We are getting ready to pour our slab in our walkout basement this spring. We want to install pex for future hydronic radiant floor heating and keep the lower level as warm as possible. Our GC is suggesting sand for the first layer, then 4″ of XPS foam with foil for a vapor barrier then pour a fiber concrete. 

I have seen so many basement slabs poured on stone, not sand. Is sand not used? Should we be using EPS foam? What would be a good design for a basement that will be finished and need to stay warm?