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Moisture accumulating before vapor barrier installed.

Knuter_63 | Posted in General Questions on

I’m in zone 7A and built an attached garage this fall: in slab heat, 2X6 walls, Rex wrap and OSB sheeting. The in floor heat got hooked up and turned on the beginning of January at the same time I began insulating the walls with unfaced fiberglass batts and faced batts in the ceiling.

Before I got poly vapor barrier up on the inside, there was a large amount of moisture (it was frost when it was cold outside) accumulating on the inside of the sheeting behind the batts.

I found this statement about the RexWrap: Micro perforated to allow moisture vapor to escape the wall cavity while holding out bulk water intrusion.

I have put up the poly vapor barrier on the inside now but my question is: have I trapped the moisture in the wall cavity or will the moisture make its way out through the OSB and Rex Wrap?

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  1. GBA Editor
    Martin Holladay | | #1

    Rex wrap appears to be a type of housewrap. Where did you install the housewrap -- between the studs and the wall sheathing, or on the exterior side of the wall sheathing? (It should be installed on the exterior side of the wall sheathing. Eventually, it needs to be covered with siding.)

    You have performed a science experiment showing why most building scientists now advise cold-climate builders to install a layer of rigid foam sheathing on the exterior side of the sheathing. The rigid foam keeps the sheathing warm during the winter, preventing the kind of condensation you have discovered.

    If I were you, I would wait for warm, dry weather to finish your project. Let the moisture evaporate before insulating. Once the sheathing is dry, you can resume work. If you install an air barrier (for example, gypsum drywall) on the interior side of your walls, and if you pay attention to making this drywall airtight (especially at electrical boxes), then you won't get as much moisture on your sheathing in the future.

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