Slabs on grade are insulated from the outside, using the same techniques as exterior basement insulation and not less than 600mm (2 ft) below grade level. Extra horizontal insulation over shallow footings have permitted what we call "frost protected shallow footings". The insulation makes the footing believe it is deep in the ground, because it is in fact beyond the frost penetration level.
Some slabs on grade houses have considerable amounts of insulation in the floor, particularly in high water table areas where the water carries off so much heat. The "protectionist" school of insulation (search keyword "heat loss" for the title "BASEMENT: FULL HEIGHT OR PARTIAL HEIGHT INSULATION?") argues that there must be a deliberate thermal bridge or planned heat loss all around the house where the floor and the wall joins to prevent the footings from freezing. Many people complain about this heat loss (cold floor perimeter), but others are thankful it is there. Those who want total comfort have hydronic heating in the slab, at least around the perimeter, giving them a warm perimeter.