How to Install Rigid Foam On Top of Roof Sheathing
How to Install Rigid Foam On Top of Roof Sheathing
This type of insulated roof assembly limits thermal bridging through rafters
A roof over a vented, unconditioned attic does not need to include any insulation. However, most cathedral ceilings and low-slope (flat) roofs are insulated roof assemblies: with this kind of roof, the insulation follows the slope of the roof.
Insulated roof assemblies can be vented or unvented. There are lots of different ways to insulate this type of roof, but one of the best methods calls for the installation of rigid foam insulation above 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. .
There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing:
- Rigid foam above the roof sheathing 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.
- Rigid foam above the roof sheathing keeps the sheathing warmer and drier than it would be if all the insulation were on the interior side of the roof sheathing.
How much foam do I need?
If you plan to install rigid foam above your roof sheathing, you have two choices:
- Option 1: You can install all of the insulation above the roof sheathing (in which case the rigid foam has to meet minimum code requirements for ceiling R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ); or
- Option 2: You can install some of the insulation above the roof sheathing, and the rest of the insulation underneath the roof sheathing (and in direct contact with the roof sheathing).
If you choose Option 1, your rigid foam will be fairly thick:
- In Climate Zone 1, you’ll need R-30 of rigid foam (about 8 or 8.5 inches of EPS, 6 inches of 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., or 5 inches of polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. );
- In Climate Zones 2 and 3, you’ll need R-38 of rigid foam (about 10 or 11 inches of EPS, 8 inches of XPS, or 7 inches of polyiso); and
- In Climate Zones 4 through 8, you’ll need R-49 of rigid foam (about 12.5 or 14 inches of EPS, 10 inches of XPS, or about 9 inches of polyiso).
If you choose Option 2, the code dictates the minimum thickness of your rigid foam layer. According to section R806.4 of the 2009 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., this approach requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” (In the 2012 IRC, the comparable code requirements can be found in Section R806.5.)
According to the relevant code table:
The purpose of the requirement that rigid foam installed above roof sheathing meet certain minimum R-values is to ensure that the roof sheathing stays warm enough during the winter to avoid moisture accumulation and possible sheathing rot. (If the rigid foam layer is too thin and the sheathing is too cold, the sheathing can absorb moisture from the home's interior. Although this process is often referred to as condensation, it is more accurately referred to as sorption.)
Note that if you follow this path (Option 2), the minimum R-values for the rigid foam layer don’t satisfy the full insulation requirement of the building code. You’ll still have to install some type of insulation under (and in direct contact with) the roof sheathing to make sure that the R-value of the assembly meets the R-30 requirement (in zone 1), the R-38 requirement (in zones 2 and 3), or the R-49 requirement (in zones 4 through 8).
If you follow Option 2, a wide variety of insulation materials can be installed under the roof sheathing. Among the possibilities: fiberglass batts, mineral wool, cellulose, or open-cell spray foam. (Closed-cell spray foam is not recommended for this purpose, since closed-cell spray foam prevents the roof sheathing from drying toward the interior if it ever gets damp. For the same reason, this type of roof assembly should never include an interior polyethylene vapor barrier.)
What if the R-value of your roof exceeds minimum code requirements?
One more caveat: If you are planning to thicken the insulation installed under the sheathing in order to achieve a total R-value that exceeds code-minimum requirements, you'll need to also thicken your above-sheathing foam layer to keep the ratio of above-sheathing insulation to below-sheathing insulation in the proper proportion.
For roofs with above-code levels of insulation, use the table reproduced as Image #4, below, to determine the minimum thickness of the rigid foam layer. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.
Cold-weather performance of polyisocyanurate
While polyiso insulation has an R-value of about R-6 or R-6.5 per inch, this value only holds for temperatures above about 40°F. In lower temperatures, especially temperatures below 25°F, the effective R-value of polyiso drops noticeably, to a value that is closer to R-4.5 or R-4.0 per inch. (For more information on this topic, see In Cold Climates, R-5 Foam Beats R-6.)
Because of this fact, the use of polyiso to insulate roofs is best restricted to hot climates (for example, Florida or Texas). Cold-climate builders would be better off installing EPS or XPS rather than polyiso. (Note that most green builders try to avoid using XPS because the blowing agents used to manufacture XPS have a high global warming potential.)
If you are a cold-climate builder who wants to use polyiso, one possible approach is to create a foam sandwich, with polyiso on the bottom and EPS on the top. The EPS will keep the polyiso warm, so that the performance of the polyiso won’t be as affected by cold temperatures as it would if the polyiso were on the top of the sandwich.
For a more detailed discussion of the ramifications of the drop in performance of polyiso at cold temperatures, see Dana Dorsett's advice in Comment #3, below.
You need an air barrier at the bottom of the assembly
Before installing the rigid foam, make sure that the roof sheathing is airtight (or that you install an air barrier immediately above the roof sheathing).
There are at least two ways to do this:
- If your roof has board sheathing, install an airtight membrane (for example, Solitex Mento, a product available from 475 High Performance Building Supply), a layer of synthetic roofing underlayment, or a peel-and-stick membrane.
- If your roof has OSB or plywood sheathing, the panel seams can be taped (for example, with Zip System tape or Siga Wigluv tape); after taping the panel seams, install the roofing underlayment of your choice (for example, asphalt felt).
Multiple layers with staggered seams
The best way to install rigid foam above roof sheathing is to include at least two layers of foam with staggered seams. Staggering the seams improves airtightness and reduces the chance that heat will leak through the foam seams, causing embarrassing stripe marks (melt patterns) through thin layers of frost.
The first layer of rigid foam can be installed with cap nails. The second layer of rigid foam will probably need to be secured with a few long screws equipped with washers or roofing buttons. When the foam is first installed, it only needs to be held in place with a few fasteners. The layer above the rigid foam (either a series of 2x4s or an upper layer of roof sheathing) will hold the foam in place permanently.
To improve the airtightness of the insulation layer, it’s a good idea to tape the seams of the upper layer of rigid foam. Foil-faced polyiso seams can be taped with housewrap tape, while EPS or XPS seams are best taped with Siga Wigluv.
Some builders wonder whether there will be any problems if two or more layers of rigid foam have multiple vapor barriers (as happens when several layers of foil-faced polyiso are stacked on top of each other). The answer is no — this won’t cause any problems.
Once your final layer of rigid foam has been installed, it’s time to install either a second layer of roof sheathing (plywood or OSB) or 2x4s that establish a ventilation gap.
Do I need ventilation channels above the foam?
If you live in a snowy climate where ice dams are fairly common, you probably want to install ventilation channels above your rigid foam. During the winter, when the wind is blowing, air will exit the ventilation channels at the ridge vent, causing cold outdoor air to enter the ventilation channels through the soffit vents. This cold outdoor air lowers the temperature of the upper layer of roof sheathing, reducing the chance that there will be melting at the bottom of the layer of snow sitting on the roof. That reduces the chance of ice dams.
The usual way to ventilate this type of roof is to install 2x4s on the flat (creating a 1.5-inch-deep ventilation gap), with a 2x4 located above each rafter, extending from the eaves to the ridge. Depending on your rafter spacing, you’ll end up with 2x4s that are 16 inches or 24 inches on center. (Lstiburek calls this a “vented over-roof.” However, this approach doesn’t really create an over-roof; all it creates is ventilation channels.)
The 2x4s are fastened to the rafters below with long screws extending through the rigid foam.
How closely should I space the screws?
According to building scientist Joe Lstiburek, you don’t need as many screws to hold down these 2x4s as you would need to fasten furring strips to a wall. (For information on wall screws, see Fastening Furring Strips to a Foam-Sheathed Wall.)
Here’s what Lstiburek has to say: “How many [screws] and how far apart? The good news is that this is less complicated than installing continuous insulation on walls. … Less gravity, better friction. Uplift is the problem, not slippage and bending moment of the fasteners. The folks who do commercial flat roofs have this uplift thing dialed in. The same fastening requirements for uplift for flat roofs will work for these roofs while simultaneously handling the slippage issue. Except where it snows a lot. Snow likes to stick to sloping roofs. With lots of snow things get very complicated. The good news is that structural engineers have a good handle on this — especially ones who work in ski resorts — particularly Swiss and Austrian engineers.”
To summarize: if you don’t expect a lot of snow, you can install the same number of screws (for the upper layer of plywood or OSB, or for the 2x4s, if any, installed above the rigid foam) that are recommended for furring strips installed on walls — basically, one screw every 24 inches along each rafter, with a minimum penetration into solid wood of 1½ inch — and you’ll have more than enough screws. If you expect a lot of snow, however, you should consult an engineer.
What if I don’t need ventilation channels?
If you don’t need ventilation channels, you’ll probably need to install a layer of OSB or plywood roof sheathing on top of your rigid foam. The upper layer of rigid foam is secured to the rafters below with long screws through the foam.
If you plan to install through-fastened steel roofing, you may be able to skip the upper layer of roof sheathing. Instead, install 1x4 or 2x4 purlins, 24 inches on center, parallel to the ridge. Then fasten the roofing to the purlins.
Where do I buy long screws?
Some suppliers of long screws include:
- Best Materials — Dekfast 6-inch roofing screws and Dekfast 9-inch roofing screws
- Wind-Lock (a source of long screws and plastic hold-down buttons)
- FastenMaster (a source of screws up to 18 inches long)
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. screws (often available at roofing supply outlets) come in long lengths and are often a good choice.
How hard is it to screw through thick foam?
Some builders have used the techniques described in this article to install as much as 10 inches of rigid foam above roof sheathing. That’s a lot. If you try to do that, you’ll probably need screws that are at least 12 inches long.
Trying to locate rafters underneath a thick layer of rigid foam can be tricky. While experience helps, it’s never a particularly easy task, and some find the work extremely frustrating.
When Alex Cheimets performed at deep-energy retrofit at his house in Arlington, Massachusetts, he specified 6 inches of rigid foam should be installed above the roof sheathing. As GBA reported, “A box of broken 10-inch screws, a new set of impact drivers, and a week’s worth of frustration later, [the contractors] wondered if it was worth it. In the end, the roof worked out well, but [some of the team members felt that] the extra effort and cost were hard to justify.”
Nailbase or SIPs
If you don’t want to create a site-built sandwich of rigid foam and roof sheathing, you might want to consider installing structural insulated panels (SIPs) or nailbase instead.
A structural insulated panel is a sandwich panel made out of rigid foam (usually EPS) faced with OSB on both sides.
Nailbase is similar, except that nailbase has OSB on only one side of the foam.
If you decide to install SIPs, you should follow the recommendations of the SIP manufacturer. However, SIPs come with a few quirks. SIP seams are more vulnerable to air leaks than the seams between rigid foam panels in site-built assemblies, because it’s impossible to stagger the seams with SIPs. Some SIP roofs have experienced OSB rot near the seams; this occurs when indoor air has access to air channels that connect with the upper layer of OSB at the seam. To address these potential problems, installers should (a) always tape SIP seams on the interior and seal SIP seams with canned spray foam; and (b) strongly consider installing ventilation channels above the SIPs, especially in cold climates.
Nailbase is available in a variety of thicknesses from several manufacturers. For example, ABT Foam sells 9-inch-thick nailbase panels rated at R-33.
If you can’t find nailbase with a high enough R-value to meet your needs, you can install nailbase above a layer of rigid foam. This method has an important advantage: it allows you to stagger the seams between the two layers.
Martin Holladay’s previous blog: “Walls With Interior Rigid Foam.”
- Image #1: Green Building Advisor - Alex Cheimets job
- Image #2: Fine Homebuilding
- Image #3: Sipschool.files.wordpress.com
- Image #4: Martin Holladay
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