How to Insulate the Rim Joist in the Basement With Rigid Foam Insulation

Measure the height and width of the rim joist between two of the floor joists. Since the rim joists serve as an attachment point for the joists that support the first floor, you'll be applying small pieces of rigid foam insulation board to cover each segment of rim joist in between the floor joists. The long walls parallel to the floor joists can take longer strips of insulation, since the length isn't broken up by the ends of the floor joists. Clean any dirt or cobwebs away from the joists to ensure the best contact between the foam insulation and the rim joist.

Mark the measurements onto a rigid foam insulation panel with a pencil. Lay a straight edge against the lines and run the blade of a utility knife along the lines to score the surface of the panel. Snap the foam panel piece off with your hands.

Apply construction adhesive to the back of the piece of rigid foam insulation. Place the adhesive against the rim joist and hold until it sticks in place. If it is difficult to fit the piece of insulation in place, cut the measurements 1/8 inch smaller on both the length and width.

Measure and cut your next piece of foam panel and apply it to the rim joist. Continue until you've covered all of the rim joists with foam.

Place your silicone caulk into a caulk gun and cut off the tip at a shallow angle. Apply silicone caulk around the edges of each of the foam panels in a continuous bead. This helps prevent air and moisture leakage from around the edges of the panels.