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Community and Q&A

Vapor Retarder on Exterior Wall, Part Two

JimmyCoro | Posted in Energy Efficiency and Durability on

Hello, in my quest to work out kinks in this basement in terms of vapor retarders and barriers, I have discovered another scenario that isn’t clear to me with regard to where one places vapor retarders on an exterior wall. The home in question is in zone 6 (WI).

This situation involves a bathroom, and this bathroom has one wall that is an exterior wall.  This exterior wall is half below grade and half above. The half below grade has 1-1/2″ XPS glued to the foundation wall. There is a 3/4″ gap between the wall XPS and 2×4 stud wall, and this stud wall is filled with fiberglass and extends to the ceiling in part of the room.

Now…where would we want to provide VR? I have come to understand (or think I do) that it’s generally not recommended to use VR on inside walls below grade. I think it will be not that easy to somehow “separate” or seal off the below- and above-grade sections of this wall.

Is there a typical way to handle something like this? Does it make any sense whatsoever to install VR on upper part of the wall only, and leave the bottom open, even though the stud cavities run from floor to ceiling?

Thank you.

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  1. Jon_R | | #1

    I'd air and and vapor separate the two sections. Because the ground is quite damp and the XPS is about .7 perms, probably allowing too much ground moisture to enter the cavity, rise and then condense/sorb on the cold sheathing.

    If you absolutely can't do this, then taped Intello on the whole wall is probably least risky.

  2. JimmyCoro | | #2

    Thank you for the suggestion, I will attempt it the best I can. Spray foam might be the only way to really seal it off after I wood block it as much as possible.

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