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Community and Q&A

Ideas for insulating cinder block wall interior

giantsean | Posted in General Questions on

Hi. We are renovating a bathroom on a 1950’s brick house in Connecticut, which has one westward facing cinder block and brick wall. It is brick facade, a 3/4″ air gap, then a layer of 4″ cinder block. Understood that the ideal situation is to insulate from exterior but this is not an option for us, so we are looking to do it properly from the inside.

The old interior wall was 1/2 gypsum board on 3/4″ furring strips which were nailed directly into the wall, creating an air gap between the board and cinder block. The type board was plastered with an additional brown and finish coat of 3/4″ total. The wall was in good shape and I can see no evidence of moisture issues on the inside or brick damage on the outside.

I see a lot of conflicting information online about how to deal with this. My original plan was to attach 2×3’s on the flat to the block, then put rigid foam (EPS or Polyiso) between them. On the other hand I have seen info to say the foam should cover the entire block wall and THEN build a wall in front. We do not want to make the wall too thick because it will interfere toilet placement.

Is it possible to attach the foam to the block without studs, then attach furring strips through the foam into the block wall (even glueing them on might work). I can have a total wall thickness of about 1.5″ max excluding drywall before I run into problems. Would I need any additional vapor barrier with less than 2″ of rigid foam? I’m open to and appreciate any other suggestions.


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  1. GBA Editor
    Martin Holladay | | #1

    You're going to have to compromise, since you don't have enough room to insulate this wall properly. Moreover, unless you gut all of the walls of your house, you will have thermal bridges on either side of your bathroom wall -- meaning that during the winter, heat will be leaving your house through your uninsulated walls, and these uninsulated walls will be cooling the edges of your (thinly insulated) new bathroom wall.

    The best way to install rigid foam on a concrete block wall is as a continuous layer. Make this layer of rigid foam as thick as possible. Then install 1x4 furring strips, 16 inches on center, on the interior side of this continuous layer of rigid foam. Fasten the furring strips to the concrete blocks with TapCons or similar masonry screws.

    If you want, you can also fill the gaps between the furring strips with strips of 3/4-inch-thick rigid foam.

  2. giantsean | | #2

    Thank you Martin. Any recommendation for the type of foam (polyiso or EPS?) Or should I just go for the best overall R value? (cost is not a huge concern w/ such a small wall).

    Also is an additional vapor barrier needed? I have some Certainteed Membrain available to use if needed.

    To the point on attachment to the wall, I left out the part that we need to frame a soffit on the ceiling to cover new plumbing, so I will have to ask the carpenter if it can attach over the foam (I don't see why not). I also plan to air seal with foam as much as I can but now sure if there is a chance to go too far.

    Many thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    Either EPS or polyiso will work fine in this location. You don't need a separate vapor barrier for this wall assembly -- rigid foam is already a vapor retarder (and foil-faced polyiso is a vapor barrier). Just pay close attention to airtightness.

  4. Expert Member
    Dana Dorsett | | #4

    A continuous layer of 3" foil faced polyiso strapped to the CMU wall with 1x4 furring will meet or exceed current IRC code-minimums for wall performance in US climate zone 5 (CT) on a U-factor basis. Take a look at TABLE N1102.1.3 (R402.1.3):

    In zone 5 it takes a U-factor of U0.057 or less, which is a "whole-wall-R" of about R17.5. Derated to R5/inch for climate, the polyiso alone delivers R15, the brick is good for at least R0.5, the air gap between the brick & CMU is another R0.5, the 4" CMU is about R1, and the inteiror side gypsum is good for about R1, which adds up to about R18. To that you can add about R0.8 for the interior & exterior air films for a whole-wall R of about R19 which is U0.053. If you use foil-faced goods you would get a performance boost out of the air-gap between the gypsum & foam too, call it R20 whole-wall, or U0.050

    In fact, it would do a bit better than that on average, since during the shoulder seasons the polyiso itself would be running about R18.5-R19 on it's own.

    To get to U0.057 with EPS would take a minimum of 3.25".

    In very depth-constrained spaces you can dado in channels for the 1x furring, so that the wallboard is tight to the foam. Done that way with 3" foam it would be 3.5" from the CMU surface to the interior paint, and it would still hit IRC code performance.

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