Slabs for Colder Climates, Part 3: Installing Frost-Protected Shallow Foundations for Unheated Buildings

The use of FPSFs for unheated buildings provides the greatest advantages in unheated and semi-heated structures attached to a heated structure, such as an attached garage or front porch, although the IRC specifically disallows this unless designed by a professional engineer in compliance with ASCE 32-01. The added insulation and excavation required can diminish some of the cost advantages of this approach, but in some climate zones it still makes sense, and builders use it because it provides a more reliable and better insulated base under slabs.

Figure 1. Frost depth varies by ground cover and soil composition. Note how the dotted line, representing frost, varies across the landscape. You will see that humus retains geothermal heat very well, and the frost depth in the humus remains relatively shallow compared to that in the gravel, which allows cold to penetrate and does not retain geothermal heat.

In an unheated building (or unheated portion of a heated building), the insulation must cover the entire foundation floor area and, depending on the climate zone, extend beyond the perimeter of the building up to 4'. Additionally, the subsurface fill must allow moisture to drain away from the foundation, because the conditions required to create frost heave include cold, moisture, and frost-susceptible soil. Dry gravel, for example, will not heave in frost. So the combination of an insulating blanket with proper sub-grade preparation, which includes no frost-susceptible soils, often conspires to make this technology more expensive than a conventional foundation. However, when building over very hard, rocky ground, such as that found in areas of the northeast, or when desiring to maximize insulation or minimize excavation and ground disturbance, this alternative becomes economically viable.

Figure 2. Detail of typical FPSF design for unheated building.

Figure 3. Alternate view of unheated building section using FPSF.

The insulation chosen for frost protection in unheated buildings must be adequate for subsurface placement, must meet R-value requirements, and, as it will lie under the footings, must also be able to support light structural loads. Guidance for the selection of insulation and fill is provided by ASCE 32-01

Figure 4. Insulation under stairways and driveways and over utility lines provides a number of cost-effective ways to prevent frost heave in secondary applications.

Other uses of this geothermal-capture technology include column base footings, utility trenches (a potentially money-saving application for sewer and water service), driveways, patios, and for building projections such as stairways. These applications, although useful, are not referenced by the IRC or ASCE 32-01. However, you can obtain guidance from a HUD translation of the Norwegian guidelines for FPSF or a monograph of the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) titled, “European foundation designs for seasonally frozen ground” by Omar Farouki (Cornell University 2010).

Employees at Bill Eich Construction Co. in Spirit Lake, Iowa, prepare the base for the FPSF of an unheated, attached garage. Although the extensive preparation and insulation add significant costs compared to simply digging frost-depth footings and pouring a slab directly on the ground, Eich prefers the warmer slab and claims that his concrete does not crack or heave despite Iowa’s extreme winter temperature swings, which can range in degrees Fahrenheit from the mid 40s to negative 30 within a 24-hour period.

In an unheated building (or an unheated portion of a heated building), the insulation must cover the entire foundation floor area. Insulation in the photo above covers the entire garage slab surface area, including the area below footings. Compacted fill not susceptible to frost will cover the insulation, and then technicians will carve trench footings to suport the structure, taking care not to damage the insulation underneath.