Monolithic slab - Thickened edge

This method works best  with a heated structure. It relies on the use of two sheets of rigid, polystyrene insulation-one on the outside of the foundation wall and the other laid flat on a bed of gravel at the base of the wall-to prevent freezing, which can be a problem with monolithic foundations in areas with frost. The insulation holds heat from the structure in the ground under the footings and prevents heat loss from the edge of the slab. This heat keeps the ground temperature around the footings above freezing.