I wanted to record my floor rot experience in case it helps others. My floor is dry, and the solum is bone dry. I decided to add some solid wall insulation to the inside of an external wall (this is incidental to what occurred). I decided to continue the insulation through the floor so that there wouldn't be bit of cold floorboard to gather interstitial condensation; to do this, I lifted the board adjacent to the wall and cut it narrower. At this point, I found that the ends of my joists resting on the dwarf wall were like paper, and I had to sister several of them to galvanised steel angle iron to strengthen them. The root cause was that inner layer of old-style wet plaster had crumbled to sand behind the skirting board, dropped, and filled the air gap between the joist end and the exterior wall. As a result, water was able to wick into the joist, and, also, air wasn't able to circulate over the joist end to dry it. It was a lot of work to fix something caused by such a trivial thing. I imagine lots of old houses with solid brick walls and old plaster could suffer from the same problem, and you probably would be unaware until the floor sinks.
