Here we are sealing the sub-slab assembly completely with 2″ of 2lb sprayfoam — we achieve an R value of 14, and with the “welding” of it to all slab penetrations and to our ICF foundation around the perimeter are completely vapor impermeable, and have a beautifully continuous thermal enclosure. What does this mean? Water AND radon can NOT get into this home from below and the heated slab will have a very tough time losing any of its heat to the earth (allowing it to most efficiently dissipate its heat to the intended living space above.)
This is an awesome new product we are using for our sub slab insulation: it is a spray foam called Cor Bond 3 that is made right here in Belgrade Montana. It has an initial R-value of 7.3/inch and has the highest aged R-value for spray foam on the market at 7.0/inch (spray foams have two ratings, initial and aged because after they are sprayed in place, some r value is lost with UV exposure and curing process) Cor Bond 3 has an incredible psi rating as well — around 32 PSI — compared to blue board which has a psi rating of 25 or less). The cost of this foam as a material are a touch more, but it yields higher, making it as cost effective as the previous spray foam we had been using.