Cutting the very ends off some floorboards. The room I've been working in is less drafty now so I decided to explore a particular area from the room above. Sure enough, multiple gaps in the brickwork but behind the joist so there's less than an inch between the wood and the brickwork. Spraying foam in from below obviously stopped the cold air seeping downwards, so now I just need to clear out the space as best I can and spray foam in from above to seal all the gaps and stop air going anywhere else.
With the benefit of hindsight I wish I'd done this work before spraying foam, it's just that I thought the foam I sprayed filled the gap rather than just the bottom half of the gap. I'll need to find something long and narrow to reach into the space from above to fish out the odd scraps of floorboard that fell into the gap, then blow it all out with an air compressor, then I can get busy spraying some more foam. While I was looking closely at everything I spotted a couple of other dodgy bits of cement work so I can get those fixed up. I've got some errands to run today and no light in the upper room so some of that work will need to wait for another day.
