How to Install Fiberglass Insulation

In this video, This Old House host Kevin O’Connor shows how to fill gaps and make a space cozier and energy efficient.

Steps for Installing Fiberglass Insulation

1. Set an unfaced fiberglass batt into the stud bay, tight to the top plate. Press the batt into place, but don’t compress it too tightly.
2. Use a utility knife to trim the lower end of the batt along the bottom plate. Cut the batt a bit longer than necessary to ensure a tight friction fit between the studs.
3. When you encounter an electrical box, pull apart the thickness of the batt and stuff the rear-half thickness of the insulation behind the box.
4. Now cut the front-half thickness of the insulation along the top and bottom of the electrical box. Fold in the cut tab of insulation and press it lightly into the stud bay.
5. If there’s an air-conditioning line running vertically down a stud bay, peel the insulation batt down the middle, splitting it into two thinner batts.
6. Slip one thin batt behind the air-conditioning line, and press the second thin batt over the line. Repeat the previous five steps to insulate the remaining stud bays.
7. Insulate around all window frames with low-expansion aerosol foam insulation. Start applying the foam along the top of each window, then fill the spaces along the sides and bottom.
8. Cut short pieces of paper-faced fiberglass insulation, called blockers, and set them between the floor joists and tight against the outside wall. Be sure the paper surface faces out into the room.
9. Cover the unfaced fiberglass wall insulation with a 3-mil polyethylene vapor barrier. Staple the polyethylene to the wall framing with a hammer tacker and ¼-inch-long staples.

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