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Strategies for internal rigid foam

giantsean | Posted in General Questions on


First, thank you for all the great advice on this site and your willingness to help. This is a wonderful resource!

I have a brick on 4″ cinder block wall that I am looking to insulate from the interior. I am 95% sold on polyiso vs XPS due to the higher R value and the fact that it is mounted on an interior wall and hopefully will not be subject to the reduced R value as temps drop. I have about 1.5″ with which to work due to plumbing code (toilet to sidewall)

Because I want to minimize the wood involved to the bare minimum, I am thinking of putting 1″ of polyiso against the wall, then 3/4″ furring strips over the 1″ and securing through the foam with tapcons. Then filling the gaps with 1/2″ polyiso. I am not sure of two things:

– Will the increased R-value as polyiso thickens be realized here (not sure if it requires one solid piece)
– Will the extra layers of foil cause issues?

Finally, does anyone have experience with thinner than usual drywall covering rigid foam? If I can get away with 1/4″ drywall with no adverse effects I would consider it. Hoping that the foam can provide a bit of support but also worried that it may create waves. It is not a long wall however.

Thanks again!

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to fill in the areas between your 3/4" furring strips with rigid foam, why not use 3/4" thick polyiso instead of 1/2" polyiso? Here is a link to a supplier of 3/4" polyiso:;jsessionid=6082A7A844CC7ADB6A727036F3541219.eklka-prodapp4-external-mcom

    Multiple layers of foil facing in this stack-up will not cause any problems.

    I have never installed 1/4-inch drywall. I imagine it would work, especially if you glue the drywall to a co-planar substrate. (That means that your rigid foam thickness has to equal your furring strip thickness.) Any waviness in the drywall can be corrected when you spread your mud, assuming that you know how to use a wide drywall knife properly.

  2. Expert Member

    I'd avoid 1/4" drywall. It's generally reserved for building up curves with multiple layers. Very fragile and hard to get the fasteners to countersink without tear through. !/4" drywall on furring, secured through foam to a CMU wall, will magnify any irregularities. You will end up using a lot more screws and mud.
    The boards account for a quite small portion of a drywall quote. I'd be surprised if there was a noticeable difference in going with 1/4" on a job.

  3. Expert Member
    Dana Dorsett | | #3

    For the section behind the toilet and to a foot either side & above it, glue the 1" polyiso to the CMU with foam-board construction adhesive, and glue 1/2" drywall to the foam board.

    Use 1x furring through-screwed to the CMU a few inches away from the toilet perimeter and then go higher-R around it, making it a small inset cove for the toilet rather than condemning the entire wall to a sub-code thermal performance.

    What climate zone is this?

  4. giantsean | | #4

    Thank you all.

    Dana, it is zone 5 (So. Connecticut). If we are talking about a matter of fractional inches I'm hoping our local building inspector will look the other way in favor of better insulation. Especially given that this is an older house (1951) with upgraded-in-place plumbing. I thought of the cove as well as some other ideas like reframing the partition wall on the other side of the toilet to 2x2 construction, but I also have to fit a vanity in there. However there is a window and some other reasons like overall depth of the room which keep me from doing something like framing a regular 2x4 wall against the masonry.

    Either way, is it a workable option to glue the entire length of the wal?l (drywall to foam). It is a roughly 10 x 7 wall (accounting for a soffit box @ ceiling)

  5. Expert Member
    Dana Dorsett | | #5

    If you make just 10 square feet of wall a continuous R6 + R-0.5 gypsum + R2 CMU+ brick= R8.5-9-ish whole wall it's the thermal equivalent of 3 square feet of code-min window, so making it a shallow cubby around the toilet isn't a big deal. But making the entire wall R6 would be a bigger deal.

    To hit code-min type performance in zone 5 would take 2.5-3" of continuous layer of polyiso, which is a bit thinner than a 2x4 wall butted up against the CMU. Even 2" of polyiso would bring that up to R15-ish whole-wall, which is pretty close to current code min, slightly exceeds 1980s & '90s code-min type wall performance (eg. vinyl sided 2x6 / R19 construction), which was about R13-R14 whole-wall. Can you spare another inch?

    The wall board can be secured to the CMU with 4" cap-screws 24" o.c. to avoid having a furring layer. Use 5/8" gypsum to keep it flatter and limit the cracking. Alternatively you could dado out channels in the foam 24" o.c. for some 1x4 furring and screw the furring to the CMU or do 1" layers with cut'n'cobbled 3/4" foam between the furring, but that's more work, and lower performance.

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