Concrete integrity with rigid foam underneath - GreenBuildingAdvisor

Concrete integrity with rigid foam underneath wanmi | Posted in General Questions on April 20, 2026 08:26pm Hi all,  Building slab-on-grade home in Zone 6. No basement or crawl due to high water table.  Bought reclaimed 4″ xps to go under and around the slab. Realized later that 2 layers is better, so purchased recycled high-density eps, 1.5″ + 2.5″ to stagger and tape the seams. Have to choose now which to use under the slab (and use the other batch insulating a barn, none goes to waste). Here is why: The eps is not exactly 1.5 and 2.5″. Some pieces of the 2.5″ are up to 1/4″ thicker. Furthermore, the density varies between higher than box store eps to rock hard density, 3x the weight. Will the difference in density and up to 1/4″ difference in thickness compromise the slab? Happy to use the single layer of 4″ xps if so and use the eps in the barn.  I should mention the slab will be my finished floor.   Either way, all of it is in perfect condition and together is still less cost than had I bought either one new.  Thanks for your insights! Jim Answer GBA Detail Library A collection of one thousand construction details organized by climate and house part Search and download construction details Join the leading community of building science experts Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field. Start free trial Related Questions Waterproofing* old basement with rigid foam Rigid foam under...