How to Install Insulation in Basement Rafters

Wear a long-sleeved shirt, safety goggles, dust mask and gloves when working around fiberglass insulation. Fibers quickly become airborne and present a respiratory hazard.

Cut pieces of batting insulation that will fit between the floor joist bays along the rim joist, the framing lumber that runs along the perimeter of the basement at the top of basement walls. Use a utility knife to make the cuts. You may need to make several passes to get through the fiberglass material. Gently press the pieces into place around the perimeter. Use caution not to squeeze the insulation material too much as it needs loft to work properly.

Cut lengths of batting long enough to reach from one rim joist to the other following a joist bay. Fit the insulation into the bay. If you are using Kraft-paper insulation, place the paper side up, toward the main floor’s subfloor. The paper is a vapor barrier, and vapor barriers should be installed against the warmer space. Push the insulation up just enough to eliminate any air gaps between the paper and the subfloor without compressing the insulation.

Work around wires and pipes that run through the bays by cutting slits in the insulation for those obstacles to pass through.

Drive nails part way into the joists starting around 6 inches from one end. Hammer in more nails about 24 inches apart down each joist. Twist a piece of stiff wire around one nail, stretch it across the insulation to its corresponding nail and twist the other end of the wire around that nail. Make sure the wire doesn’t push up into the insulation. It should have just enough tension to hold the batting in place.

Repeat the process of fitting insulation and securing it with stiff wires until all the bays are completed.