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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 user-756436

UPDATED on April 8, 2016

Although the 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.”

Tags: , , , , , , , ,

Image Credits:

  1. Peter Yost
  2. Fine Homebuilding

Nov 18, 2011 7:16 AM ET

Vented Over-Roof Hybrid
by homedesign

For Affordable High Performance...the closing sentence in Martin's Article is worthy of a Rant.
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
same article at Fine Homebuilding
here is a good video and discussion about roof ventilation
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.

Nov 18, 2011 9:12 AM ET

Image 1
by homedesign

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.

Nov 18, 2011 9:38 AM ET

Rafter vent channels
by user-723121

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.

Nov 18, 2011 9:41 AM ET

other photos of this roof failure
by Peterbilt

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.


Nov 18, 2011 9:42 AM ET

Edited Feb 4, 2014 1:16 PM ET.

Response to Doug McEvers - & a video for cold-climate builders
by user-756436

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.

Nov 18, 2011 10:10 AM ET

Dense pack cellulose in vented athedral ceilings
by user-723121


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.

Nov 18, 2011 10:18 AM ET

Edited Feb 4, 2014 1:16 PM ET.

Response to Doug McEvers
by user-756436

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.

Nov 18, 2011 10:52 AM ET

A comment from Bill Rose
by user-756436

[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

Nov 18, 2011 10:53 AM ET

Response to Bill
by user-756436

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

Nov 18, 2011 3:10 PM ET

Air tightness
by ken levenson

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 A robust air-tight insulation assembly that doesn't require foam.


Nov 18, 2011 3:21 PM ET

Response to Ken Levenson
by user-756436

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.

Nov 18, 2011 3:54 PM ET

vent channels and installs
by ken levenson

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.

Nov 18, 2011 5:36 PM ET

Vent channels
by user-723121


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.

Nov 18, 2011 5:52 PM ET

new products
by ken levenson

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.

Nov 19, 2011 10:35 AM ET

by user-788447

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.

Nov 19, 2011 1:01 PM ET

Where are failures
by DancingDan

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.

Nov 19, 2011 1:10 PM ET

Vented ridge only
by user-723121

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?

Nov 19, 2011 8:09 PM ET

Dense-packing accuvent
by user-945061

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.

Nov 20, 2011 9:46 AM ET

About "Where are the failures?"
by user-756436

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.

Nov 20, 2011 10:50 AM ET

Why not just do it right plus
by wjrobinson

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

Nov 20, 2011 12:09 PM ET

Edited Nov 20, 2011 12:19 PM ET.

Concerning post #19
by homedesign

Perhaps your "memory" is from Joe's Don't Be Dense Insight and Bill Hulstrunk's comments

see post #9

Nov 20, 2011 12:23 PM ET

Plywood is better than wuzwood
by homedesign

It won't/didn't save a bad design like Image 1

Nov 20, 2011 12:27 PM ET

by user-716970

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.

Nov 21, 2011 9:20 AM ET

Edited Nov 21, 2011 9:29 AM ET.

Looking at the fatal pic I
by wjrobinson

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.

Nov 21, 2011 8:59 PM ET

aj builder
by user-723121

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.

Nov 22, 2011 1:28 AM ET

Whew, I'm confused now.
by MPFadR5c6L

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.

Nov 22, 2011 5:36 AM ET

Response to Deniz Bilge
by user-756436

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.

Nov 22, 2011 10:35 AM ET

Doug I agree, too many wrongs
by wjrobinson

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.

Nov 22, 2011 11:33 AM ET

Response to Deniz
by user-869687

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.

Nov 22, 2011 7:05 PM ET

Edited Nov 22, 2011 7:44 PM ET.

Science-code disconnect
by QorfMYBwTx

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,


Nov 23, 2011 12:26 PM ET

What about an insulated nailbase product like Cool-Vent?
by xZnysF2mcR

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?


Cool-Vent literature pdf.pdf 845.56 KB

Nov 23, 2011 1:15 PM ET

Response to James Martin
by user-869687

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.

Nov 23, 2011 3:34 PM ET

Flat roofs, spray foam and vents
by kim_shanahan

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.

Nov 23, 2011 8:33 PM ET

Condensation control
by user-731553

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.

Nov 24, 2011 9:55 AM ET

alternatives to spray foam underneath sheathing
by user-735498

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.

Nov 25, 2011 1:33 PM ET

Take another look at the no foam option:
by albertrooks

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."

PCUN.jpeg PCUN2.jpeg PCUN3.jpeg PCUN4.jpeg

Nov 26, 2011 5:09 AM ET

Response to James Martin
by user-756436

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.

Nov 26, 2011 5:27 AM ET

Response to Malka van Bemmelen
by user-756436

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.

Nov 26, 2011 5:34 AM ET

Response to Katy Hollbacher
by user-756436

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.

Nov 26, 2011 5:41 AM ET

Response to Kim Shanahan
by user-756436

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.

Nov 26, 2011 5:44 AM ET

Response to James Brown
by user-756436

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.

Nov 26, 2011 5:48 AM ET

Response to Albert Rooks
by user-756436

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.

Nov 26, 2011 8:57 AM ET

Edited Nov 26, 2011 9:16 AM ET.

by homedesign

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


Nov 26, 2011 10:58 PM ET

Martin & John: Yep. Your right of course...
by albertrooks

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.

Nov 27, 2011 7:54 AM ET

Not really Vaulted
by homedesign

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.


Nov 27, 2011 10:16 AM ET

Edited Nov 27, 2011 10:24 AM ET.

Vaulted, compact, & sealed but drying
by albertrooks


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!

Image 8.jpg

Nov 27, 2011 10:46 AM ET

aha.....Parallel Chord trusses
by homedesign

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

Nov 27, 2011 1:55 PM ET

Yeah... I could be more clear
by albertrooks

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.

Nov 28, 2011 6:55 PM ET

Experience in Yukon
by spbzrUaeti

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.

Nov 30, 2011 1:25 AM ET

Better check the walls too
by JohnWalker

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).


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

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