One false conception regarding the placement of foam under a concrete floor is that it stops frost, which in turn, stops ground movement; therefore, eliminating cracking due to the movement.  Foam placed under the slab alone will not stop cracking, in fact, it may very well increase it.  The foam will reduce the frost depth toward the center of the floor but will do nothing for the frost depth at the edges.  The inconsistent frost depths lead to differential movement which will increase the cracking, not decrease.  To highly reduce, or eliminate, frost movement, under-slab foam should be combined with vertical and perimeter foam known as a Shallow Frost Foundation.