Shallow Frost Protected Foundations a more economical option on the right site
When building a home in a cold climate, the foundation is traditionally placed on undisturbed or compacted soils below the frost line to better resist the effects of ground freezing and frost heaving. In Alaska, every region has building codes or generally accepted design standards that specify the depth of the local frost line. In Fairbanks, the design depth for footings is a minimum of 42 inches below grade. However, installing footings and a foundation wall to this depth can be expensive. That’s where shallow frost-protected foundations come in, presenting a more economic option if you’re on stable ground (in other words, not permafrost).
Unlike a standard foundation, a shallow frost-protected foundation (SFPF) can sit well above the frost line, often at depths of about 16 inches below grade, and in some cases less. Since the foundation now rests on soils that would normally freeze seasonally, the key issue is to keep the ground underneath and on the sides of the foundation from freezing. These designs usually depend on foam board insulation laid out far enough horizontally around the perimeter of the footing to ensure that the ground underneath remains thawed year-round, no matter how cold it gets. In essence, the horizontal insulation creates a “heat bubble” under the building. A frost-protected foundation can accommodate a variety of designs including thickened edge/monolithic slabs and shallow footings.
By code, the horizontal foam board insulation needs to be protected from sunlight and physical damage. Typically this means the insulation will get covered with a layer of backfill thick enough to protect it for the life of the structure–although concrete or pavement coverings may also be options (in a high traffic area, for example). Typically foundations including SFPF systems should extend a minimum of 6 inches above grade to keep the home’s wood framing away from ground moisture. Any vertical area above the horizontal insulation must also be well insulated.
Because SFPF systems are fairly new in Interior Alaska, a professionally engineered design will give you peace of mind. Due to site-specific variations in soils conditions and foundation designs, a professional engineer will best be able to calculate the insulation values and installation methods to ensure the foundation will perform properly.