It’s becoming increasingly common for builders to install one or more layers of rigid foam on the exterior side of wall sheathing 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 GBA articles:
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-value 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 effect) 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 2×4 or 2×6 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…