Question: Are polystyrene foam boards adequate for perimeter insulation for slabs on grade and if so how should they be installed?
Answer: It has been the practice in the past to use one-inch-thick boards for this purpose. Increasing emphasis on energy conservation may justify using thicker boards, at least in cold climates.
They are installed in any of several ways:
vertically on the inside of the perimeter wall, from the top of the slab to the top of the footing
vertically for only the thickness of the slab and also horizontally under the outside two feet of the slab but above the gravel basewith perimeter heating, vertically from below the duct to a point a foot or more above floor level, and horizontally under the duct (but spaced at least 1 ½ inches below it) for a distance several inches past the inside edge of the duct.