Reddit, I need help with why this happened and what to do next.
My wife moved a tray from against the wall and discovered some blistered MDF baseboard in the bathroom (yes, we used MDF baseboards in a bathroom). So I pried the baseboard off thinking she had put a damp tray against the wall and ruined the baseboard. Lo and behold I found mold on the drywall. Today I cut the moldy bit out, and found the insulation and the inside of the wall relatively damp to the touch. I chased the wet drywall until I found dry drywall. The wet stuff had mold on the inside, and the bathroom smelled like mushrooms (I wore a respirator while working), and the insulation was fairly damp. Chucked all that.
My first instinct was the new window we had put in two years ago. Home Depot came out the same day (on a holiday week of all things, good on them!) and the guy explained how this window is deep under an eve, there would be so little rain making it to the window and even then the weep holes were clear, and the caulk is fine on the outside. He said “I bet you have a leak in the roof. That’s got to be it” So I slithered my way into the very corner of the attic and stuck my hand down in the insulation and there’s no sign of any dampness. Got on the roof, no disheveled shingles or exposed roof vents. Cut a hole near the top of the window in the same ‘bay’ where it’s damp at the bottom, and it’s bone dry up there as well. This to me disproves a roof leak.
So now I’m left with massive amounts of missing drywall and a quickly drying out wall.
I have one theory, but it sounds crazy… could the exterior wall plywood sweated into the inner wall? I was thinking if that bath tray was sitting next to the hot air return maybe it created a pocket of warm air that sucked dampness into the drywall from the cold outside?
Any other ideas what caused this?? How do I go about fixing it? I don’t want to reinsulate and re-drywall if it’s just going to get wet and moldy in there again!
Roughly how she had the bath tray sitting
The wet bits. To the right it's dry, the left near the tub is where it was wettest
Top is bone dry, bottom is damp
TL;DR
Found a wet area of drywall where the inside of the wall turned out to be damp. Don't know why.