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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.

Steps on Insulation Installation

1. Cut to Width

Man Trimming Insulation With Utility Knife Photo by Ryan Benyi
  • 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.

2. Trim to Length

Man Trimming Length Of Insulation Photo by Ryan Benyi
  • Tuck the insulation into the cavity, snug against the top of the stud bay and edges flush with the studs. The batt should not be tightly compressed against the studs; that will reduce its R-value.
  • For an accurate fit against the bottom of the cavity, let the insulation run long, then cut it against the bottom plate of the wall framing with your utility knife.

3. Deal with Obstacles

Man Halves Batt’s Thickness And Slides Wire In Front Of Other Half For Wall Insulation Photo by Ryan Benyi
  • For wires, halve the batt's thickness by pulling it apart. Then slide one half behind the wire and lay the other half in front.
  • For electrical outlet boxes, install the batt, then cut a notch in its edge, using the box as a guide.
  • For plumbing supply pipes, work the full thickness of the insulation behind the pipe to help prevent it from freezing.

4. Add the Vapor Barrier

Man Adds Vapor Retarder Over Insulation Photo by Ryan Benyi
  • In cold climates, a vapor retarder on the insulation's inside face prevents condensation that robs R-value and encourages mold. Batts faced with paper or foil have the retarder built in. Unfaced batts should be covered with 6-mil plastic or MemBrain (shown), a breathable sheet.
  • Apply a bead of sealant to the top plate and to any studs where the sheets overlap, then tack the sheet every 12 to 24 inches to the top plate, studs, and bottom plate.

More Articles: All About Insulation; Adding Attic Insulation