How to Insulate a Wall
The drafty season is upon us. Learn how to beef up your wall's weather protection in four easy steps
Q: I'd like to add insulation to a wall during a renovation. What should I keep in mind?
—Peter Wefers, North Andover, Mass.
Tom Silva replies: Filling walls with a fluffy layer of fiberglass insulation is one of the easiest and least expensive ways to make your home more energy efficient. All it takes is a utility knife, a straightedge, and a little patience to make sure the insulation completely fills the stud cavity, side to side and top to bottom, without being overly compressed. Work with batts made to match the depth of the studs and the width of the stud bays; for typical 2x4 construction, that's 3½ inches deep and 15¼ inches wide with an R-value of 15. Fiberglass is easy to cut, so if your stud spacing is irregular, buy batts that will fill the widest bays, and trim the rest to fit.
When you're ready to begin, protect your skin, eyes, and lungs by wearing a long-sleeve shirt, pants, gloves, safety glasses, and dust mask. Then follow these basic installation tips to help the work go faster and make sure your insulation does its job.
Cut to Width
For narrow stud bays, you have to trim down the batt's width. To do that, lay a batt on the floor in front of the cavity, as shown, and line up one edge with the inside face of the stud bay. Then lay a straight 2x4 in line with the inside face of the neighboring stud and press down on it with one knee. Run a utility knife alongside the 2x4, trimming the batt with a series of shallow cuts.