Introduction
Fiberglass batts are the cheapest, easiest way to insulate new walls. However, they're often installed poorly and even small gaps can reduce efficiency as much as 25 percent. In this article, we'll show you how to avoid those mistakes, how to cut and fit fiberglass batts and how to work around electrical outlets and cables.The key to a quality insulating job is tight-fitting batts that completely fill the stud cavity, with no voids or gaps.
You can do top quality work with only a few basic tools. You’ll need a utility knife with a good supply of sharp blades, a tape measure and a straightedge, and a three- or four-inch putty knife for stuffing insulation around doors and windows.
Fiberglass can irritate your throat and skin, so wear protective gear. Buy a two-strap mask rated for fiberglass insulation (look for an N95 rating), and wear a hat, gloves, a long-sleeve shirt and goggles. The latter will keep fibers out of your eyes.
We’re using unfaced batt insulation sized to friction-fit into standard stud spaces (either 16-in. or 24-in. on-center studs). They’re also available precut to lengths that fit standard 8-ft. and 9-ft. walls.
Buying precut batts eliminates some work, but you’ll still have to cut some batts to length. You could measure the space and cut the batt to fit, but you’ll find a quicker method that’s just as accurate shown below. Leave an extra 1/2-in. of length for a snug fit.
Accurate cutting is essential. A batt cut too small leaves gaps, and one cut too large bunches up and leaves voids. Slightly oversized batts are best.
We’re using unfaced batts because they’re easier to cut and install. In most climates, you’ll have to staple 4-mil plastic sheeting over the batts to form a vapor barrier. Check with your local building inspector for the recommended practice in your area.
The photos show two methods of cutting batt insulation to width. If you’re having trouble getting an accurate cut with the “eyeballing” technique, measure the width of the stud space and use the straightedge method instead. Add about 1/2-in. to the width to ensure a tight fit.
It’s better to compress the batts a little than to leave gaps. Don’t worry if the batts bulge out a bit. The drywall will compress them tightly.
