Combining Exterior Rigid Foam With Fluffy Insulation
Combining Exterior Rigid Foam With Fluffy Insulation
Figuring out how thick your rigid foam layer needs to be can be tricky
It’s becoming increasingly common for builders to install one or more layers of rigid foam on the exterior side of wall 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. and roof sheathing. Typically, these walls and roofs also include some type of air-permeable insulation (fiberglass, cellulose, or mineral wool) between the studs or rafters.
These wall assemblies and roof assemblies perform extremely well, as long as the rigid foam is thick enough to keep the sheathing above the dew point during the winter. (Exterior foam reduces the ability of sheathing to dry to the exterior. Thin rigid foam is more dangerous than thick rigid foam because it isn't thick enough to prevent moisture accumulation in the sheathing during the winter; however, it's just thick enough to lower the rate of outward drying.)
Guidelines for determining the thickness of exterior rigid foam are discussed in several GBAGreenBuildingAdvisor.com articles:
- For wall foam guidelines, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
- For roof foam guidelines, see How to Install Rigid Foam On Top of Roof Sheathing.
The thickness of the rigid foam varies by climate zone
To prevent moisture accumulation, cold-climate builders need to install thicker rigid foam than warm-climate builders. So the minimum R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the exterior rigid foam in these assemblies varies by climate zone.
Guidelines for roof assemblies aren’t the same as guidelines for wall assemblies. Why? Because (due to the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.) roof sheathing is at greater risk for moisture accumulation than wall sheathing. Moreover, because of nighttime radiation cooling, roof sheathing gets colder at night than wall sheathing, increasing the chance of moisture accumulation in the roof sheathing. These facts lead to more conservative guidelines for roof assemblies than those used for wall assemblies.
What about walls with above-code levels of insulation?
If you plan to build a 2x4 or 2x6 wall, the guidelines for the minimum R-value of exterior rigid foam shown in Table R702.7.1 in the 2012 International Residential Code — guidelines that I summarized in my article, “Calculating the Minimum Thickness of Rigid Foam Sheathing” — are fairly easy to follow. (Watch out for a few traps, however, including the “R-20+5” trap in Climate Zones 6, 7, and 8. For more on the “R-20+5” trap, see The 2012 Code Encourages Risky Wall Strategies.)
Things get complicated, however, if you plan to build a wall system with above-code levels of insulation. If your framed wall will have more insulation than the R-20 assumed by code for 2x6 walls, you’ll need thicker rigid foam than the amount required by Table R702.7.1.
Similarly, if you plan to build a roof system with above-code levels of insulation, you won’t be able to depend on Section R806.5 of the 2012 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.. You’ll probably need thicker rigid foam.
In either of these cases, you need to figure out how much rigid foam you need to stay out of trouble. To make these calculations easier, I’ve created two tables. The tables are based on the nominal value of the insulation layers, without taking into account the effects of 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 studs. The tables show the minimum percentage of the total R-value of the assembly that needs to come from the rigid foam layer.
First, the table for walls.
If you are planning to design a wall with above-code levels of insulation, use the percentage numbers in the last column. For example, let’s say you want to build a wall in Climate Zone 5 that will have 2x8 studs filled with fluffy insulation rated at R-26. How much rigid foam would that wall need? The table shows that 27% of the wall’s insulation needs to come from the rigid foam. That means that the remaining 73% of the wall’s insulation needs to come from the air-permeable insulation (which we know has a nominal R-value of R-26). If we divide R-26 by 0.73, we discover that the entire wall has an R-value of R-36. So the rigid foam needs to have a minimum R-value of 0.27 times R-36, or about R-10.
Next, I’ll present my table for roofs.
Again, I’ll walk through a typical example. Let’s say that you are planning to build an R-60 roof assembly in Climate Zone 6, using 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. and fiberglass. In that climate zone, you need to make sure that 51% of the total insulation comes from rigid foam. That amounts to about R-31 of rigid foam (about 8 inches of EPS). To complete the assembly, you would need to install about R-29 (about 8 inches) of fiberglass insulation under (and in direct contact with) the roof sheathing.
Thicker rigid foam is perfectly OK
It's important to emphasize that these calculations determine the minimum R-value for exterior rigid foam. Whether we're talking about walls or roofs, there is no harm in increasing the thickness of the rigid foam beyond these minimum requirements.
Remember: Thicker rigid foam makes the assembly safer, but thicker fluffy insulation makes the assembly more risky. Installing a higher R-value of rigid foam keeps the sheathing warmer during the winter, and that moves the wall assembly in the direction of lower risk. On the other hand, installing a higher R-value of air-permeable insulation makes the the sheathing cooler during the winter, moving the wall assembly in the direction of higher risk.
Martin Holladay’s previous blog: “How to Design a Wall.”
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