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Thread: Insulation where floor joists overhang foundation

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    DIY Senior Member pmayer's Avatar
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    Jul 2006

    Default Insulation where floor joists overhang foundation

    I am continuing my basement remodel and following the great advice that I have received here in the past regarding insulation. In fact I should mention that the previous basement room that I finished is by far the most comfortable room in the house thanks to you guys. In the area that I am currently finishing there are a couple places where the floor joists extend a couple feet past the foundation wall (one is for a bay window and the other is for a patio door bump-out thing. Currently there are just unfaced fiberglass bats in these cavities, installed with the bats laid flat apparently to allow some air flow and to make a nice comfy home for mice. Now that I am replacing all of the fiberglass that is up against the rim joist with Thermax (inspector won't allow unfaced XPS), but I won't be able to reach in far enough to install this on the protruding rim joist in these two areas. So, my question is, how should I insulate in these areas? Do I need to use spray foam? Or replace the unfaced bats with paper back ones? What about the air flow that appears to be currently in place; is this important?

    Also, on the exterior side of 2 of my foundation walls there is a layer of 6 mil plastic directly against the foundation, and 1" of XPS on the outside of that. Am I OK to install another 1" XPS against the inside of the foundation wall? The other wall that I am finishing had no XPS or plastic on the outside and I am adding 2" of XPS on the inside foundation wall as I did on my previous remodel.


    Last edited by pmayer; 01-01-2013 at 05:30 PM.

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