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Community and Q&A

Insulating Cement-Block Basement Walls

Evan_C_B | Posted in General Questions on

I live in southeast Michigan and just bought a house that was built in 1943. The basement was finished when I bought but was suspicious of how they finished it off. I started tearing drywall off the studs and realized they had butted the studs right up against the concrete block and insulated with batt which was also right up against the block. The bottom plates were all rotted out and the batt insulation also had mold on it as well as in various spots on the block wall. I want to refinish the basement again and have been looking into closed cell spray foam insulation. As far as I can see there does not look like there are any water leaks or really any real sign of water pooling on the floor but I know the block walls will absorb water from the exterior and interior and some moisture will be present sometimes. Can shimming the bottom plate and building the studs about an inch of the wall then using closed cell spray foam in-between everything work? Is it a good idea for the foam to be right up against the block? Is there any other options to prevent mold and rot? I have been told poly vapor barriers are a bad idea.

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  1. Expert Member
    BILL WICHERS | | #1

    Ideally you want rigid foam between the studs and the block. The studs should not be directly against the block. If you have rotting wood in there, I'd take out the old studs, insulate the wall with code levels of rigid foam, then build the studwall back. Polyiso will give you the most R per inch, EPS is safest in terms of possible future moisture issues. I would go with EPS if you have risk of future moisture seeping on ocassion, otherwise I'd go with polyiso if you know you have a dry basement.

    I would not use batts in the stud bays. If you use enough rigid foam, you can use batts in the stud bays for additional R value, but I personally don't trust this approach and prefer to put ALL the required R value in the rigid foam. You can then put 2x3s "on the flat" against the rigid foam to save on space, and use 1.5" deep 4" square electrical boxes for any wiring. Use mud rings to bring the devices up from the box to be flush with the finished drywall surface.


    1. Evan_C_B | | #2

      Thank you very much for the advice. Is it still ok to use the rigid foam even though not all the blocks are flush with eachother? When they built the foundation it looks like some of the blocks aren't perfectly straight or just not lined up flush. It's not a lot but there are various places where it's like that. Also, the past owners painted the block. Should the paint be stripped or does that not matter?

      1. Expert Member
        BILL WICHERS | | #4

        The paint on the block doesn't matter.

        As long as the blocks are flat enough that you can put the rigid foam up without breaking it, you should be OK. Ideally you want to avoid any gaps/voids between the insulation and the block, but small gaps here and there on a slightly out of plane block wall shouldn't make much difference.


  2. GBA Editor
    Kiley Jacques | | #3

    Hi Evan,

    You’ll want to read this article: How to Insulate a Basement Wall.

  3. MarieMar | | #5

    I also have cinderblock walls that I was going to insulate.

    I've seen it said that these wall should be coated with something like FlexSeal paint before adding the insulation.

    Do I have this right?

    Thank you.

    1. Expert Member
      BILL WICHERS | | #6

      No, there is no need to paint the walls with anything prior to insulating. The rigid foam itself will act as a vapor barrier in most cases anyway, and will act to "seal" the wall. There is no need for anything extra.

      If you have bulk water problems, I'd use a dimple mat and perimeter drain, but ideally you want to fix bulk water problems on the exterior and not rely on interior side waterproofing paints.


      1. Evan_C_B | | #7

        Thank you very much for the insight Bill

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