Frost-Protected Shallow Foundations (FPSF) provides protection against frost damage without the need for excavating below the frost line. An FPSF has insulation placed strategically around the outside of a foundation to direct heat loss from the building toward the foundation, and also to use the earth's natural geothermal energy.

Casting footings for a FPSF.

Traditionally, foundations are protected from frost-heaving damage by placing the footing below the frost line. Because FPSF are protected from freezing by thermal insulation, bottoms of footings can be just twelve to sixteen inches below grade. This reduces excavation costs, making this an economical alternative for protecting foundations against frost damage.

Insulated footings had been used as early as the 1930s by Frank Lloyd Wright in the Chicago area. There are now over one million homes in Norway, Sweden, and Finland with insulated shallow footings, recognized by their building codes as standard practice. It is estimated that there are over 5,000 buildings in the United States that have successfully used frost protected shallow foundations.

Frost-protected shallow foundations are similar to conventional foundations except for insulation placement and footing depth. Bottoms of FPSF footings are placed about 12 to 16 inches below grade. FPSF have vertical insulation placed at the outside edge of the foundation extending from above grade to the bottom of the footing. When required in colder climates, "Wing" insulation extends outward horizontally from the footing. The colder the climate, the further the wing insulation is extended. Wing insulation is unnecessary in moderate climates.

The insulation used in FPSF is commonly rigid expanded or extruded polystyrene foam suitable for below grade application, and it must be in compliance with ASTM C 578 Standard. FPSF can be used for both heated and unheated portions of a building.

Frost-protected shallow foundations (FPSF) allow bottoms of footings to be shallower than the normal code-required frost depth. This reduces the amount of excavation and concrete or masonry required, which reduces labor and material costs. Heating energy costs are also reduced because of increased perimeter foundation insulation.

Insulation around the perimeter of the foundation reduces the amount of heat loss from the warm interior to the cold exterior in the winter, which would normally pass through the slab or floor, and out through the foundation. Heating energy costs are thus reduced.

 

Frost-protected shallow foundations rely on building and geothermal heat to keep the earth around the foundation warm enough to preclude frost formation that might heave the structure. The foundation is wrapped with polystyrene foam that provides a thermal barrier and channels the escaping heat to warm the earth beneath the foundation. The method allows footing or foundation depth to be reduced from 48" to 16" in colder climates; representing a 2/3 savings in the amount of concrete needed, and with less excavation, disturbing less of the surrounding earth.

 

Cost is highly dependent on building size and climate. Savings of $635-4750 have been reported.

A FPSF can be incorporated into slab-on-ground construction, but it can also be used for stem wall, floating slab, and unvented crawlspace foundations.

The NAHB Research Center published a "Design Guide for Frost Protected Shallow Foundations" to help determine proper foundation design and insulation details. This design guide contains detailed drawings of the prescribed methods of construction. In addition, the National Association of Homebuilders (NAHB) has published brochures describing FPSF.

Care should be taken that the foundation edge is completely insulated with no uninsulated area through which heat can pass. As with any foundation type good drainage is important so that water does not collect below the footing and freeze.

Source: NAHB Research Center



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