Polystyrene or polyurethane foam boards provide superior insulating R-values for relatively little thickness. They can be used for nearly any part of a building and are especially effective for insulating exterior wall sheathing, interior sheathing for basement walls, and in special applications such as attic hatches or air gaps where floor joists meet foundations. They offer a good way to insulate the spaces between roof rafters in unvented attics.



When used on interior wall applications, foam panels must be covered with half-inch-thick gypsum board panels​ or another approved building material.



Foam board provides excellent thermal resistance, as much as twice that for other materials of the same thickness. They can easily be trimmed to size for various applications.