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.
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.
