has anyone used this for underslab insulation, does it work?
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"smoke and mirrors" ......severe embellishments of the truth. Notice they do not quote an actual tested R value to ASTM Standard 236. This is the accepted method to determine R. But even if this was tested to the standard, it would be an overstated R since some of the tested R in the "ideal" lab conditions are not found under the slab! Stick with real foam board insulation under the slab; at least R10 if you're in a cold location and you have hydronic radiant heat.
Another thread in this section has a bit of banter about foil insulations. Started out discussing "foam verus batts", I believe.