It’s easy to insulate your walls with fiberglass insulation (at least when they’re open!), but the job still requires attention to detail to get the maximum benefit. Every gap and compressed batt leaves a path for heat or cold to escape.
Do measure and cut the fiberglass batt for an exact fit. Add about 1/4 in. to the measurement to ensure a snug fit. Use a 4- to 6-in.-wide board or strip of plywood as a straightedge to guide your utility knife. Line up the edge of the board at the proper width, compress the insulation and cut it with a sharp utility knife. A scrap of plywood under the batt will protect finished floors and keep the blade from dulling on concrete.
Don't stuff full-width batts into spaces that are too narrow. Crumpling batts to fit narrow spaces creates uninsulated air pockets. And packed insulation has a lower R-value.
Do protect your skin, eyes and lungs when you're working with fiberglass. If you're installing a lot of it, consider wearing a disposable coverall (inexpensive at paint stores and home centers).
Do seal around window and door jambs with expanding spray foam. The main purpose of the spray foam is to seal the space around the window to prevent air infiltration. Use foam that’s labeled for window and door insulating. This “minimal-expanding” type reduces the chance of warping the jamb. If there’s still space around the window after the foam cures, lightly stuff the remaining space with strips of fiberglass insulation.
Do notch fiberglass batts around electrical boxes. Put the batt in place, and use a scissors to snip around the box. Tuck the snipped-out plug of insulation behind the box. Don’t wrap fiberglass batts around electrical boxes or stuff full batts behind them. That creates gaps and air convection routes around the box.
Do split apart the batts to fit around wires and pipes to get the full value of the insulation.
Don't tuck full-thickness batts behind pipes and cables. Compressing the fiberglass decreases its insulating value and creates voids between the insulation and the drywall.
Do plug holes in the top and bottom plates with expanding spray foam. Even small holes can let a lot of air escape. Don't leave gaps around wires, pipes or ducts unplugged. These gaps create pathways for warm interior air to leak into the attic, wasting energy and causing attic condensation or even ice dams in cold climates.
Don't buy paper-faced insulation for standard wall insulating jobs. The paper facing makes cutting the batts difficult. And it's hard to create a tight vapor retarder with paper-faced batts.
Do buy unfaced friction-fit batts and seal the walls with a 4-mil poly vapor retarder. Seal the gap between the bottom plate of the wall and the floor with acoustical sealant or caulk. Press the poly into the sealant. Use special airtight electrical boxes (see photo, Tip 5) or seal the poly to the electrical box with acoustical sealant. Tape the seams in the poly with sheathing tape.