Use hedge shears
Here’s a faster, cleaner way to cut fiberglass insulation. Use a hedge shears to slice through the insulation. Unlike a utility knife, the shears won’t spread loose tufts of insulation all over, and the best part is, you stay itch free.
Easier fiberglass cutting
Flatten with scrap plywood
For easier cutting, temporarily flatten unruly fiberglass insulation with a piece of scrap plywood. Just cut a 1-1/2-in.-wide slot in the center of a 16 x 30-in. board, use the slot as a straightedge and get a clean cut every time.
Fill all voids
Push batts to the back of the stud space
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.
Fit batts tightly around electrical cables and boxes
Split batts around electrical cables
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.
Split and cut batts around electrical boxes
Split and cut batts to fit behind and around electrical boxes. Slide half the batt behind the box. Then trim the insulation to fit snugly around the box. 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.
Video: Working With Mineral Wool Insulation
Fit first, then cut to length
Cut to length against the bottom plate
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.
Trim batts in place
One technique is to “eyeball” it
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.
Or use the straightedge technique
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.
Fill gaps around windows and doors
Use a putty knife to stuff gaps with fiberglass
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.
Choose the correct R- value
Balance price, insulation type and R-value
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.