What are Frost-Protected Shallow Footings and Why Are They Used?

Most building codes in cold-climates require foundation footings be placed below the frost line, which can be about 4-feet deep in the northern United States. The goal is to protect foundations from frost heaving.

There is an exception to this standard: many codes permit foundations to lie above the frost line as long as they're "protected from frost." However, approval depends on local code officials, and may require special engineering. The 1995 edition of the Council of American Building Officials (CABO) One and Two-Family Dwelling Code includes simplified guidelines for building slab-on-grade homes with shallow foundations that are protected from frost by rigid foam insulation.

A frost protected shallow foundation (FPSF) is a practical alternative to deeper, more-costly foundations in cold regions with seasonal ground freezing and the potential for frost heave.

Figure 1 shows an FPSF and a conventional foundation. An FPSF incorporates strategically placed insulation to raise the frost depth around a building, thereby allowing foundation depths as shallow as 16 inches, even in the most severe climates. The most extensive use has been in the Nordic countries, where over one million FPSF homes have been constructed successfully over the last 40 years. The FPSF is considered standard practice for residential buildings in Scandinavia.