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Community and Q&A

Sealing transition between open CMU first floor and stick framed second floor

Eric Barker | Posted in General Questions on

I’m finally doing some much needed air sealing and insulating work on my 1950s ranch in NJ. The first floor of the house is all CMU construction, with thin furring strips for attachment of drywall on the interior, and ugly vinyl siding over thin EPS foam on the exterior.

As part of the insulation work, I have a contractor converting a previously unfinished/unconditioned part of the attic into a conditioned space by removing the existing floor insulation and adding close cell foam to the underside of the roof sheathing. Now that the previous floor insulation is removed it’s easy to see one of the major problems. The first floor CMU’s are unfilled and there’s nothing in place to seal that open cavity, or the gap between CMU and drywall, from the attic space. You can see down to 

Even though the attic is now within the conditioned space of the house, I still want to seal this area in order to reduce the stack effect as well as for another layer of mouse/insect barrier.  I momentarily considered filling the blocks from the top with concrete, but I’m sure there are many holes in the blocks that would lead to a gigantic mess. 

So how best to seal this from the top? Cover with rigid foam and seal in place? Mostly concerned about not having gaps where mice and insects can make their way through, but obviously air sealing is a priority as well.

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  1. Expert Member
    Akos | | #1

    The best would be to insulate the CMU cavity, this bumps up the R value of them a fair bit. If they are clear, it might be worth it as a retrofit.

    As for sealing, have the SPF folks flash the area with about 1" of foam. It will seal everything up nice and tight.

    1. Eric Barker | | #2

      Thanks for the suggestion. They've done that in other areas (behind kneewalls in finished space), but weren't planning on it here. I will mention it.

      To insulate the CMU cavity would you use a pour-able foam? Those seem hard to come by and, from what I recall reading, have a high global warming potential (not that I'm saint with the SPF, but seemed to be the best overall compromise.) Can't see what other type of product would work.

      Longer term I plan to add a significant amount of rigid foam to the exterior of the CMU walls, but that's going to have to wait until I can afford to do that and replace the windows at the same time.

      1. Expert Member
        Akos | | #3

        Insulating CMU is not cheap, the stuff I've come across is with perlite, but usually cheaper to insulate with batts on the inside.

        Your insulation dollars go much further with the exterior foam. If the plan is to update the exterior insulation, save your money for that.

        1. Eric Barker | | #4

          Makes sense. I think it would extremely challenging to do a good job insulating the interior of the blocks. I'll seal where the wall meets the attic and address the wall assembly down the line.

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