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Community and Q&A

Mold in fiberglass insulation

Meep | Posted in General Questions on

I have a lookout basement, built in 2009 in climate zone 6, currently unfinished. The above grade framed section of the lookout wall is framed with 2x6s and insulated with fiberglass batts and a poly vapor barrier on the interior side of the wall covering the fiberglass. I have noticed small dark areas in the fiberglass throughout the wall where it meets the poly sheet. The dark spots are very sporadic and appear at first glance to be mold to me, although i have not confirmed this.

Assuming this is a mold issue, what would be the preferred method of replacing the insulation system?


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  1. user-2310254 | | #1

    This article by GBA's Martin Holladay covers basement insulation ( You will note he admonishes against two common building practices: installing air permeable insulation against the basement wall and covering it with polyethylene.

  2. GBA Editor
    Martin Holladay | | #2

    We get this question every few months. Here is a link to a Q&A Spotlight article on the topic:
    Why Is This Sheathing Moldy?

    Here is a link to another Q&A thread on the topic: Basement insulation in Minnesota.

    The mechanism is pretty simple. Indoor air is warm and humid. The wall sheathing (and some of the insulation fibers) are cold. The walls don't have an effective air barrier, so moisture in the indoor air is able to reach cold surfaces and condense, leading to dampness and mold.

    Installing an effective interior air barrier (taped drywall) can in some cases reduce the problem. (Or it can simply hide it.)

    In general, as we've been preaching at GBA for years, the right place for insulation is on the exterior side of the wall sheathing, not the interior side of the wall sheathing. If the sheathing is warm, you won't get condensation.

  3. GBA Editor
    Martin Holladay | | #3

    If you are willing to remove the existing moldy fiberglass, and wait for warm weather -- allowing the sheathing to warm up and dry out -- you can replace the fiberglass with closed-cell spray foam. That will solve the problem. The closed-cell spray foam is an air barrier, a vapor barrier, and an insulation with significant R-value.

  4. Jon_R | | #4

    Here is CertainTeed's solution (replace poly with MemBrain).

  5. GBA Editor
    Martin Holladay | | #5

    Jon R,
    It should come as no surprise that CertainTeed (a major manufacturer of fiberglass insulation) is eager to sell fiberglass insulation.

    But their video is flat-out wrong. This method is not a "best practice recommendation." A concrete-block basement wall can get wet from either direction -- from contact with the damp soil, or from the condensation of humid indoor air. Installing MemBrain won't prevent mold in this type of wall. The images in the video make me shudder.


  6. Meep | | #6

    Thanks, I havent inspected the sheathing yet, but all of the suspected mold that i can see at this point is in between the poly sheet which is the innermost layer of the wall and the fiberglass, not between the sheathing and the fiberglass. Wouldnt the mechanism for condensation here be the warm humid outside air condensing on the relatively cool plastic sheet?

    I am thinking of pulling all of the fiberglass and spraying foam then put up drywall. Sounds like this would be an acceptable solution.

  7. GBA Editor
    Martin Holladay | | #7

    First of all, it's always possible that what you are seeing isn't mold. It might just be dirt, or something harmless.

    If your basement is cool (or air conditioned during the summer), you could be looking at mold that formed when humidity in the wall assembly condensed on the chilled polyethylene.

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