Rigid foam can be used in walls, roofs, and foundations, for retrofits or new construction. Most varieties of foam have a higher R-value per inch than fiberglass, cotton, or cellulose.
There are three main types of rigid foam insulation: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate. Key differences are R-value per inch, water resistance, compressive strength, permeability to water vapor, facings, and of course, cost. All foam insulation products are petroleum-derived.
Rigid foam sheets are sold in several thicknesses; most lumberyards carry insulation ranging from 1/2 in. to 2 in. thick. Thicker sheets (up to 6 in. thick) are usually available by special order.
Rigid foam can solve thermal bridging problems
Because a stud connects the inside of a house to the outside, it can act as a bridge for heat to escape (studs have lower R-values than insulation). Thermal bridging through the studs significantly degrades the thermal performance of the wall. In all climates, exterior foam sheathing improves a wall’s performance.
Installing rigid foam insulation over wall or roof framing reduces this thermal bridging, raises the R-value of the wall or roof assembly, and can eliminate or reduce air leaks. Building scientists Joseph Lstiburek and Peter Baker have reported that adding 1 in. of R-5 insulation to a 2×6 wall insulated with fiberglass batts increases the effective R-value of the wall from 14.4 to 19.4—a 35% gain with only a 15% increase in wall thickness. Adding 2 in. of foam raises the R-value from 14.4 to 23.8, an improvement of 65%.
A layer of insulating foam on the outside of exterior walls also helps the framing stay dry by raising the dew point of the…
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