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Community and Q&A

Insulating and Sealing Concrete Block Cavities at Sill Plate

acretejr | Posted in General Questions on

Long time reader, first time poster. I am using 2 inch XPS and spray foam to insulate my rim joists with R15 unfaced mineral wool on the warm side. My sill plate is sitting on concrete block with about 2-4 inches of the block cavity exposed. I’ve read solutions on filling those cavities with loose batt insulation and sealing off with spray foam. I am wondering is it possible to just use blueskin or similar peel and stick to cover the cavity holes. Is it necessary to stuff loose insulation? Alternatively, could I lay a sill plate gasket to cover the cavities and secure using a 1×2. I’m not opposed to spray foaming each cavity but rolling out the peel and stick or sill gasket would be much quicker. I am in upstate NY/zone 6. Any thoughts or suggestions would be great. Thanks!

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  1. GBA Editor
    Kiley Jacques | | #1

    Welcome to GBA! I’m giving your question a bump. Given your climate zone, I suspect you will be advised by those who know best to insulate, not just cap those cavities. I quote Martin Holladay, who weighed in on a somewhat similar situation by saying: “Concrete is a conductor, so it acts as a thermal bridge. You can't just insulate the rim joist area if you want the insulation to be effective; you need to bring the insulation down to the footings.”

  2. Expert Member
    Michael Maines | | #2

    Acrete, Blueskin or other membranes should work just fine; stuffing cavities with fluffy batts is just a fast and cheap approach that insulators use regularly. Spray foam is hot when installed (or shortly after installation) so it's best to use a material that won't melt.

  3. Expert Member
    BILL WICHERS | | #3

    I've usually boxed over the top of the block using the same rigid foam I'm putting against the rim joist itself. This is easy to do, either glue together "L" shape rigid foam pieces pre-cut to fit, or glue in the lower piece over the block after you've foamed the first piece against the rim joist. Either way works.

    In my expierience, there is usually random detritis on top of the block which is likely to make any self adhesive membrane not stick very well. If you try a membrane, you'll probably want to vacuum off the top of the block first to get a clean surface for the membrane to adhere to.


  4. acretejr | | #4

    Thanks! The xps sits back in the cavity and the mineral wool brings it forward to the face of the sill. I have about r25 in total. I think using the old batt insulation in the block cavities and placing a peel and stick membrane over top to seal off. That is provided its clear of debris and it actually adhere (otherwise back to the drawing board). Thanks for the suggestions.

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