Such a boring and pedestrian thing to have to do, but once you start air-sealing a house you need a system to get as much moisture out of it as possible. Bathrooms generate more moisture than anywhere else, hence, bathroom fans. Neither bathroom had them.
One was already sort-of done; it had been wired in and installed when we did the electricity. I'll talk about that one later. The other...well.
The downstairs bathroom is tiny. It has enough room for the sink, toilet, and shower, and then to turn around. That's it. The one exterior wall has a window, a corner/wall from the addition, and an eave that made for basically zero space to get a fan out. The joists ran the wrong way to run venting. Every idea we had was thwarted.
there were a bunch of these old masonry nails hanging out in the wall for some reason. don't they look like shoe nails?
I had hired a handyman to come help out, and then worked alongside him to get this done, and it was complicated.
Step 1: Cut a hole in the existing ceiling to make room for the fan + venting.
Step 2: Drop a new ceiling with 2x4 beams to make enough room for the fan, complete with a corner chase
Step 3: Install a stupid complicated venting system.
Step 4: New drywall on the ceiling
(Step 3.5 was to strip wallpaper and repaint the bathroom, about which more later, because since we were basically redoing the bathroom, why not make it look better at the same time?)
Step 5: As much as it physically pained me, we had to trim about half an inch off the bathroom door in order to get the right "draw" for the fan to actually be able to pull air out of the bathroom.
Steps I did not include: cutting a hole in the exterior and installing the vent outside, and painting the bathroom ceiling. You'll just have to trust me that we did that, too.
Total PITA, but on the plus side, when we turn on the bathroom fan during and after a shower (the switch is a timer) there's barely any fog on the mirror during the shower, and it vanishes shortly after the shower is over. Huzzah for that.