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?

  • Why are you worried about 2 1/2 inches? Is your house that tiny?
    – crip659
    Jul 30, 2022 at 20:07
  • 1
    If asking this at the design stage (which is the right time to ask) just push the concrete wall out far enough to give the desired insulation value and space inside.
    – Ecnerwal
    Jul 31, 2022 at 2:20
  • @Ecnerwal Yes this is the design stage but it's for an ADU but for various reasons including that it's a California ADU and in a side setback, it's limited to 800sqft (this is a hard limit) and also constrained in width. That square footage limit is measured from the exterior of the walls so pushing the concrete wall out doesn't actually work
    – gmbuell
    Jul 31, 2022 at 2:40
  • @crip659 Yes, see my comment to Ecnerwal above. The exterior dimensions have a hard limit of 16' x 50' so 2.5 inches causes me to loose 10sqft of interior area (which isn't that bad but because that 2.5 inches is coming out of that 16', it really starts to cause a problem with the design. It might be my only option but I'll take the 10sqft extra if I can get it.
    – gmbuell
    Jul 31, 2022 at 2:42

2 Answers 2


You might want to look at one of the integrated insulation/studding systems. Install with construction adhesive, wire as appropriate, surface with plasterboard, apply a bit more adhesive or spray foam to seal top and bottom, done. Used one of these (InSoFast) in my own basement; fairly easy and minimizes added thickness for decent insulation. I haven't used other products so this is an observation, not necessarily a recommendation.

This Old House has a video of using this. They also have one a less fancy version of this basic idea: apply foam-board insulation to walls with construction adhesive, apply two crossed layers of furring over that with tapcons (one will support wallboard, the other provides space for running wire), wire and wallboard. Recommended viewing to understand the tradeoffs. Basically the integrated system is a bit easier to install, may support shelves and the like better, and saves you an inch or so of floor space, but costs a bit more and generates more foam particles for static electricity to get stuck to everything.


Typically you would attach the furring strips to the CMU (16" centers for 1/2" drywall) and cut the rigid insulation between those furring strips. After checking your climate zone it seems you are in a moderate zone where this could be OK. This would save on your overall wall thickness.

If maximum insulation is necessary or desired then you could fasten the insulation and then put the furring strips over that (with tapcons or an equivalent). Concerns would be overtightening the tapcons and compressing the insulation resulting in a wavy surface for your drywall. This also adds to your wall thickness.

Another option would be to apply your rigid insulation to the walls- it would have to be a non-foil encased type of foam- and then applying a Sto-type modified plaster wall finish (this is a brand name- not a process name- I am unaffiliated with them) . This is often used for plaster/stucco exteriors. It consists of a modified plaster base coat with fiberglass mesh imbedded and then a finish coat (with many different options for integral color and texture). This would be by far the most expensive option but would give you the most minimal wall thickness.

If having good insulation is your paramount goal, I would suggest using 2x4 furring strips at 16" centers attached to the CMU. Then you could use 1 1/2" insulation between those furring strips and 1/2" drywall on top of that. 1x2 furring strips are only 3/4" and would not work with your 1" insulation. This would also allow for some room (although pretty tight) for electric outlets and switches. This, of course, makes your walls thicker but would give better insulation.

This all assumes your sub grade walls are perfectly sealed and moisture proof because any of these options will not do well with any moisture that can come through.

Bottom line is that if you need/ want insulation then your walls are going to thicken. How much they thicken is your call based on your insulation desires.

  • Thanks for the in depth answer. Because of the energy efficiency requirements of California Title 24, I can't install the rigid foam between the furring strips. It has to be R-5 continuous insulation to work (the alternative is non-continuous insulation but with a much higher R value which is an even wider wall assembly). For that Sto-type modified plaster wall finish - How do you attach the fiberglass mesh to the foam? Any idea if it that qualifies as a covering for rigid foam for fire code purposes?
    – gmbuell
    Jul 31, 2022 at 2:46

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