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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 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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Image Credits:

  1. Peter Yost
  2. Fine Homebuilding

Jul 10, 2016 9:55 PM ET

Re: Replies 196,197,198 Stephen,Brad,Martin
by Chuck Collins

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

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

Jan 13, 2017 1:18 PM ET

If simply blocked the soffit/ridge vents, would I be better off?
by Brian Lengel

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

Jan 13, 2017 2:22 PM ET

Response to Brian Lengel
by Martin Holladay

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

Feb 21, 2017 11:24 AM ET

Old capes?
by Tyler Keniston

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

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

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

Feb 21, 2017 11:33 AM ET

Edited Feb 21, 2017 11:35 AM ET.

Response to Tyler Keniston
by Martin Holladay

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

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

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

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

-- Martin Holladay

Feb 21, 2017 12:31 PM ET

Response to Martin Holladay
by Tyler Keniston

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


Apr 26, 2017 7:47 AM ET

T&G under closed cell spray foam
by Steve Harris

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

Apr 26, 2017 8:59 AM ET

Closed cell foam at code-min R is an air barrier.
by Dana Dorsett

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

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

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

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

Apr 26, 2017 10:23 AM ET

Response to Steve Harris (Comment #207)
by Martin Holladay

Dana Dorsett gave you good advice.

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

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

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

Jun 1, 2017 12:24 PM ET

I'm in zone 6. New structure
by N/A N/A

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

Jun 1, 2017 12:51 PM ET

Response to N/A N/A
by Martin Holladay

(It would be nice if you told us your name.)

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

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

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

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

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

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