By Ethan Davis
The objective of frost protection in foundation design is to prevent damage to the structure from frost action (heaving and thaw weakening) in frost-susceptible soils.
In northern U.S. climates, builders and designers mitigate the effects of frost heave by constructing homes with perimeter footings that extend below a locally prescribed frost depth.
Other construction methods include:
piles or caissons extending below the seasonal frost line;
mat or reinforced structural slab foundations that resist differential heave;
non-frost-susceptible fills and drainage; and
adjustable foundation supports.
The local building department typically sets required frost depths. Often, the depths are highly conservative in accordance with frost depths experienced in applications not relevant to residential foundations. The local design frost depth can vary significantly from that required by actual climate, soil, and application conditions. One exception occurs in Alaska, where it is common to specify different frost depths for “warm,” “cold,” and “interior” foundations. For homes in the Anchorage, Alaska, area, the perimeter foundation is generally classified as warm, with a required depth of 4 or 5 feet. Interior footings may be required to be 8 inches deep. On the other hand, “cold” foundations, including outside columns, may be required to be as much as 10 feet deep. In the contiguous 48 states, depths for footings range from a minimum 12 inches in the South to as much as 6 feet in some northern localities.
Based on the air-freezing index, Table 4.8 presents minimum “safe” frost depths for residential foundations. Figure 4.12 depicts the air-freezing index, a climate index closely associated with ground freezing depth. The most frost-susceptible soils are silty soils or mixtures that contain a large fraction of silt-sized particles. Generally, soils or fill materials with less than 6% fines (as measured by a #200 sieve) are considered non-frost-susceptible. Proper surface water and foundation drainage are also important factors where frost heave is a concern. The designer should recognize that many soils may not be frost-susceptible in their natural state (e.g., sand, gravel, or other well-drained soils that are typically low in moisture content). However, for those that are frost-susceptible, the consequences can be significant and costly if not properly considered in the foundation design.
Frost-Protected Shallow Foundations
A frost-protected shallow foundation (FPSF) is a practical alternative to deeper foundations in cold regions characterized by seasonal ground freezing and the potential for frost heave. FPSFs are best suited to slab-on-grade homes on relatively flat sites. The FPSF method may, however, be used effectively with walkout basements by insulating the foundation on the downhill side of the house, thus eliminating the need for a stepped footing.
An FPSF is constructed by using strategically placed vertical and horizontal insulation to insulate the footings around the building, thereby allowing foundation depths as shallow as 12 inches in very cold climates. The frost-protected shallow foundation technology recognizes earth as a heat source that repels frost. Heat input to the ground from buildings therefore contributes to the thermal environment around the foundation.
The thickness of the insulation and the horizontal distance that the insulation must extend away from the building depends primarily on the climate. In less severe cold climates, horizontal insulation is not necessary. Other factors, such as soil thermal conductivity, soil moisture content, and the internal temperature of a building are also important. Current design and construction guidelines are based on reasonable worst-case conditions.
After more than 40 years of use in the Scandinavian countries, FPSFs are now recognized in the prescriptive requirements of the International One- and Two- Family Dwelling Code. However, the code places limits on the use of foam plastic below grade in areas of noticeably high termite infestation probability. In those areas termite barriers or other details must be incorporated into the design to block hidden pathways leading from the soil into the structure between the foam insulation and the foundation wall. The exception to the code limit occurs when termite-resistant materials (e.g., concrete, steel, or preservative-treated wood) are specified for a home’s structural members.
The complete design procedure for FPSFs is detailed in Frost-Protected Shallow Foundations in Residential Construction. The first edition of this guide is available from the U.S. Department of Housing and Urban Development. Either version provides useful construction details and guidelines for determining the amount (thickness) of insulation required for a given climate or application. Acceptable insulation materials include expanded and extruded polystyrenes, although adjusted insulation values are provided for below-ground use
The designer is cautioned that the thawing of permafrost due to a building’s thermal effect on a site can quickly undermine a structure. It is critical that the presence of permafrost is properly identified through subsoil exploration. Several effective design approaches are available for building on permafrost.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)