I have a very old house with a non-insulated gap in my basement ceiling and an exterior brick wall that is at or below grade, making it prone to dampness. I’ve read that in century-old homes, it’s crucial to allow the bricks to breathe and dry out, so I’m looking for an insulation solution for the joist cavity that touches the exterior wall. I want to avoid trapping moisture or causing it to wick into the joists.

The rest of my basement ceiling is insulated with rockwool batting. Initially, I placed a plastic vapor barrier against the brick in the joist area and put rockwool behind it. However, I removed the plastic barrier to avoid trapping moisture against the brick, which could lead to deterioration. My concern is that if I leave the rockwool bare, it might absorb moisture from the walls and transfer it into the joist bay. Will it just evaporate, or will it cause problems?

What should I put in that joist bay? Is rockwool a good option, or does it have capillary action that could make the wood damp? Should I use something else instead? Any advice on how to approach this would be greatly appreciated!