Common Questions and Answers about Frost-Protected Shallow Foundations

Question No. 1: How does insulation stop frost heave from occurring?

Frost heave can only occur when all of the following three conditions are present: 1) the soil is frost susceptible (large silt fraction), 2) sufficient moisture is available (soil is above approximately 80 percent saturation), and 3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of frost damage. Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation. Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.

Question No. 2: Does the soil type or ground cover (e.g., snow) affect the amount of insulation required?

By design, the proposed insulation requirements are based on the worst-case ground condition of no snow or organic cover on the soil. Likewise, the recommended insulation will effectively prevent freezing of all frost-susceptible soils. Because of the heat absorbed (latent heat) during the freezing of water (phase change), increased amounts of soil water will tend to moderate the frost penetration or temperature change of the soil-water mass. Since soil water increases the heat capacity of the soil, it further increases the resistance to freezing by increasing the soil's "thermal mass" and adding a significant latent heat effect. Therefore, the proposed insulation requirements are based on a worst-case, silty soil condition with sufficient moisture to allow frost heave but not so much as to cause the soil itself to drastically resist the penetration of the frost line. Actually, a coarse grained soil (non-frost susceptible) which is low in moisture will freeze faster and deeper, but with no potential for frost damage. Thus, the proposed insulation recommendations effectively mitigate frost heave for all soil types under varying moisture and surface conditions.

Question No. 3: How long will the insulation protect the foundation?

This question is very important when protecting homes or other structures which have a long life expectancy. The ability of insulation to perform in below-ground conditions is dependent on the product type, grade, and moisture resistance. In Europe, polystyrene insulation has been used to protect foundations for nearly 40 years with no experience of frost heave. Thus, with proper adjustment of R-values for below-ground service conditions, both extruded polystyrene (XPS) and expanded polystyrene (EPS) can be used with assurance of performance. In the United States, XPS has been studied for Alaskan highway and pipeline projects, and it has been found that after 20 years of service and at least 5 yrs of submergence in water that the XPS maintained its R-value (ref. McFadden and Bennett, Construction in Cold Regions: A Guide for Planners, Engineers, Contractors, and Managers , J. Wiley & Sons, Inc., 1991. pp328-329). For reasons of quality assurance, both XPS and EPS can be readily identified by labelling corresponding to current ASTM standards.

Question No. 4: What happens if the heating system fails for a time during the winter?

For all types of construction, heat loss through the floor of a building contributes to geothermal heat storage under the building, which during the winter is released at the foundation perimeter. Using insulated footings will effectively regulate the stored heat loss and retard penetration of the frost line during a period of heating system failure or set-back. Conventional foundations, with typically less insulation, do not offer this level of protection and the frost may penetrate more quickly through the foundation wall and into interior areas below the floor slab. With ad-freezing (the frozen bond between the water in the soil and the foundation wall), frost does not need to penetrate below footings to be dangerous to light construction. In this sense, frost protected footings are more effective in preventing frost damage. The proposed insulation requirements are based on highly accurate climate information verified by up to 86 years of winter freezing records for over 3,000 weather stations across the United States. The insulation is sized to prevent foundation soil freezing for a 100-year return period winter freezing event with a particularly rigorous condition of no snow or ground cover. Even then, it is highly unlikely that during such an event their will be no snow cover, sufficiently high ground moisture, and an extended loss of building heat.

Question No. 5: Why are greater amounts of insulation needed at the corners of the foundation?

Heat loss occurs outward from the foundation walls and is, therefore, intensified at the proximity of an outside corner because of the combined heat loss from two adjacent wall surfaces. Consequently, to protect foundation corners from frost damage, greater amounts of insulation are required in the corner regions. Thus, an insulated footing design will provide additional protection at corners where the risk of frost damage is higher.

Question No. 6: What experience has the U.S. seen with this technology?

Frost protected insulated footings were used as early as the 1930s by Frank Lloyd Wright in the Chicago area. But since that time, the Europeans have taken the lead in applying this concept over the last 40 years. There are now over 1 million homes in Norway, Sweden, and Finland with insulated shallow footings which are recognized in the building codes as a standard practice. In the United States, insulation has been used to prevent frost heave in many special engineering projects (i.e., highways, dams, pipelines, and engineered buildings). Its use on home foundations has been accepted by local codes in Alaska, and it has seen scattered use in uncoded areas of other states. It is likely that there are several thousand homes with variations of frost protected insulated footings in the United States (including Alaska).

To verify the technology in the United States, five test homes were constructed in Vermont, Iowa, North Dakota, and Alaska. The homes were instrumented with automated data acquisition systems to monitor ground, foundation, slab, indoor, and outdoor temperatures at various locations around the foundations. The performance observed was in agreement with the European experience in that the insulated footings prevented the foundation soil from freezing and heaving even under rigorous climatic and soil conditions (ref. U.S. Department of Housing and Urban Development, "Frost Protected Shallow Foundations for Residential Construction", Washington, DC, 1993).

Question No. 7: How energy efficient and comfortable are slab foundations with frost protected footings?

The insulation requirements for frost protected footings are minimum requirements to prevent frost damage. The requirements will provide a satisfactory level of energy efficiency, comfort, and protection against moisture condensation. Since these requirements are minimums, additional insulation may be applied to meet special comfort objectives or more stringent energy codes.

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