Properly insulating and air-sealing rim joists takes patience, so most builders simply stuff in some fiberglass and walk away. If you have an unfinished basement, you can properly insulate the rim joists in two or three hours. (This will also block tiny passages where spiders and other insects enter your basement!)
Call your local building inspections department before you begin this project. The inspector may require you to cover the new insulation with drywall (as a fire block) or leave some areas uncovered to allow for termite inspections. You can insulate second-floor rim joists following the same steps shown here if you happen to tear out a ceiling during remodeling.
Rigid foam is the best insulation for rim joists. We chose 2-in.-thick (R-10) “extruded polystyrene.” Don’t use “expanded polystyrene,” which is a less effective air and moisture barrier. Cut the foam into 8-ft.-long strips 1/8 in. less than the height of the rim joist. A table saw is the fastest way to “rip” these strips, but you can also use a circular saw.
Then cut the strips to length to fit between the joists, again cutting them 1/8 in. short (Photo 1). A heavy-duty box cutter is the best knife for making short cuts and trimming foam; the long blade slices cleanly through the foam (a utility knife blade is too short). Use long sections of foam to cover the rim joists that are parallel to the floor joists (Photo 2). Don’t worry about cutting the foam for a tight fit around pipes, cables or other obstructions; you can seal large gaps with expanding foam sealant later.
It’s important to create an airtight seal around each section of foam using caulk or expanding foam (Photo 2). Otherwise, moist inside air could condense on the cold rim joist. The resulting dampness can lead to mold and rot. If you have a solid concrete foundation, also run a bead of caulk where the sill plate meets the concrete. If you have a concrete block foundation, also seal the openings on top with expanding foam. Stuff a wad of fiberglass insulation into each opening to support the foam as it hardens (see Figure A in the Horizontal Callout).