When rigid foam insulation boards are used to insulate the interior of masonry walls, they do not require added vapor retarder treatment. If foil-faced board is used, the foil side is placed toward the heated room.

To install insulation boards, wood-furring strips should be fastened to the wall first. These strips provide a nailing base for attaching interior finishes over the insulation. Fire safety codes require that a gypsum board finish, at least ½-inch thick, be placed over plastic foam insulation. The gypsum board must be attached to the wood furring strips or underlying masonry using nails or screws.

When rigid foam insulation boards are used to insulate the walls of an unventilated crawlspace, they can be bonded to the wall using recommended adhesives. Because the insulation will be exposed, be sure to check the local fire codes and the flame-spread rating of the insulation product.

If you live in a very cold region, you should continue the insulation over the floor of the crawl space for about two feet (on top of the required ground vapor retarder discussed previously). If you live in an area prone to termite damage, check with a pest control professional to see if you need to provide for termite inspections.