The physical principle behind a frostprotected shallow foundation (FPSF) is simple: About 5 feet below ground level, the earth’s temperature stays a relatively constant 50°F to 55°F yearround. Imagine snuggling under a down comforter on a cold night. Before long, you feel nice and warm; the comforter has trapped your body heat. Substitute rigid insulation for the comforter, and you can trap the earth’s heat. With a heated building, as long as you don’t overinsulate beneath a slab, enough heat escapes the building to keep the ground beneath the slab warm, and the perimeter vertical insulation keeps heat from migrating outward. That’s why this is an ideal building solution for coldclimate housing.
Cost and energy savings aside, there’s a “bonus” advantage to this method: Buildings can be built on marginal land or when unfavorable sub-surface conditions (such as below-grade ledge or a high water table) become evident after the excavator digs his bucket into the earth. The slaband-foam-form combination allows the project to move forward rather than being abandoned or delayed during a costly search for another site on the property. And you can probably get to within 3 to 4 feet of a large tree and not be concerned about disturbing the root system.
Before proceeding with an FPSF, check in with your local code enforcement officer. Although FPSFs are national building code–compliant, some jurisdictions may be unfamiliar with them and reluctant to approve their use. In Portland, Maine, I had no difficulty getting a waiver for an FPSF on an addition, but in Burlington, Vt., the codes officer at the time insisted on a frost wall with a top-of-footer depth 4 feet or more below grade.
The National Association of Home Builders Research Center offers good guidance on FPSFs in its “Revised Builder’s Guide to Frost Protected Shallow Foundations.”
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