Raised-frame floor above slab
I understand the importance of super insulating the underside of a slab with large amounts of XPS foam, especially in my very cold climate (northern Maine). I have run across a unique foundation design of a passive house in Falmouth, MA, by architect Steven Baczek (http://www.deapgroup.com/Falmouth_PH.html). It involves insulating ABOVE the slab, essentially leaving the slab out of the conditioned space. His design involves 4″ of XPS rigid foam resting on top of the slab, followed by 2×8 16″ OC boards spanning the slab, resting on a 2×4 strut. 3/4″ T+G subfloor covers the 2x8s. Blown-in cellulose fills the cavity underneath raising the R-value to tremendous amounts, which I imagine is significantly less expensive than underslab foam insulating. He weighs the loss of a thermal mass but says the loss is negligible based upon the heating load of the house and amount of insulation that otherwise would be financially unrealistic with foam underneath the slab. He doesn’t take into account the labor and extra materials required to essentially build another floor. I am guessing that this is not the best system in most climates (warmer), but in mine (Zone 7-8), I am wondering if it’s worth it? Any other testimonials or lessons learned out there? Thanks.