The requisite insulation pic. Since 1 1/2" is not sold in OK, I went with 3/4" (R5) because it is $6/sht less than 1" (R6) and the 3/4" gap between the foil and the interior wall skin makes the foil a radiant barrier (and gives me a place for wires to run). The 1/2' gap that the 1" would have left was too small to allow the foil to act as a radiant barrier. I called RMax (the mfg) and talked to their tech and he advised going this route.