The footings of most foundations are placed below the frost depth. In colder areas of the United States, this can mean excavating and pouring concrete 4 ft. or more below grade. If you include enough rigid-foam insulation around a foundation, however, you can keep the soil under the house warm enough to permit shallow excavations, which can be 12 in. or 16 in. deep, even in northern areas.
So-called frost-protected shallow foundations usually consist of a monolithic (thick-edged) slab wrapped with vertical and horizontal rigid-foam insulation. Although the International Residential Code (IRC) does not require a shallow foundation to have insulation below the slab, omitting the subslab insulation is not a good idea. After all, the more insulation you have under the slab, the less heat will leak out of your house into the soil below. Fortunately, these shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth.
Either extruded-polystyrene (XPS) or denser types of expanded-polystyrene (EPS) insulation may be used to insulate a frost-protected shallow foundation. To account for the possible performance degradation of foam insulation that remains buried for years, designers “derate” the presumed R-value of XPS from its nominal value of R-5 per in. to R-4.5 per in. The amount of insulation you’ll need depends on the air-freezing index in your area. Coincidentally, because of existing energy-code requirements, you may already be insulating your foundation walls enough to achieve the necessary R-value for a shallow foundation.
Let’s say you’re building a frost-protected shallow foundation in a Minnesota town with an air-freezing index of 2500. According to code requirements for frost-protected shallow foundations found in Table R403.3 of the IRC, the minimum R-value of the vertical insulation at the perimeter of the slab is R-6.7 (about 1-1/2 in. of XPS). Ironically, the energy section of the IRC, which applies to all types of slabs, not just those that are frost-protected, requires more slab-edge insulation, R-10, for slabs commonly built with full-depth footings in this climate zone.
The R-value for the horizontal wing insulation in this example is R-4.9. Table R403.3 also specifies the minimum width and configuration of the wings.
Now that minimum energy-code requirements for slab insulation have overtaken the design requirements for a frost-protected shallow foundation, the line between a “conventional” slab-on-grade foundation and a frost-protected shallow foundation has been blurred. As a result, almost any monolithic slab complying with energy-code requirements can be turned into a frost-protected shallow foundation by adding the required wing insulation.
For more information, see “Revised Builder’s Guide to Frost Protected Shallow Foundations” (www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf