We bought a cabin built in 1990s that has some kind of buffalo board like fiber board as sheathing and heavy rough cut lumber as siding. I discovered the fiber board when opening up the wall to put in a new window. The place has this mild, faint odor that is more like damp paper than mold or mildew, but possibly some mildew mixed in. When the windows are open it’s not even detectable but I have a super sensitive smeller and it annoys me after awhile when it’s cold out and we close up the windows. When you push on the board, it is soft, slightly mushy but not degrading. I’ve read that this stuff is supposed to be good for dealing with moisture, supposedly doesn’t support mold, and that it will dry out but in the lakeside environment it seems like it never really does, so I think that this is what the smell comes from. I’ve noticed the smell is mostly in the part of the structure where the buffalo board is used, where the older part of the cabin probably does not have it, is completely different, and much less of that smell migrates over to it. Being 50 feet from a lakeshore and on soil that must have a pretty high water table (in Wisconsin) there is so much vapor rising up into the space between the sheathing and the siding, when I reach in along the bottom edge outside it is actually wet except during the late winter into late March, when the air has been much drier than the rest of the year, even right after a lot of snow has been on the ground under it and melted away. I considered that maybe the plumbing was leaking inside the wall but the plumbing is still winterized, well pump turned off for the winter and not back on again but it’s getting wet again now. There is supposed to be a little gap between the sheathing and the siding so water can run down and out, but the vapor does seem to be likely to be all coming from below and getting trapped right in between. Inside the wall cavity there is fiberglass batting with a proper vapor barrier between it and the drywall. But I pulled a lot of it out of the several joists where the drywall is opened up and not too much sign of mold but some spots or specks where it MIGHT be minimal mold, might be just some insects got in right around the old window and nested and died. What would you do?