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Community and Q&A

Basement insulation in Minnesota

jaspar5150 | Posted in GBA Pro Help on

We recently purchased a home in MN where the basement is an unfinished walkout. Most of the walls are above grade with fiberglass batts and poly. Recently with the current cold snap and we are in the process of planning to finish basement we noticed there is condensation on the exterior sheathing. What would be the preferred method of insulation to fix this issue? Code requirements in our area is to have the vapor/air barrier on the warm side. Most posts I see is we should avoid this. How do we avoid this when the code requires? Spray foam?? Any help would be greatly appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    A wood-framed wall insulated with fiberglass batts, and covered with polyethylene on the interior side, is a bad wall. You can make this wall work, but it has a lot of disadvantages.

    If you can dry out your wall sheathing, and if you make sure that your fiberglass insulation installation and polyethylene installation jobs are impeccable, you can probably install drywall and cross your fingers. In most cases, a good interior air barrier (the airtight drywall) will prevent the condensation you see now. But it's still a lousy wall.

    Spray foam works better than the wall you describe. So does flash-and-batt. (For more information on flash-and-batt, see Flash-and-Batt Insulation.)

    Another way to improve this wall would be to remove the polyethylene and install MemBrain instead. For more information on MemBrain, see Smart Vapor Retarders.

    Still another way to improve this wall would be to remove the fiberglass batts and substitute dense-packed cellulose insulation. If you do this, you would want to use InsulWeb fabric during the cellulose installation, and then install MemBrain on the interior when the cellulose installation is complete.

    Here is a link to an article that discusses a case similar to yours: Why Is This Sheathing Moldy?

  2. jaspar5150 | | #2

    Thank you

  3. Expert Member
    Dana Dorsett | | #3

    Is the foundation insulated?

    MN code has funny particulars about how to insulate foundations that are in some ways at odds with the IRC. When you get to that point, check back in- you don't want to have a code inspector force you to tear it all out.

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