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Community and Q&A

Insulate a basement wall and joists with an angled top to the wall

Andrew207 | Posted in Green Building Techniques on

I’m in central Maine, so cold climate zone. I’m about to go ahead and insulate my basement walls using Thermax 2 inch. Once the sheathing warms up this summer I was hoping to do a cut and cobble with 2 inch Thermax on the rim joists in the basement. The problem is the top of my basement wall is angled so it would be hard to get the rigid board against it. From my understanding it is important to insulate between the rim joist insulate and the basement wall insulation. Any ideas if I were instead to leave this triangle open and just lay the bottom piece of foam on top of the sill plate and on top of the foam insulating my basement wall? Is there any way to seal this area using cut and cobble or am I going to have to spend a lot on closed cell spray foam? See the pictures for a better idea of what I’m talking about. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    A triangular void at the top of your wall won't create much of a thermal penalty, as long as all rigid foam seams are sealed against air leakage.

    Because air sealing work is hard to do perfectly, many energy nerds would use canned spray foam to fill the triangular void. But if you are conscientious in your air sealing efforts, I don't think that the void will matter much.

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