In the southern half of the U.S., it’s common to build houses on concrete slabs without a basement or crawlspace underneath. Where there’s no need or desire for basement living and storage space, slab-on-ground construction is a simple and affordable option. But in colder climates, building on a slab is more complicated. In areas where the ground freezes in winter, it’s standard practice to dig footings below the frost line to support and protect the structure against frost heave. The cost of excavating these footings as deep as 4 feet below grade can eat up much of the cost savings of slab construction, without providing the benefit of basement living space.
One alternative, which was developed in Scandinavia and used extensively in rebuilding there after World War II, is the frost-protected shallow foundation (FPSF). In this method, a monolithic slab with thickened edges serves as the foundation, and foam insulation around the perimeter keeps the ground warm enough to prevent frost heave.
In the 1980s, the U.S. plastics industry and the National Association of Home Builders Research Center became interested in FPSF and began exploring its potential use in the U.S. In the early ‘90s, U.S. HUD sponsored a verification study, building and monitoring five FPSF houses in northern states. This study led to the creation of an Air-Freezing Index map and the development of a design guide for the use of FPSF in the U.S.
The foundation system first gained CABO model code recognition in 1995, and ASCE Standard 32-01 “Design and Construction of Frost-protected Shallow Foundations” (based on the HUD design guide) was issued in 2001. The IRC now includes simplified FPSF design provisions for homes, and ASCE 32-01 has been referenced in IRC and IBC for residential and commercial applications since 2003. Market acceptance of FPSF has grown slowly, but NAHB says that thousands of structures have been built with them in the U.S. so far, and it represents a practical and energy-efficient foundation technology for affordable housing and other applications.
The idea behind FPSF is to use strategically placed rigid foam insulation to trap the heat of the earth beneath the foundation and keep the ground from freezing. The amount and placement of the insulation varies, depending on the severity of the climate. Figure 1 shows how heat flows at the base of a heated FPSF building. Heat lost through the floor slab, as well as the geothermal heat of warm soil beneath the building, combine to keep frost from forming below the slab edge. Vertical insulation is installed along the exterior of the thickened slab edge; in the coldest climate zones, additional sheets of rigid foam are placed horizontally, extending out from the base of the slab (Figure 2).
For unheated space, such as a garage or outbuilding, where the minimum average indoor temperature is less than 41° F, design guidelines call for the foam insulation to be placed under the entire slab to retain geothermal heat in the ground. Although it’s not required by code, some designers and builders opt to place horizontal insulation under the slab of heated spaces as well, to provide for tighter, more comfortable interiors.