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How to Insulate an above grade block wall

benneaf3 | Posted in General Questions on

The walls of the rental I’m rebuilding are block (built in 1950).  I have read (somewhere) with that with a block wall I should use rockwool on the inside and foam board on the outside and the R value of each should be about equal.

My project manager wants to use closed-cell foam insulation on the inside only.  He also says that it will function as an air barrier since it adheres to the block.  I just want to make sure this is accurate.  Are both ways valid?

I’m in 4A (Eastern TN outside of Knoxville)

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  1. Expert Member
    BILL WICHERS | | #1

    You can insulate either or both sides of the block. There is no need for "drying" of the block. I would not use spray foam here, I would use rigid foam board, probably polyiso if you don't have any issues with water getting into the interior of the basement. If these are all above grade walls, then I would use either polyiso (most R per inch, but expensive) or EPS (least R per inch, but relatively cheap). Ideal is probably to put all of the R value on the exterior, but if that's not practical, putting some on either side is fine.

    There is no need for spray foam as an air barrier, any of the other rigid foam panels will work too if you tape the seams between panels and seal the perimeter with canned foam or sealant (caulk).

    I consider spray foam to be more of a niche product, and try to use it only in unvented roof assemblies, irregular foundation walls (cut stone), and rim joist areas -- and not always in rim joist areas. When you have flat, regular walls, which should be the case with properly laid block, you can use rigid foam board instead for an easier assembly with much less potential risk in terms of installation issues.


    1. benneaf3 | | #4

      These are all above grade walls. The foundation is a slab, everything is above grade.

    2. benneaf3 | | #5


  2. Expert Member


    The problem with insulating old masonry walls on the inside is that they can go though freeze/thaw cycles when wet, and deteriorate as a result. That problem is much worse in cold climates, and with porous brick. I don't see any issue with just insulating on the interior, and like Bill think foam board is the way to go. You could also insulate the stud walls inside the foam with batts.

    1. benneaf3 | | #3

      I'm not sure what you mean by 'insulate the stud walls inside the foam with batts.'. The studs are up as close to the walls as possible. If the spray foam fills the stud wall cavity where would the batts go? How thick should the spray foam be?

      1. Expert Member
        MALCOLM TAYLOR | | #6


        I didn't know you had already installed the framed walls. The usual advice its to cover the interior of the block with foam board or leave a gap for the spray foam, both as a vapor-retarder to stop inward vapor drive, and to have enough impermeable insulation to stop warm interior air from coming in contact with the block and condescending. I'm not sure what I would do if the walls are tight to the block.

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