Yes ! Fond an independant study awhile back and stated that eps keeps its R longer than xps when used under ground. It also sucks up less water over time.

I was not aware of the study referred to by thagreen. I put plastspan under my slab as well. I had questions so called Plastispan in Edmonton. He was really helpful. They make a product called Insulworks for under slab that is used for radiant heating. Made installation really slick. They also make a product for the exterior walls - custom cut channels for furrow strips. Great people to work with

Green Build AB: I believe it's Beaver Plastics of Edmonton that makes Insulworks, not Plastispan (see www.beaverplastics.com). It looks like a similar EPS, but with slots for the radiant heating pipes. Thanks for pointing it out.
...Terry