I'm working on finishing my 1924 home's basement in Minnesota and am currently working on insulation. My rim-joists look like this:

2x8 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 2x8 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?

Rim Joists

Those Rim-Joist cavities look like this once cleaned out:

Rim Joist Cavity


3 Answers 3


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.


Actually you only want to use fiber insulation against cold surfaces (rim joist) when you can completely cut off the air supply from the interior. Interior air carries a lot of water. The air moves through the fiber insulation and condenses when it hits the rim joist and you have a potential issue.

Better way is to buy a sheet of foam board insulation, like the blue or pink stuff, a can of spray foam insulation, and a roll of fiber insulation. Cut the foam board so it fits loosely over the rim joist. Fill in around it with the foam in a can. Cover that whole thing with the fiber. Now the dew point is inside the foam board where the air can't reach. No condensation, really good insulation.

If you want to get all geeky, you can calculate the dew point location in the assembly based on exterior and interior temps, interior relative humidity, and insulation type and thickness, to make sure the dew point is in the foam. For example, don't use 1/4" foam board and R-30 fiber. Use 2" foam board (about R10) and 6" fiber (about R19) and you should be OK.

BTW, same principle applies to cold basement walls. A nice layer of fiber insulation against the wall commonly creates a mold problem.


Yea, just cut some small strips of R-30 and shove them on up in there. Try not to pack them in too tight though because the fluffiness is where the insulation comes from. Or I suppose you could have them spray insulated if you have the money for it.

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