The key to a quality insulating job is tight-fitting batts that completely fill the stud cavity with no voids or gaps. Push batts all the way to the back of each stud space and then pull out the front edges until they’re flush with the face of the studs. 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.
Split batts to fit around electrical cables. Tear the batt in half, starting from the bottom. Slide one half behind the cable and lay the other half over the top. 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.
Cut batts to length by setting the top of the batt into the space and cutting against the bottom plate with a sharp utility knife. Leave an extra 1/2 in. of length for a tighter fit. Unfaced batts are 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.
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. There are two different methods. For the “eyeball” method, leave the batt folded in half and hold one edge against the edge of the stud. Slice down the length while holding the top of the batt. Cut against the stud face.
If you’re having trouble getting an accurate cut with the “eyeballing” technique, press a straightedge down on the batt at the desired width and use it as a guide for the utility knife. 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.
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. The insulation should fit snugly, but don’t pack it.
Manufacturers now produce batts with higher fiberglass densities, so you can buy 3-1/2-in.-thick batts with R-11, R-13 or R-15 thermal resistance values. The higher the number, the better the insulation. The high-density R-15 batts are best, but they cost more than twice as much as R- 11 batts. Balance the price with the insulation requirements of your local building codes. In most cases, low- or medium-density insulation is adequate.