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Wet sheathing with mold on a 2×6 build R-19 Fiberglass

erikdavitt | Posted in GBA Pro Help on

Curious if anybody has any ideas on how to solve a mold issue. Recently had some electrical work done and the electrician noted mold on the inside of the wall.  The wall had a vapor barrier, R-19 batt insulation. On further inspection of the wall with removal of the insulation the sheathing is completely damp. Im assuming this is from condensation. The humidity of the inside air of the house is around 34%. Im not really sure the best way to tackle this problem. I took the insulation out so the wall sheathing could dry and am having a cleaning company come to see what they can do about the mold. However Im concerned about the problem re-occuring. Is there anything I can do differently to prevent the exterior sheathing from getting so damp on the inside of the wall?

I live in northern, NH so does get pretty cold in the winter.

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  1. Expert Member
    BILL WICHERS | | #1

    Check that you don’t have a water leak first. Usually water leaks will be localized (streaks, etc), condensation will be pretty even over the entire surface. If it’s really condensation, you probably have air leaks so that humid air is getting around that vapor barrier and into the wall. There are other possibilities too. Chances are if it’s in one stud bay, it’s probably in others too.


  2. erikdavitt | | #2

    Thanks bill, pretty sure it’s condensation. Entire sheathing is wet. And yes it’s in multiple stud bays.

  3. Expert Member


    Can you describe the wall from inside to out?

    1. erikdavitt | | #4

      Yes so it is 2x6 frame. From inside out drywall, polyurethane vapor guard (in my opinion crappy install), r19 fiberglass, OSB, sheathing, Tyvek and then cedar shakes for siding. I am guessing most likely the install of the vapor barrier may be the main issue. I guess I’m also curious if there are better wall builds to stop this condensation issue. I feel like the vapor barrier/polyurethane is at so much risk for a poor install and puncturing if I fix the wall the same way I’m going to end up with the same issues. But maybe not. On exterior walls would it be better to do a spray and batt model? If I had the time or the expertise in my area I would love to put adequate exterior foam on the outside of the sheathing.

      1. Expert Member
        BILL WICHERS | | #6

        Do you mean you have a polyETHYLENE vapor barrier behind your drywall? Polyethylene is the clear(ish) plastic sheet that is commonly used. Some areas require it, but it has been known to cause problems. Smart vapor retarders are usually a better choice.

        If you actually do have polyURETHANE (which probably wouldn’t be a clear(ish) material, it would probably be opaque) sheet as a vapor barrier, then you have a product I’m not familiar with. I would expect such a material to perform about the same as polyethylene though.


        1. erikdavitt | | #7

          Bill, it is clear so must be polyethylene. Do you think once the mold is cleaned out using a smart vapor guard will prevent the condensation issue or would you do the wall differently?

  4. the74impala | | #5

    Is this a gable end or is there a soffit? If you have had ice damming this could happen to the wall. Don't ask how I know. Huge mess.

    1. erikdavitt | | #8


      It is the soffit end. No ice dams this year. Last year though we did have extensive ice dams with a lot of bulk water coming into the house. Some of the exterior walls had to come down. We had a cleaning company come in and scan the walls for moisture content where there wasn’t obvious water and they said it was dry. It’s possible the start of the mold could of been water they missed and now the condensation on the sheathing just exacerbated the issue.

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