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Community and Q&A

Best way to cap CMUs/cement blocks?

CrisPA | Posted in General Questions on

Hello all,

Following the recommendations here, I’m working to improve my house’s efficiency by stopping the air leaks first. I’ll closed off some real doozies (and apparently closed off the mouse gateway in the process, a source of much rejoicing).

The lower level of the house (1950s,  SE PA, zone 4A almost 5) is uninsulated concrete block. The tops of visible blocks have been capped with mortar/grout but those that are above eye-level or blocked by a joist are open and the voids make excellent reservoirs for air at close to outside temperatures.

I’ve tried jamming various things into the holes to provide a platform for an inch or so of spray foam but haven’t found anything that works well; it needs to be a little springy to hold onto the sides of the holes without falling down. Handfuls of old fiberglass batts seem to be the least-bad solution I’ve found but I’m hoping someone has a better idea.


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  1. Expert Member
    BILL WICHERS | | #1

    Mineral wool will work lots better than fiberglass for this because mineral wool has “shape”. Mineral wool is much stiffer and denser than fiberglass so it will work better to stuff in the holes of the blocks.

    The other thing I’ve done in the past is to use pieces of thin (1/2” or the 1/4” fanfold stuff) XPS cut to be a tight fit in the blocks. It helps to cut the pieces so the sides are a bit small, but leave “fingers” at or near the corners that stick out farther and hold tight. Your spray foam can then cover the thin piece of foam board.


  2. GBA Editor
    Martin Holladay | | #2

    You wrote, "Handfuls of old fiberglass batts seem to be the least-bad solution."

    Don't sell yourself short. You stumbled on the classic solution to this age-old problem. Weatherization workers have been using handfuls of old fiberglass batts to solve this problem for decades.

    Handfuls of fiberglass insulation work -- and as Bill points out, handfuls of mineral wool insulation work a little better.

  3. CrisPA | | #3

    Thank you, gentlemen.

    Bill, perhaps you have a business opportunity in "cookie" cutters for XPS for this application.

    1. Expert Member
      BILL WICHERS | | #5

      Hehe, thanks for the thought, but it’s probably a limited market. I’ve actually had custom dies made before (to punch some special size/shape oval holes in metal brackets), and it’s not cheap for production-quality dies. If there was a GBA group buy then sure, I’d do it, but we’ll all probably just stick to the time-tested “stuff the holes with what we have available and then grumble about it not working very well” method :-)


  4. user-723121 | | #4

    If your floor joists sit on a sill plate you may consider rigid insulation sealed to the edge of the sill plate and the top of the block. I do this when I am insulating and air sealing rim joists. You now also have a thermal break on top of the block wall. Add rigid insulation to the interior of the foundation walls if possible. If left exposed, some Polyiso foams may meet fire code.

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