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 2×4 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