This past week the extruded polystyrene (XPS) is being laid down over a bed of gravel and sand, within the insulated foundation walls of our house. This 10.5-inch layer of XPS will be topped by the actual concrete slab, which will "float" upon the XPS, resulting in a thermally decoupled slab!
Structural Insulated Panels (SIPS) will be erected upon the foundation walls. Adding 10.5 inches of insulation under the floor slab is radical, at least when compared to current practice, which is typically a one-inch layer. The resulting "R" value, will be about R-50, pretty much the same for the 12" thick walls and ceiling SIPs.
Anyone who has sat on a carpeted floor in a typical Midwest house in winter can attest to the fact that one inch is not sufficient to prevent heat loss through the floor.
We had to insist that the workers laying the XPS, lap the pieces so that there were no deep cracks that would allow air or moisture to move from layer to layer more easily.
We were glad to find that we could get reclaimed XPS from Plywood King in Spencer, IN. Though Marko Speigel, our energy modeler and guru of all things green built, was skeptical when we proposed using reclaimed XPS, the quality of the material we received was excellent. We had very little waste XPS remaining after the installation. The XPS will be disposed of by incineration; when burned, XPS is about the same as burning wood, and therefore emits no dioxin products into the air.