I'm working on finishing my 1924 home's basement in Minnesota and am currently working on insulation. My rim-joists look like this:
2×8 Joists with a board between each joist. Behind that board is a bit of concrete and a bunch of sand, rocks, construction debris from when the house was built. A few of the sections have penetrations already for faucets, venting, electrical, A/C, etc.
- The open space above the concrete is ~4 inches high.
- The sub-floor seen is the primary living area.
- At the back of the rim-joist is another 2×8 and stucco exterior.
No further insulation.
Should I remove the boards and sand to do this, or leave them as-is and insulate over the wood?
What is my best bet for insulating this?
Those Rim-Joist cavities look like this once cleaned out:
Best Answer
What you have there is what's known as a mud sill. Mortar was set around the joist for whatever reason. My previous home was exactly like this. I simply cut 16 in fiberglass batts into roughly two foot lengths and folded them into the space to create something like an R-30 thickness. It's not really important how you fit the fiberglass in the cavity as long as it's filling all the void without any air pockets and it's not packed tight.
Assuming that you don't also need to solve airflow / leakage issues this will be just fine. It's substantially better than what's probably in your walls. I don't have any concerns with you removing the cross blocks. With the joists being fastened to the subfloor and locked in with the mortar there's really nowhere they can go.
Alternatively you could have someone come in and spray foam into each cavity. This would give you an equivalent R-value and would also seal it up tightly against air flow. It would be fairly expensive, though.