Scrap the Roxul. Fibrous insulation in the rim joists allows the buildup of moisture and mould. Instead, use foam or XPS caulked and sealed.

Gluing XPS to poured walls generally doesn't work as the walls are too irregular. In any case, it may not meet some Codes, which require mechanical attachment. Instead, you can attach each foam board with concrete screws and fender washers or run strapping across multiple foam boards. Fibrous insulation to the interior can add up to R13.65 for a 2x4 wall.

Use shiplap XPS or tape the seams for optimal airtightness. In some markets, you can obtain XPS in 4' wide sizes.

Here is a link that might be useful: Building Science Corp. on Rim Joists

worthy,

I know that in my city, we are currently required to follow the IRC (or at least that's what the building inspectors follow). It's nice to know I don't need to firebreak the foam/xps in the rim joists.

Do you recommend having the rim joist spray foamed? This is right below my kitchen and the kitchen is the furthest heat run from the furnace. Would this help with heating/cooling this area?

Can you provide the link to the mechanical attachment document that would help? I am able to get 4'x8'x2" XPS foam for $26 a sheet.

You should probably "re-do" a basement and have a link to exactly how you would do everything. It would probably save on answers..

Thanks for your help. It's really useful for a new DIY project.

Early blower doors tests in the 1980s revealed that leaky rim joist areas accounted for unexpectedly large amounts of air infiltration. You can seal rim joist areas with caulking and polyurethane foams (Great Stuff, for instance) and then either insulate with boards or several inches of closed or open celll foams.

Spray or seal with foam board?" is essentially an economic question.

What makes sense for a do-it-yourselfer homeowner may be impractical for a homebuilder.

As a general rule, foaming is faster and tighter with fewer air leaks and thermal bridges than achieved by applying boards. But it's more expensive.

As a sort of middle ground, diy foam kits sold in the big-box stores can be useful for doing the rims. (I've not personally tried them.)

The mechanical attachment of the boards is straightforward. You can use concrete screws (Tapcon is a well-known brand) and fender washers on individual boards. Or you can run strapping secured by concrete screws across several boards.

Here is a link that might be useful: Attaching and Taping XPS to Basement Walls