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I am finishing the above-ground basement in my 1950's house, which has 2x4 framing, 16" o.c. I want to insulate as much as possible without breaking the bank (i.e. sprayfoam is out of the question). My current plan is to insulate with rockwool comfortbatt between the studs which should give me an r-value of ~14. I can't stuff any more insulation between the studs, so I'm wondering if there's any problem with me putting 2" rigid foam board (XPS) insulation over top of the studs on the interior side of the wall to get a higher r-value. I'd then drywall directly over top of the XPS foam board. Wondering if this is a recipe for disaster?

For what it's worth, I live in the pacific northwest where temperatures are very moderate in the winter.

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I've seen that done plenty of times and I don't think you'll encounter any issues with that plan. Be sure, however, to include a vapor barrier between the foam board and the drywall.

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  • Interesting - I was under the impression the rigid foam board acts like a vapour barrier on its own, as long as its seams are sealed.
    – TarnTarnTurtleneck
    Commented Sep 29, 2020 at 23:24
  • Check the manufacturer's specifications on this. If they say it's suitable for use as a vapor barrier, then if you install it according to their instructions, you will be good.
    – jwh20
    Commented Sep 30, 2020 at 9:12
  • Thanks! One clarifying point - I have an earthquake retrofit company adding 5/8" plywood over specific sections of the exterior wall to make "shear walls". Would it be better to do a vapor barrier underneath that and then the XPS over top, or should I just use vapor-barrier rated XPS overtop the plywood?
    – TarnTarnTurtleneck
    Commented Sep 30, 2020 at 16:38
  • The vapor barrier must be between the insulation and the heated/cooled space. Usually that means just under the drywall. You don't want moisture from the living space getting into the insulation where it will cool and saturate it.
    – jwh20
    Commented Sep 30, 2020 at 16:44
  • I think I understand, but to clarify, are you saying the following stack would be appropriate for the "shear walls" (from interior to exterior): drywall, vapor barrier, rigid foam board, plywood shear wall, stud bays with rock wool, exterior sheathing, tar paper, stucco. What might not have been clear in my original comment was the shear wall plywood was being installed on the interior side of the studs
    – TarnTarnTurtleneck
    Commented Oct 1, 2020 at 22:53

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