An FPSF often improves the energy efficiency of a typical home, because it requires more foundation insulation than many codes. The concept is simple. Instead of placing footings below the frost line , the FPSF uses insulation and drainage techniques to raise the frost line to just below the surface. Even in the coldest climates, this permits footing depths as shallow as 12 inches. Compressive strength is an important characteristic for these below-grade applications.
Compressive strength is related to foam density.
Naturally, foundation applications require stouter material. CABO permits 2.0 pcf extruded polystyrene for horizontal insulation and 2.0 pcf extruded or expanded polystyrene for vertical insulation. This exceeds the underlying soil's bearing capacity of around 2500 psf. If you prefer a margin of safety, higher density foams are available. Designing Good Drainage Insulation is only half the equation.