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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 May 31, 2018 with information on the cut-and-cobble method.

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.

Minimum R-value requirements

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

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

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

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

Does a cathedral ceiling need to be vented?

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

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

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

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

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

How do I build a vented cathedral ceiling?

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

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

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

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

Eventually, manufacturers began offering stiffer alternatives that are better able to resist the pressures of dense-packed insulation. These products come and go, and many are no longer available. At one time or another, it was possible to buy baffles made of polystyrene, cardboard, 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 (IRC), “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.” Such a vent space can be created by installing 1 inch by 1 inch “sticks” in the upper corners of each rafter bay, followed by stiff cardboard, thin plywood, OSB, fiberboard sheathing, or panels of rigid foam insulation. (If you use rigid foam for your baffles, it probably makes more sense to choose thin 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. (In this case, the closed-cell spray foam prevents the roof sheathing from drying toward the interior if the sheathing gets damp. But wintertime condensation is theoretically possible on the interior side of the cured spray foam, especially if the spray foam layer has thin areas. Because of this possibility, it's best to allow inward drying.)

      What about the cut-and-cobble method?

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

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

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

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

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

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

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

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

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

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

      For more on this topic, see these two articles:

      If you want to install a combination of closed-cell spray-foam on the underside of the roof sheathing and air-permeable insulation between your rafters — an approach sometimes called “flash and batt” — the building code requires that spray foam (or, arguably, rigid foam insulation) be “applied in direct contact with the underside of the structural roof sheathing” and that the foam insulation meet the requirements “specified in Table R806.4 for condensation control.” These are the same minimum R-value requirements mentioned above, ranging from R-5 in zone 1 to R-35 in zone 8. Moreover, "The air-permeable insulation [for example, fiberglass batts or cellulose 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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Mar 28, 2015 5:01 AM ET

Response to Jim Boyd
by Martin Holladay

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

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

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

Oct 22, 2015 5:51 PM ET

Mansard roof question
by Seth K

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

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

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

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

Oct 23, 2015 6:17 AM ET

Response to Seth K
by Martin Holladay

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

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

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

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

Oct 26, 2015 12:17 PM ET

by Seth K

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

Thanks again, Seth

Oct 26, 2015 12:26 PM ET

Edited Oct 26, 2015 12:27 PM ET.

Response to Seth K
by Martin Holladay

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

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

Oct 26, 2015 1:57 PM ET

by Seth K

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

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

Thanks again

Oct 26, 2015 2:11 PM ET

Response to Seth K
by Martin Holladay

I'm not sure if you are interested in a building science answer or a code answer.

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

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

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

Oct 28, 2015 4:00 PM ET

by Seth K

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

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

Thanks again.

Oct 28, 2015 4:06 PM ET

Response to Seth K
by Martin Holladay

You wrote, "I was under the impression blown cellulose prevents moisture migration."

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

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

Nov 7, 2015 8:16 AM ET

Closed Cell + Fiberglass + EPS?
by Peter Matt

Great article!!!

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

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

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

Nov 7, 2015 9:36 AM ET

It's a bit risky doing it with foam board.
by D Dorsett

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

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

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

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

Nov 12, 2015 4:51 PM ET

by John Poulin

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

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

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

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

Thanks for your time,

Nov 13, 2015 6:12 AM ET

Response to John Poulin
by Martin Holladay

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

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

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

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

Feb 24, 2016 4:41 PM ET

Edited Feb 24, 2016 4:52 PM ET.

urgently need advice re: cathedral ceiling insulation
by Debra R

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

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

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

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

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

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

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

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

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

Thanks very much for any advice.

2016-02-22 08.21.34.jpg 2016-02-22 08.21.04.jpg 2016-02-22 08.21.08.jpg 2016-02-24 12.23.31.jpg 2016-02-24 12.24.26.jpg

Feb 24, 2016 5:11 PM ET

Response to Debra R
by Martin Holladay

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

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

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

Feb 24, 2016 5:16 PM ET

Is your contractor doing it for free?
by stephen sheehy

If not, he doesn't get to refuse to put foam above the sheathing, which would be better and cheaper than fitting it between the beams.

Feb 25, 2016 12:03 AM ET

Edited Feb 25, 2016 1:39 AM ET.

thanks, we will reconsider
by Debra R

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

A few follow-ups:

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

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

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

Thanks again. Appreciate your valuable input.

Feb 25, 2016 5:38 AM ET

Response to Debra R.
by Martin Holladay

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

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

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

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

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

Feb 25, 2016 12:23 PM ET

attic questions
by Debra R

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

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

Thanks again.

Feb 25, 2016 12:37 PM ET

Response to Debra R
by Martin Holladay

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

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

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

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

Feb 25, 2016 1:43 PM ET

attic, etc.
by Debra R

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

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

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

Feb 25, 2016 2:09 PM ET

Response to Debra R
by Martin Holladay

Either (a) or (b) is possible.

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

Feb 25, 2016 4:14 PM ET

roof assembly stack-up
by Debra R

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

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

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

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

Thank you for the generosity of your time and advice!

Feb 25, 2016 4:30 PM ET

Response to Debra R
by Martin Holladay

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

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

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

Feb 28, 2016 6:37 PM ET

Edited Feb 28, 2016 8:03 PM ET.

air barrier, vapor barrier, etc.
by Debra R

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

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

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

Thanks very much again.

Feb 29, 2016 8:20 AM ET

Response to Debra R
by Martin Holladay

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

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

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

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

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

Feb 29, 2016 3:22 PM ET

are R-values from separate compartments of insulation additive?
by Debra R

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

Thank you.

Feb 29, 2016 3:36 PM ET

Response to Debra R
by Martin Holladay

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

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

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

Feb 29, 2016 4:23 PM ET

Response to Debra R
by Charlie Sullivan

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

Mar 3, 2016 6:29 PM ET

Cathedral roof vent
by George Baum

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


Mar 3, 2016 10:13 PM ET

Sorry for beating a dead horse
by Alan B
This roof assembly looks pretty cathedral but if i understand correctly its not vented at all, but has generous above sheathing foam. Is this considered a vapour safe assembly considering this article says

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

Mar 4, 2016 7:25 AM ET

Response to George Baum (Comment #180)
by Martin Holladay

It looks like you posted your questions on two different pages. I have provided an answer on the other page where you posted. Here is the link to my answer:

Mar 4, 2016 7:37 AM ET

Edited Mar 4, 2016 7:39 AM ET.

Response to Alan B (Comment #181)
by Martin Holladay

You provided a link, but you never explained which illustration you were referring to in the article you linked to. I'm guessing that you were talking about the illustration that I have reproduced below.

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

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

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

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


Insulated roof assembly from Building Science Corporation.jpg

Mar 9, 2016 10:14 PM ET

Edited Mar 9, 2016 11:05 PM ET.

roof assembly stack-up, again
by Debra R

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

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

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

Thanks much.

Mar 10, 2016 6:04 AM ET

Response to Debra R
by Martin Holladay

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

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

May 22, 2016 11:55 AM ET

Edited May 22, 2016 1:13 PM ET.

New build on '47 foundation
by Ryan Nelson


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

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

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

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

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

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

Ryan Nelson

May 22, 2016 12:38 PM ET

Response to Ryan Nelson
by Martin Holladay

The best insulated sloped roof, in my opinion, is described in these two articles:

How to Install Rigid Foam On Top of Roof Sheathing

Combining Exterior Rigid Foam With Fluffy Insulation.

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

1. Roofing.

2. Roofing underlayment.

3. Roof sheathing.

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

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

6. An air barrier.

7. Roof sheathing.

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

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

May 22, 2016 2:03 PM ET

New build on '47 foundation
by Ryan Nelson


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

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

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

Thanks again,

May 22, 2016 2:39 PM ET

Edited May 22, 2016 2:53 PM ET.

I mis-read you,
by Ryan Nelson

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

Jun 8, 2016 9:11 PM ET

Edited Jun 9, 2016 6:52 AM ET.

Venting over roof sheathing
by Dylan Kinsey

Hi Martin,

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


Jun 9, 2016 4:55 AM ET

Response to Dylan Kinsey
by Martin Holladay

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

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

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

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

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

Jun 9, 2016 8:29 AM ET

Martin, I see your point. I
by Dylan Kinsey


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

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

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

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

Jun 9, 2016 9:11 AM ET

Response to Dylan Kinsey
by Martin Holladay

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

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

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

Jun 9, 2016 8:22 PM ET

Response to Martin
by Dylan Kinsey


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


Jul 5, 2016 12:50 PM ET

wood cathedral ceiling?
by Chuck Collins

Hi Martin,

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

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

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

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

Any suggestions will be greatly appreciated

Jul 5, 2016 1:39 PM ET

Reclaimed foam
by stephen sheehy

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

Jul 5, 2016 7:42 PM ET

Edited Jul 5, 2016 7:44 PM ET.

Reclaimed foam and exposed pine board ceiling
by Brad Hardie


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

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

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

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

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

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

Jul 6, 2016 8:17 AM ET

Edited Jul 6, 2016 8:17 AM ET.

Response to Chuck Collins (Comment #195)
by Martin Holladay

Q. "Can this work to have an unfinished wood ceiling?"

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

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

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

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

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

Jul 8, 2016 2:23 PM ET

Edited Jul 8, 2016 2:47 PM ET.

Adding insulation
by Peter Anderson

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

Jul 8, 2016 2:37 PM ET

Edited Jul 8, 2016 2:57 PM ET.

Response to Peter Anderson
by Martin Holladay

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

A. Yes.

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

A. No -- no problem.

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

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

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