Thermal Conductivity of Rigid Foam Board

Closed-cell foams beat fibrous insulation because the cells trap a blowing agent with lower conductivity than air. Fresh polyisocyanurate can test near R-7/in; after five to fifteen years of gas diffusion the long-term thermal resistance settles nearer R-5.6/in, and manufacturers now publish that aged value. XPS is about R-5/in, EPS about R-4/in and stable because it is blown with air-like gases from the start.

Polyiso has a second and less advertised behaviour: its conductivity worsens at low temperature. Below roughly 10 °C the blowing agent begins to condense in the cells, and measured R-value can fall from R-6 to R-4.5 per inch on the coldest nights: exactly when a roof or wall assembly is being asked to perform. Continuous exterior foam is still the right answer for thermal bridging, but in cold climates designers either use XPS or mineral wool outboard, or derate polyiso. All rigid foams also need fire protection and UV protection, and their R-value assumes the boards are gap-free and taped.