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Exterior XPS when wall has kraft paper on inside

user-669103 | Posted in Green Building Techniques on

This is for a retrofit.

I plan to fit 4″ of XPS on the exterior of two walls. Both of these walls are 2×4 with fiberglass batts with craft facing on the inside then sheet rock.

Removing the sheet rock to remove the craft paper is not an option on at least one wall.

To improve the wall’s breath-ability I was considering using 2″ of unfaced EPS and 2″ of XPS on top.
The rational is that EPS is two to three times more permeable per unit R-value.

I suspect that I may be over thinking this, so maybe I should just stick with two x 2″ XPS.
However, forethought is the antidote to hindsight!

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  1. GBA Editor
    Martin Holladay | | #1

    You don't mention your climate. It's almost always important to note one's climate when asking such questions.

    However, I don't think it matters much in this instance. Kraft paper allows drying to the interior. Like MemBrain, it is a "smart" vapor barrier -- that is, as the humidity level increases, it becomes more permeable. Moreover, four inches of foam provides a fairly high R-value, preventing condensation on the interior side of the wall sheathing in all but the very coldest climates. So I think you're fine.

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