How to Seal and Insulate Basement Rim Joists

RIM JOISTS are the primary culprit behind cold basement floors and drafty main levels. Because this area sits right at the intersection of your foundation and the wooden frame of your house, it is riddled with small gaps where plumbing, wiring, and framing meet, acting as a chimney for outside air to infiltrate your living space. Done well, this project transforms a basement from a chilly storage area into a thermal envelope. You are looking for a complete airtight seal followed by a thick layer of insulation. If you finish this correctly, you will stop the stack effect, where warm air rises through your home and pulls freezing air into the basement rim cavity.

1. Start with a clean slate. Remove any old fiberglass batts currently stuffed into the rim joist cavities. Vacuum out all dust, cobwebs, and debris to ensure the foam adhesive will stick to the joists and the masonry foundation.
2. Fill every gap and crack. Use a can of expanding foam to seal the perimeter where the sill plate meets the concrete foundation, and around any pipes or wires entering from outside. Apply a bead to every joint, seam, and hole you can see.
3. Measure twice, cut once. Measure the height and width of each joist bay. Since foundations are rarely perfectly square, measure every single cavity individually rather than assuming they are all identical.
4. Size boards for tight fit. Cut your rigid foam insulation boards to match the dimensions of your joist bays. Aim for a friction fit, leaving about an eighth of an inch gap around the edges for sealant.
5. Lock panels in place. Apply a bead of construction adhesive to the back of the foam panel and press it into the rim joist cavity. Ensure it is flush with the inside face of the foundation wall.
6. Create an airtight barrier. Apply a bead of spray foam around the entire perimeter of the rigid foam board where it meets the joists and the foundation. This creates a permanent airtight air barrier.