Under the concrete we first placed 2 ” of Polyiso Foil Faced Foam Board from AtlasRoofing. The product will provide an R-value of 6.5 per inch under the slab. It was also a bit hard to find, but we were steered in that direction after the August 1st article from Environmental Building News described the numerous downsides of Polystyrene Insulation which is still the dominant market product in below-grade applications.
Polyiso avoids many of the Polystyrene problems and the foil facing will help reflect the radiant heat from the radiant coils which were embedded in the concrete.
Since the concrete is going to be the finished floor we added ‘limestone’ color to lighten the look.