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I am currently working in the cinder block basement of a small cabin in a climate with very cold winters. The convective cold from the cinder block walls in the winter is quite chilly. I want to attach one inch thick XPS to the walls to aid in heat retention in the winter. We have a propane fireplace for the basement.

Generally speaking however, I really have no plans to "finish" the walls. The basement is kind of living overflow/utility area most of the time. We kind of enjoy it in its rugged state. With that said, the major goal of this task is warmth!

I do realize though that there are building codes and that something needs to go over the foam as a fire barrier/retardant etc. So with that in mind I don't really want drywall down here. Its a cabin, so something tongue and groove, or shiplap or something with a cabin look would be more appropriate.

Unfortunately there just isn't a lot of room. If I made 2x4 walls on the interior side of the perimeter it would severely cut down on our space. Even adding furring strips over the foam seems too much.

What options and methods do I have for just covering the foam board directly to satisfy code and provide a finished look? Can I put tongue and groove wood right onto the foam? If so, should I simply go through the foam and anchor it into the concrete?

If furring strips are the best space saving option, say so and I will go that route. Looking for options given this unique situation.

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  • You can, but consider the thermal expansion of the wood
    – Traveler
    Oct 28, 2022 at 19:38
  • You could use the "flat side" of the 2x4 if saving an inch or so makes any difference. How small is this room? You need something rigid to anchor the wooden wall boards to other than compressible styrofoam. The added spacing between the wood planks and the cold block may help keep the room warm too.
    – gnicko
    Oct 28, 2022 at 22:12
  • @gnicko I don't mind that idea. What would my base plate strategy be? Could I just use a 1x2 I guess?
    – absentx
    Oct 29, 2022 at 1:18
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    Keep in mind that drywall is a known quantity re: its ability to hold flames back (vs. wood paneling, which'll burn through much more quickly) Oct 29, 2022 at 2:06
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    If adding fitting strips is too much lost space, you might want to consider one of the insulation systems with integral "studs". I used InSoFast and mostly liked it, even if it is EPS rather than XPS. They claim it can carry as much weight as steel studs when properly adhered to the wall; I was nervous and put the heavy lumber rack on long tapcons anyway. I haven't worked with alternatives so can't compare; this is an observation, not a plug.
    – keshlam
    Nov 28, 2022 at 10:47

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Even adding furring strips over the foam seems too much.

I needed a thin insulation for some places, especially around windows, so I used PU foam panels glued to the wall with PU foam. These panels are often sold to be put under concrete slabs and have a layer of aluminium to act as vapor barrier. For the same thickness, it insulates much better than polystyrene (lambda PU = 0.022 W/m.K, lambda XPS=0.030-0.038 W/m.K, so PU has 25-45% less thermal loss).

Cinderblocks aren't airtight so it's important to block air drafts with more PU foam on the edges, plug any holes, etc.

To cover these panels, I didn't use drywall since I had only 15-20mm total thickness available (to still be able to open the window) so one 13mm drywall would mean almost no insulation which would cause a cold spot and thus risk of condensation and mold. So I applied some universal primer, then drywall mud, fiberglass fabric (the kind for wall renovation), and more drywall mud. The result is pretty tough considering it's about 3mm thick, and can be finished smooth.

If you want tongue and groove wood, you could just glue it on top of the insulation but I think it'd be more practical to glue some wood strips perpendicular on the PU panels, then nail/staple the decorative wood on top.

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    The plaster-and-fiberglass solution is creative, but may not meet code. Of course in some areas you don't absolutely have to meet code until you are selling the place or trying to place an insurance claim, but code usually has good reasons behind it -- firestop, in this case -- so be very cautious about where and how you cheat.
    – keshlam
    Nov 28, 2022 at 15:00

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