This foundation is when the slab and the footings are poured all at the same time. It saves some time as all the concrete is poured at once. It does make layout of the internal plumbing and other items thatcome out of the slab more difficult to place. There is also the concern that even with exterior slab insulation there is still heat loss through the bottom of the slab. This type of foundation cannot be used with radiant floor heating.