There's something to be said for building a house from scratch.
The upstairs bath had been remodeled sometime in the 80's. I'm not sure of the extent of the renovations, but obviously a medicine cabinet had been abandoned and a mirror plastered over the open spot. No problem, what do you do with an exposed area, you fill it with insulation.
Once the room was gutted during this renovation, we see what the real problem was. The only insulation in the exterior wall was the little bit where the medicine cabinet had been abandoned.