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Community and Q&A

Mold on inside of external OSB wall

Dennis44 | Posted in General Questions on

Hello there,

Upon pulling a carpet in our newly-acquired home, I’ve noticed some mold under the window (1st pic).
After removing a piece of drywall I found what seems a big patch of mold on the inside of the exterior OSB wall. Interestingly, there is no mold on the fiberglass R-19 insulation.
There was no any vapor barrier or retarder behind the drywall and the house is in the climate zone 5.

What would be a proper course of action to proceed in this case?
Open drywall on the remaining exterior walls and put a foam / rock wool insulation in case this was cause by the vapor diffusion?

Do we need professionals to clean this up? The mold seems to be going behind the beams where it would be difficult to cleanup without removing the siding and the exterior walls which I would like to avoid.

Thank you

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  1. user-2310254 | | #1


    If it were me, I would start to run a dehumidifier to reduce the indoor humidity levels. I would also want to identify the source of this moisture problem. From the pictures, I would be suspicious that the window openings were not properly flashed. I would also check the gutter (if present) to make sure they were dumping water against the sheathing.

    If the windows are the problem, you will need to remove them so the opening can be properly prepped. Otherwise, you will never resolve this issue.

  2. Dennis44 | | #2

    Thanks Steve.
    What bothers me is that the mold is closer to the floor than to the window. If the window would be the source, wouldn't it be the other way around?
    This window is on the second floor, so the gutters are not the problem in this case.

  3. user-2310254 | | #3

    Water always takes the course of least resistance. So I would be surprised if it entered the room and then flowed to a low spot to pool. Of course, the windows could be fine, but the staining underneath the one in the picture is suspicious.

  4. GBA Editor
    Martin Holladay | | #4

    Steve Knapp is right: the first step is to identify the source of the moisture. It's hard for us to do this type of diagnostic work over the internet.

    Note that these two spots of mold may have different causes. The wall sheathing mold looks like a classic case of water entry under a poorly flashed window; that hypothesis can be verified by investigation with a garden hose. (Spray the window and see what happens in the stud cavity).

    You haven't told us anything about the home's foundation. If the house is on piers or over a crawl space, the subflooring mold may be due to summertime moisture accumulation (which can occur in air conditioned homes).

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