The placement of foam vertically around the perimeter of a concrete slab enables easier heating to achieve a more comfortable floor temperature along the edges and is an integral part of a “shallow frost foundation”.  Typically, the thickness of foam used for this purpose is 2” but if an engineer is used to figure the construction, a different thickness may be specified for the project.  Foam is not UV stable, which means ultraviolet light degrades the product.  To combat this negative effect, all exposed foam must be covered.  Water must also be directed to the front of the foam.  Flashing can be used for this dual purpose.