How to Insulate a Return Vent Joist

Measure the distance from the outer edges of the two joists that contain the return vent. If the joists sit on 16-inch centers, such that the center of one joist to the center of the next joist measures 16 inches, the distance between the outer edges will be very close to 17-1/2 inches.

Transfer the measurement noted in Step 1 to a 1 1/2-inch rigid foam insulation board and use a straightedge to cut a strip of the foam board to size. The boards come in 2-by-8 and 4-by-8 dimensions and can be cut with a sharp utility knife.

Apply a 1/4-inch bead of foam-safe construction adhesive down the center of both joists and fit the cut foam panel over the joists. Rigid foam panels are lightweight, and the adhesive will probably hold the panel in place by itself, but it’s a good idea to use a few nails to reinforce the panel.

Drive a 2 1/2-inch plastic head nail 1/2-inch away from the edge of the rigid foam board and into the joist. Tap the nail head lightly until the plastic ring is flush with the surface of the foam. Do not drive the nail any deeper. Insert additional nails about every 12 inches apart along both edges.

Cut and attach additional rigid foam boards to cover the rest of the joist vent in the same way and butt the panels snugly together.

Apply rigid foam tape over the seams in the panels for added insulation.