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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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Image Credits:

  1. Peter Yost
  2. Fine Homebuilding

Nov 30, 2011 5:40 AM ET

Response to John
by Martin Holladay

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

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

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

Nov 30, 2011 2:33 PM ET

Snow Country Cathedral Roofs
by Ken Watts

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

Dec 5, 2011 5:26 PM ET

Edited Dec 5, 2011 5:27 PM ET.

Unvented cathedral questions
by John W

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

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

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

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

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

Dec 5, 2011 5:32 PM ET

Response to John W.
by Martin Holladay

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

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

Dec 5, 2011 8:38 PM ET

OK, I was figuring that the
by John W

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

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

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

Thanks for your response Martin.

Dec 6, 2011 5:42 AM ET

More answers for John W.
by Martin Holladay

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

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

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

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

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

A. Ideally, yes.

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

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

Dec 6, 2011 9:30 AM ET

Thanks again for the answers.
by John W

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

Dec 8, 2011 8:00 AM ET

Zip Roof over 16" Heeled Trusses
by Jim Peck

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

Dec 8, 2011 8:12 AM ET

Zip Roof Assembly
by Jim Peck

As in this.

Ceiling Assembly.jpg

Dec 8, 2011 9:20 AM ET

Response to Jim Peck
by Martin Holladay

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

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

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

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

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

Dec 8, 2011 11:47 AM ET

What formula to calculate the
by Jim Peck

What formula to calculate the amount of ventilation?

Dec 8, 2011 12:01 PM ET

Edited Dec 8, 2011 12:02 PM ET.

Response to Jim Peck
by Martin Holladay

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

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

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

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

Dec 9, 2011 7:33 AM ET

by Jim Peck

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

Dec 9, 2011 6:22 PM ET

Unvented Cathedral questions
by Michael Collins

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

Dec 9, 2011 7:10 PM ET

Response to Michael Collins
by Martin Holladay

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

Dec 9, 2011 9:02 PM ET

Thanks so much
by Michael Collins


Dec 16, 2011 1:18 AM ET

Edited Dec 16, 2011 2:37 AM ET.

Advice Needed - Retrofit Solution for 1915 House Zone 5/6
by Daniel Grade

I have a series of questions that incorporate many of the topics that you have covered in these articles, but I am trying to apply them in a retrofit situation.

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

Did I mention that money is tight?

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

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

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

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

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

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

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

Thanks for your time and your advice! Dan

Dec 16, 2011 7:01 AM ET

Edited Dec 16, 2011 7:03 AM ET.

Response to Daniel Grade
by Martin Holladay

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

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

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

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

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

Good luck.

Dec 16, 2011 2:28 PM ET

Response to Martin Holladay
by Daniel Grade

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

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

Thanks for sharing your expertise,

Dec 16, 2011 2:44 PM ET

Edited Dec 16, 2011 2:45 PM ET.

Second response to Daniel Grade
by Martin Holladay

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

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

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

Jan 11, 2012 12:38 PM ET

Response to Martin
by james brown

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

Jan 11, 2012 12:47 PM ET

Edited Jan 11, 2012 12:48 PM ET.

Response to James Brown
by Martin Holladay

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

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

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

Feb 24, 2012 9:06 AM ET

Proper vapor barrier for vented?
by Jaime Masick

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

Feb 24, 2012 10:14 AM ET

Response to Jaime Masick
by Martin Holladay

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

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

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

Mar 9, 2012 9:20 PM ET

vented cathedral ceiling in climate zone 3, edge to zone 2
by Michael Meyer

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

Mar 10, 2012 5:34 AM ET

Response to Michael Meyer
by Martin Holladay

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

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

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

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

Mar 15, 2012 10:41 AM ET

Edited Mar 15, 2012 11:44 AM ET.

Does Open Cell foam qualify as an air permeable insulation
by kimbark smith

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

Mar 15, 2012 10:45 AM ET

Response to Kimbark Smith
by Martin Holladay

Q. "Does open-cell foam qualify as an air-permeable insulation?"

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

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

A. Yes.

Mar 15, 2012 12:02 PM ET

by kimbark smith

I assume using closed cell would create the dreaded sandwich?

Mar 15, 2012 5:08 PM ET

Response to Kimbark Smith
by Martin Holladay

Q. "I assume using closed cell would create the dreaded sandwich?"

A. Yes indeed.

Apr 11, 2012 10:34 PM ET

Hot humid climate-all metal cathedral roof-must use R-11 batts
by PJ PR

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

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

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

This is where I got the above idea. It's the "Double Layer banding system."

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

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

Answers to any of the above are very appreciated.

Roof1.jpg Roof2.jpg

Apr 12, 2012 5:34 AM ET

Response to PJ PR
by Martin Holladay

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

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

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

Apr 12, 2012 10:25 AM ET

Hi Martin, Thanks for the
by PJ PR

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

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

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

Apr 12, 2012 10:39 AM ET

Response to PJ
by Martin Holladay

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

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

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

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

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

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

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

Apr 14, 2012 12:23 AM ET

Light on regarding vapor & air barriers
by PJ PR

Aha....I get it now.

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

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

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


Apr 25, 2012 2:50 PM ET

1st layer of R11 going on ...
by PJ PR

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

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

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

Thoughts? ideas? PJ


Jun 13, 2012 12:54 AM ET

Flat roof cathedral ceiling
by Bryce Nesbitt

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

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

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

Jun 13, 2012 4:29 AM ET

Response to Bryce Nesbitt
by Martin Holladay

Q. "Should we vent the space above the sheetrock to the interior?"

A. No, that won't be necessary.

Jul 4, 2012 10:53 AM ET

Old cottage cathedral ceiling
by thomas kral

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

Sep 22, 2012 4:34 PM ET

Question: Bunkhouse
by Melissa MacMillian

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

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

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

Sep 23, 2012 6:45 AM ET

Edited Sep 23, 2012 6:46 AM ET.

Response to Mellisa Macmillan
by Martin Holladay

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

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

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

Sep 23, 2012 3:05 PM ET

Thanks Martin! Yeah, I agree
by Melissa MacMillian

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

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

Oct 12, 2012 2:30 PM ET

by Jim Bohrer

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

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

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

Oct 12, 2012 3:11 PM ET

Response to Jim Bohrer
by Martin Holladay

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

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

Nov 28, 2012 9:29 PM ET

Closed cell foam and a roof leak
by bo jespersen

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

Nov 29, 2012 6:08 AM ET

Response to Bo Jespersen
by Martin Holladay

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

1. Stop using spray foam.

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

Nov 29, 2012 9:15 PM ET

I like it
by bo jespersen

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

Thank you very much Martin.

Jan 29, 2013 9:44 PM ET

Insulating existing ceiling
by Joe Lethert

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

Jan 30, 2013 7:03 AM ET

Edited Jan 30, 2013 7:05 AM ET.

Response to Joe Lethert
by Martin Holladay

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

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

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

Assuming I got that right, here is my advice.

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

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

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

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

5. Install your new ceiling.

Feb 14, 2013 8:42 PM ET

unvented attic and rigid foam in rafter bays
by Erich Riesenberg

Thank you for this detailed article and discussion.

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

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

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

Thank you.

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

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