Currently in the final planning stages of my new house ("raised ranch" style), and the first floor is going to be slab-on-grade. My current insulation plan has 2" of XPS under the slab across its entire area (with a vapor barrier and 4" of 2B stone beneath). However, when I went to put the compliance information into ResCheck, that application only had parameters for entering perimeter insulation (e.g. from the edge of the slab, vertically down to the footers). That leads me to believe entire-slab insulation isn't that common.
My question is, from a thermodynamic standpoint, is it pointless to insulate the field of the slab if the perimeter is insulated, or do you still get a lot of energy migration down through the stone into the dry earth below? Does most of the energy loss in a basement occur from conduction of the slab to the foundation walls to the exterior, or is there still a significant amount of vertical conduction into the soil?