Posted By Lbear on 19 Sep 2013 01:54 PM
Dana, Is slab EPS foam worth it for a Zone 4 climate?

From a both a comfort & energy cost point of view, R10 at the slab-edge is worth it, as enshrined in IRC 2012.

The folks at BSC seem to think that there's a long term rationale for as much as R7.5  full-field across the slab in zone 4, and R5 in zone 3- see Table 2 p.10.   (Curiously, the BSC slab edge R for zone 4 is only R7.5, which is below the IRC 2012 code min, but may extend deeper than the 2' specified in the IRC.)

But it depends on your actual design whether the expense of the sub-slab foam might otherwise be better spent on other thermal enhancements. R8 EPS costs about 75-80 cents per square foot, call it a buck a foot installed, with seams taped.

In Zone 4A where the summertime dew points are high there is some rationale for sub-slab foam from a moisture handling point of view. A cardboard box or rug resting on an uninsulated slab can develop mold since the slab temp can otherwise fall near or below the dew point of the summertime air.  In zone 4B that aspect is somewhat out of the picture.  In zone 4 it doesn't take anything like R7.5 just for dew point control at the carpet underlayment though- half that should be plenty.