Frost-Protected Shallow Foundations (FPSF)

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.

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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.

FPSF Resources

History of frost-protected shallow foundations

HUD FPSF Study Findings

Advantages of FPSF

Building Codes and FPSF

Frost Action and Foundations (the nitty gritty on how frost heave works)

Types of Insulation Allowed for FPSF

Types of FPSF

Applications and Limitations of FPSF

FPSF in Heated Buildings

FPSF in Unheated Buildings

Recommended Construction Methods and Details

Simplified Design Method

Detailed Method for Heated Buildings

How FPSF Works

The frost protected shallow foundation technology recognizes the thermal interaction of building foundations with the ground. Heat input to the ground from buildings effectively raises the frost depth at the perimeter of the foundation. This effect and other conditions that regulate frost penetration into the ground are illustrated in Figure 2.

It is important to note that the frost line rises near a foundation if the building is heated. This effect is magnified when insulation is strategically placed around the foundation. The FPSF also works on an unheated building by conserving geothermal heat below the building. Unheated areas of homes such as garages may be constructed in this manner.

Figure 3 illustrates the heat exchange process in an FPSF, which results in a higher frost depth around the building. The insulation around the foundation perimeter conserves and redirects heat loss through the slab toward the soil below the foundation. Geothermal heat from the underlying ground also helps to raise the frost depth around the building.

FPSFs are most suitable for slab-on-grade homes on sites with moderate to low sloping grades. The method may, however, be used effectively with walk-out basements by insulating the foundation on the downhill side of the house, thus eliminating the need for a stepped footing. FPSFs are also useful for remodeling projects in part because they minimize site disturbance. In addition to residential, commercial, and agricultural buildings, the technology has been applied to highways, dams, underground utilities, railroads, and earth embankments.

More Common Questions and Answers

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.

FPSF Construction Issues

These issues apply to the construction of any FPSF:

Cold bridges. Cold bridges are created when building materials with high thermal conductivity, such as concrete, are directly exposed to outside temperatures. Foundation insulation should be placed such that continuity is maintained with the insulation of the house envelope. Cold bridges may increase the potential for frost heave, or at the least, create localized lower temperatures or condensation on the slab surface. Care must be taken during construction to ensure proper installation of the insulation.

Drainage. Good drainage is important with any foundation and FPSF is no exception. Insulation performs better in drier soil conditions. Ensure that ground insulation is adequately protected from excessive moisture through sound drainage practices, such as sloping the grade away from the building.

Insulation should always be placed above the level of the ground water table. A layer of gravel, sand, or similar material is recommended for improved drainage as well as to provide a smooth surface for placement of any horizontal wing insulation. A minimum 6-inch drain layer is required for unheated FPSF designs. Beyond the 12-inch minimum foundation depth required by building codes, the additional foundation depth required by an FPSF design may be made up of compacted, non-frost susceptible fill material such as gravel, sand, or crushed rock.

Slab surface temperatures (moisture, comfort, and energy efficiency). The minimum insulation levels prescribed in this design procedure protect the foundation soil from frost. They also provide satisfactory slab surface temperatures to prevent moisture condensation and satisfy a minimum degree of thermal comfort. Since the design procedure provides minimum insulation requirements, the foundation insulation may be increased to meet special needs concerning these issues and energy efficiency. Successfully limiting cold bridging is critical -- use of the stem wall and slab technique, in effect, adds a second thermal break between the slab and stem wall. Increasing the vertical wall insulation thickness above the minimum requirements for frost protection will also improve energy efficiency and thermal comfort. Selection of a finish floor material such as carpeting decreases the surface contact between occupant and the slab, giving a warmer feel.

Heated slabs and energy efficiency. FPSF design procedure can be applied to all slab-on-grade techniques, including those with in-slab heat which provide excellent thermal comfort. If an in-slab heating system is used, additional insulation below the slab and around the perimeter is recommended for improved energy efficiency.

Protecting the insulation.Because the vertical wall insulation around a foundation extends above grade and is subject to ultraviolet radiation and physical abuse, that portion must be protected with a coating or covering that is both tough and durable. Some methods to consider are a stucco finish system or similar brush-on coatings, pre-coated insulation products, flashings, and pressure treated plywood. The builder should always verify that such materials are compatible with the insulation board. The protective finish should be applied before backfilling, since it must extend at least four inches below grade. Also, polystyrene insulation is easily broken down by hydrocarbon solvents such as gasoline, benzene, diesel fuel, and tar. Care should be taken to prevent insulation damage during handling, storage, and backfilling. Also, where termites are a concern, standard preventative practice such as soil treatment, termite shields, etc. is suggested.

Insulation specifications.Because some insulation materials resist water absorption less effectively than others, which in turn degrades their thermal resistance (R-values), insulation material should be specified carefully. The following effective R-values shall be used to determine insulation thicknesses required for this application: Type II expanded polystyrene - 2.4 R per inch; Types IV, V, VI, VII extruded polystyrene - 4.5 R per inch; Type IX expanded polystyrene - 3.2 R per inch. Special applications, such as bearing structural loads from footings, may require higher density polystyrenes for the required compressive strengths. The builder is referred to manufacturers for product-specific information.

Doorways and Thresholds. At doorways where the threshold overhangs the vertical wall insulation, the insulation should be cut out as required to provide solid blocking for adequate bearing and fastening of the threshold. The size of the cut-outs should be minimized.

Landscaping and wing insulation. In situations where wide horizontal wing insulation is required (e.g., greater than 3 to 4 foot widths), this may deter the location of large plantings close to the home. In some of these cases, using thicker wing insulation or increasing the foundation depth will decrease the required width of the wing insulation.

Foundation height. Given that most polystyrene insulation boards are typically available in 24 inch and 48 inch widths, 24 inches becomes a practical height for many foundations. This provides 16 inches of the foundation below grade and 8 inches above grade.

Excavation. Generally, lightweight equipment is adequate for FPSFs because little excavation is required. As with any foundation, organic soil layers (top soil) should be removed to allow the foundation to bear on firm soil or compacted fills.

Construction scheduling. The foundation should be completed and the building enclosed and heated prior to the freezing weather, similar to conventional construction practice.