Does the IRC require a slab-on-ground floor to have R-20 under-slab insulation in addition to the R-10 perimeter insulation extending 3 feet down from the surface? Or does "floor" in table N1102.1.3 actually mean "non-slab-on-ground floor", so no under-slab insulation is required at all?
I think it's the former, but the homebuilder for my custom house thinks it's the latter.