Many builders add one or more layers of rigid foam insulation to the outside of a house to lower heat losses. Rigid insulation has an R-value of up to 6.5 per inch, but it also can be an effective vapor retarder.
Ed Welch touched off an extended discussion in the Green Building Advisor’s Q&A section when he asked whether the foam would trap moisture inside walls, creating mold as well as the potential for structural decay.
His concerns seem well placed. In a cold climate during the heating season, moisture vapor inside a building is driven outward into exterior walls. When it reaches a surface that’s below the dew point, the vapor condenses into a liquid. That surface is typically the back side of the exterior sheathing.
Rigid foam board, especially foil-faced polyisocyanurate, creates a vapor-impermeable barrier, so the wall would have limited drying potential. Even more permeable types of insulation, such as expanded polystyrene, are vapor barriers when the installation is thick enough.
No one argues this point of view more forcefully than Robert Riversong of Vermont, who has been building high-performance houses for many years. Riversong notes the exterior skin of a house should be at least five times as vapor-permeable as the interior. Yet 2 in. of extruded polystyrene insulation — the amount it would take to keep sheathing above the dew point in a cold climate — create an effective vapor retarder.
Further, Riversong argues that long-term studies have shown foam-clad walls have no way of drying out in the event of even minor rain penetration. He suggests there are better ways of increasing R-values and reducing thermal bridging, such as double-framed walls or using foam board on the interior.
He also favors cellulose insulation and other natural materials over petrochemical plastics because they are more…