How Much Force Does a 4” Concrete Slab Exert on the Insulation Below?
A 4” concrete slab actually only exerts 50 pounds per square foot (Or 0.35 pounds per square inch) on the foam board insulation below. And because the slab itself is quite stiff, any point load on the slab will be distributed. By the time that load reaches the foam board insulation, it will actually be much less than what it is on the surface.
This is why for many applications a foam board insulation, like Halo Subterra, with a compressive strength of 16 psi is usually sufficient.