mar 26, 2015 4:40 PM ET

Unvented attic
by Edward Skakie

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.
Mar 27, 2015 4:19 AM ET

Response to Edward Skakie
by Martin Holladay

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.
Mar 27, 2015 7:34 PM ET

Cut and cobble
by Jim Boyd

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.]


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