Cutting batts of fiberglass insulation takes forever because the batts are so thick and fuzzy.
What Can I Use to Cut Fiberglass Insulation?
You can use a sharp utility knife to cut fiberglass batt insulation. You’ll also want to have a scrap piece of plywood handy to help speed up the process and produce a clean, neat cut every time.
Tom Silva, This Old House general contractor, advises: “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.”
How to Cut Fiberglass Batt Insulation
- Lay the insulation on top of a scrap piece of plywood with its foil or kraft-paper vapor barrier facing up. Then cut a 2X4 at least 12 inches longer than the width of the insulation.
- Place the 2X4 across the batt on the cut line. Compress the insulation by kneeling on one end of the 2X4 and pressing down on the other end with your palm.
- Then use a sharp utility knife to slice through the batt in a single stroke.
For narrow stud bays where you need to trim a batt’s width rather than its length, lay the batt on the floor in front of the cavity and line up one edge with the inside face of the stud bay. Then lay a straight 2×4 in line with the inside face of the neighboring stud, press down on it with one knee, and run a utility knife alongside the 2×4, trimming the batt with a series of shallow cuts rather than trying to slice through in one pass.
When fitting the cut batt into the stud cavity, tuck it snug against the top of the bay and let the bottom run slightly long. Then trim it against the bottom plate of the wall framing for an accurate fit. The batt should not be tightly compressed against the studs—that will reduce its R-value. As one This Old House insulation installer explains, “Don’t push it in too much, or you’re gonna compress the R-value. We like it nice and fluffy.”
