The simple answer to this question is that the temperature of the foundation must remain slightly above freezing. It does not have to be very much above, but it must be above. Frost problems often occur when clay soil is saturated with water and it freezes. It creates ice lenses that expand in the direction of heat loss. If the basement is not heated at all, these ice lenses expand toward the basement and can push the wall in. If the basement is heated to about 10°C (50°F), heat moves out from the basement and pushes the ice lenses harmlessly toward the surface of the soil.