The National Association of Home Builders’ Research Center has developed a unique way to reduce the amount of concrete in foundations. Rather than pour footings below the frost line in cold climates, researchers placed footings just 2 feet below grade and then insulated the slabs in two ways. Walls were insulated conventionally with 2 inches of rigid closed-cell extruded polystyrene foam. Then, horizontally, a section of rigid foam 2 feet wide was placed around the perimeter of the slab 24 inches below grade.
These added layers of insulation keep frost from being driven deep into the ground where it could crack the slab. Model building codes have recognized frost-protected shallow foundation design principles for more than a decade. In most regions of the continental United States, these foundations can reliably be placed as shallow as 16 inches below grade. Performance has been proven in Europe, and frost-protected foundations have been used in very cold climates in the United States—in North Dakota, for instance—with great results.
It takes a lot less concrete to form one of these slabs when compared to the more conventional approach of placing concrete stem walls below the frost line before the slab is poured. That means savings for the buyer, as well as less energy and material use—all without sacrificing performance.