One culprit is pictured here.
Over the last 130 years, parts of the east wall have migrated about half an inch further east, resulting in a crack along the side of the stairway and damaging the plaster adjacent to the joists. The result is that air from between the layers of brick, redolent with 130 years of wet spiderwebs and other organic debris, can escape into the interior of the house. At the extreme upper right, you see part of the ductwork that had been nailed between two joists; the problem with this is that the last time the upstairs apartment was renovated, they clearly forgot that there was a register there, and left it covered with linoleum without cutting and replacing the register.
No, the top of the stairs is not a good place for that.
And this last week, the inevitable finally happened: an innocent bystander put their foot through it. Once it was open, the smell from the aforementioned joist flaws wafted up through the register hole... I'd been working on this all week, actually, trying to locate the mold smell and get it under control. The large area of exposed brick you see in the picture up there was after I took a hammer to the loose plaster.
And here you see it after I'd applied the base coat. Isn't that much nicer? Anyway, this picture also shows the wire coming in for the security light. My goal was to put a switch on the inside so I could turn that off without a ladder - so that little arched area was still open. And after all the rain, it didn't smell so hot, either.
The answer was clearly to replace the wooden insert (which I cleverly saved - although it took a 15-minute search to find it, since I'd taken it out about a year and a half ago).
Here's the result - I wasn't sure how best to anchor the wooden insert, so I toenailed it with finishing nails, then drove in a couple of shims at the top to really hold it in place. It seems to have made it solid enough.
I also stuck in some fiberglass insulation. After I get back to Menard's I'll squirt in some foam, too (the fiberglass didn't really get down into the corners at all).
So. My first real post in months. Didn't that feel good?