How to Lay a Shallow Frost-Protected Foundation

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Cross-section showing how a shallow frost-protected foundation is laidA shallow, frost-protected foundation uses both horizontal and vertical insulation to retain extra heat around the building, reducing the depth of the frost line. This, in turn, allows a building to stand on shallower foundations and improves energy efficiency.

The insulation used in this modern innovation is water-resistant rigid foam. It must be rated for burial. Polystyrene products are the best option, with the better choice among those being beadboard (molded expanded rather than extruded polystyrene) because it doesn't use ozone-depleting HCFCs in its manufacture.

The foundations are laid in the same way as normal, deep foundations. Ideally, a slab-on-grade design is used, though the frost protection will work on most types. The foundations themselves are usually made of concrete, though other materials could be used. Once the foundation is in place, the insulation is added – the vertical part of this is the same as usual. Vertical insulation ranges from R4 to R10 values, depending on the average ambient temperature, and footing depth is from 12 to 16 inches.

The additional insulation – the "wing" insulation that juts out the side – varies from none (in the warmest climates) to R14 values in the coldest. The width also varies depending on climate, going from a foot to five feet in reach. Drainage is also extremely important as the drier the ground is, the better the heat retention will be. The wings are therefore laid on top of a six-inch layer of crushed rock, gravel or sand to suck the moisture away from the insulation.

Comparing how heat flow is affected by foundation insulationIt is this "wing" insulation that holds extra heat close to the building and helps resist frost, as you can see in the diagram. The practical result is that the ground around the home is a lot warmer than usual, reducing the depth of the frost line where the foundations are laid. This means that in areas where foundations normally have to penetrate five or six feet into the ground (to reach below the frost line), a shallow foundation of 16 to 24 inches is sufficient. Shallow foundations can mean savings of 15% to 25% on construction.

As an extra bonus, the warmer ground around the home reduces the delta-T (the temperature difference) between the inside and outside of the building, so heat loss is also reduced. Homes with such foundations are more energy-efficient and easier to heat.

If you are considering a shallow, frost-protected foundation for a new build, you should refer to the excellent document created by the National Association of Home Builders' Reasearch Center for the US Government. It's called " Design Guide for Frost-Protected Shallow Foundations " and is available for free download (in PDF format). The guide provides step-by-step guidelines and instructions for laying this type of foundation.