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Community and Q&A

Insulation Against Concrete and AAC Wall

Michael Grundvig | Posted in General Questions on

Hi all; with the help of the answers to my previous question (thanks!!) We’ve attached 3″ of XPS and EPS foam against the interior of my foundation walls. We used a combination of glue and screws/washers to hold it against the walls. All told it seems to have worked nicely. Thanks for the suggestions on that one! Now to my next round of questions:

1) In the attached photo (sorry for the screwy colors, it was getting pretty dark when I took it) you can see that I taped a couple of the seams. It seemed like a good idea but I wanted to know if I should bother doing it for all the rest of the foam. Is the tape adding anything or just a waste of materials?

2) I read a fair bit about the need to fire block foam when installed like this. I used 2x4s at 10′ horizontal and 8′ vertical spacing. When I frame out the wall against the foam, will I need to put additional fire block (beyond the horizontal strips in the framing) in the wall cavity as well or is the existing fire blocking with the foam all that is required?

3) I’m thinking about using spray foam from the top of the existing foam up to the bottom of the floor deck. It would be quite hard to get rigid foam up into the truss and the ledger board. Spray foam would fill in very well and seal it all up nicely. Any issues with this plan?

Thanks very much in advance for any tips and help. This site, and community, have been very helpful!

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  1. GBA Editor
    Martin Holladay | | #1

    Yes, it's important to tape the seams.

    I'm not sure if I understand your question about fireblocking -- but if you are asking whether an additional 2x4 wall, installed on the interior side of everything we are looking at, needs fireblocking at the top of the new wall, the answer is yes.

    I'm not sure why you didn't insulate the exposed concrete above the rigid foam in the photo -- but if you plan to insulate the exposed concrete with spray foam, you'll be OK.

  2. Michael Grundvig | | #2

    Thanks Martin, I appreciate the help. The thinking for the spray foam vs. just XPS was that it seemed an easier way to seal the ledger board to the wall. It also seemed like it would work better on the mortar seam between the AAC and concrete wall (not visible in the picture). Thanks again!

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