Fill all voids
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 3- or 4-in. 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 N-95 rating) and wear a hat, gloves, a long-sleeve shirt and goggles to keep fibers out of your eyes.
Fit batts tightly around electrical cables and boxes
Running a full batt in front of electrical cables leaves an uninsulated space behind. Avoid this by splitting the batt as shown. Then when you come to an electrical box, trim the insulation to fit snugly around it. Run your knife blade against the outside of the box to guide the cut. But don't cut too deep or you risk nicking the wires. If you have plumbing pipes on an outside wall, insulate behind them, but leave the side facing the interior uncovered to allow heat from the house to keep the pipes warm.
Fit first, then cut to length
We're using unfaced batts that are 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 a quicker method that's just as accurate is shown below. Leave an extra 1/2 in. of length for a snug fit.
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.
Trim batts in place
Accurate cutting is essential(actually, slightly oversized batts are best). A batt cut too small leaves gaps and one cut too large bunches up and leaves voids.
The photos show two methods of cutting batts 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.
Fill gaps around windows and doors
The shim space around windows and doors is a prime spot for air leakage. Stop these leaks by reaching to the back of this space with the straw-type nozzle included with a can of expanding foam insulation and applying a bead around the perimeter. Let it cure at least an hour before stuffing the remaining space with a thin strip of fiberglass. Don't pack the fiberglass too tight or it will bow the jambs and cause trouble with the operation of the window.