by Brian Hval
Frost protected shallow foundations (FPSF) are used in cold climates because they are a cost-effective alternative to conventional deep foundations. Over 1 million homes have used this technique worldwide. It is accepted as the standard foundation design method in Scandinavian countries.
FPSF homes are often cheaper to build as they eliminate deep excavations and use less foundation material.
Heated homes and other structures are protected against frost heave by insulating the outside of the foundation below grade. A short wing of foam insulation stretches out from the foundation to make a frost barrier like this:
Expanded PolyStyrene foam sheets are usually used.
The thickness and extent of foam required are determined by historical climatology data like this:
Some building design approval jurisdictions may require a professional engineer to review the calculations and stamp the foundation drawings. But the calculations are simple to do. There is also a wide range of foam installation options to give the required insulation R-value.
If you live in a cold climate, FPSF is well worth looking into. It applies to both renovations and new construction. I have used it in Canada on two projects. One was to install a patio door in an existing basement foundation to make a basement walk-out like this:
Foam wings on a gravel bed extend outwards and just below the threshold of the door. FPSF dramatically lowered the cost of making this renovation. The second project was for a new oceanfront home where it was essential to minimize disturbance to the soil.
FPSF foam wings around the wall perimeter provided the needed frost protection. Soil integrity and stability was maintained and the overall cost was much less than a traditional foundation.