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I am partially gutting a bathroom. It has been redone once. When I tore up where the sink was, underneath the Ditra on the layer of OSB that was laid down there was water damage. So I tore that up, the original subfloor was also rotten in that area. It is an exterior corner, the house is brick. In this area of the house it's a 2x4 fiberglass insulated wall with some type of MDF or similar facing, an air space, and then brick. In the joist bays it seems like the insulation runs right up to the brick/ no air space.. when I look in the space where the subfloor was underneath the wall, it looks like a portion of the MDF is also broken down.

My thought was to just try and put a piece of roofing felt or something moisture tolerant where the MDF is gone and also at the end of the joist bays. Another thought was to put pressure treated ply as the subfloor in that area? Is there a better way to seal / prevent moisture problems.

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  • It might be good to include a picture of the "some type of MDF or similar facing". It's likely someone will be able to ID it to tell you whether you should be concerned about this kind of rot from moisture coming through the brick elsewhere in the house, too.
    – FreeMan
    Jan 28 at 13:35

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I can't tell what the true wall composition is, other than your telling us it is brick. Using a moisture meter can give you a picture of where the moisture is coming from. Then a plan to correct that issue can be formed.

As for correcting the flooring, I would remove all the OSB and put down 3/4 plywood. This is the perfect example of why OSB should never be used in any area that may get wet. ( kitchens, baths, entry, mudrooms, laundry...I never use it for anything.) The expensive Ditra didn't help the OSB at all.

Get some moisture reading and edit your question for a better idea of what to do further.

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