I'm in California, IBC climate zone 3B, California climate zone 8, very high termite area.
I'm trying to determine the smallest width (for the total wall assembly) code compliant way to insulate (to R-5) a below grade 8" CMU wall. This is not a basement, it is a habitable area.
The smallest width option I can find for R-5 insulation is 1" XPS (or Neopor GPS) rigid foam boards. I believe I have to insulate on the inside because generally rigid foam isn't great below grade on the outside with termites. Since I have to insulate on the inside and it's rigid foam, I have to have 1/2" of gypsum board for fire code.
This gives me a total wall assembly width of 8" CMU + 1" XPS + 1/2" drywall = 9.5". Any smaller options?
Secondly how do I attach the drywall to the CMU through the XPS? Even ignoring the XPS, the California Residential Code only seems to list approved attachment methods for gypsum board on wood or steel studs.
Practically speaking I believe I could use glue and flat head tapcons to do it but I'm not certain that's code compliant. Do I have to use furring strips? Do furring strips count as a wood studs for attaching drywall? Adding 1"x2" furring strips to the assembly increases my width to 10.25". I'd love to reduce that. Any suggestions?