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Wet basement OSB. Remedy?

jkrahn24 | Posted in General Questions on

I started finishing my basement and noticed slight mild behind the vapor plastic barrier. After removing the plastic, the insulation between the studs, i noticed the OSB was sopping wet. Basement is half exposed and this occurs above the ground level.
I don’t understand the cause but would like to determine a solution and move forward with the remodel. Advice?

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  1. GBA Editor
    Martin Holladay | | #1

    It's always a good idea to tell us your climate zone, or where your house is located.

    In most cases, installing interior polyethylene is a bad idea. Here is a link to an article with more information: Do I Need a Vapor Retarder?

    The reason that your OSB sheathing is wet is simple: the sheathing is cold, and the interior air is warm and humid. The cold OSB is a condensing surface.

    The best solution is to install a sufficiently thick layer of rigid foam insulation on the exterior side of the wall sheathing. This keeps the OSB warm and dry all winter long. But in your case, it's probably too late for that.

    In your case, you should probably open everything up (remove the polyethylene and insulation) and wait a few months. In warmer weather, everything will dry out. Then you can insulate your stud bays and protect everything with an airtight layer of drywall. If you do a good job, your wall sheathing will stay "dry enough" -- not perfectly dry, but dry enough to avoid problems.

    Your problem is common. Here are links to an article and some Q&A threads about cases almost exactly like yours:

    Why Is This Sheathing Moldy?

    Getting heavy condensation on the inside of the exterior boards

    Basement insulation in Minnesota

  2. jkrahn24 | | #2

    Thank you Martin. Those links demonstrate my issues perfectly. I live in Green Bay Wisconsin, zone 6 i believe. I should have mentioned i purchased the home with the plastic already on. I don't know if the exterior is properly insulated. From your advice and the links i am going to remove insulation and plastic, dry it out, then install new fiberglass and install drywall directly over. Thanks.

  3. charlie_sullivan | | #3

    I'm guessing this is a semi-basement--foundation walls ~4 ft. high and then stud walls for the remaining 4'? And the part you are talking about is the top 4 feet? What about the bottom 4 feet? What is it now and what are your plans? Or maybe read this:

  4. jkrahn24 | | #4

    Thanks Charlie for the input. The lower 4ft concrete wall i have purchased 3/4" rigid insulation, then will have 1/2" gap, then 2x4 studs, r-11 non krafted, then drywall.

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