How to Cut Insulation Around Rafters in an Attic

Measure the distance to the rafter that you need to fill with insulation. Transfer this measurement to a piece of insulation. If your insulation has paper vapor barrier backing, measure with this side up and place a pen mark to note the measurement. If your insulation has no vapor barrier, use a utility knife to make a small cut at the appropriate measurement. Make two measurement marks on the insulation, one on the insulation's left edge and one on the right edge.

Measure the insulation's width and add 12 inches. Transfer this measurement to a 2-inch by 4-inch board. Use a handsaw to cut the board straight across at this measurement. It is not necessary to make a perfectly straight cut.

Set the insulation on a piece of plywood. Put the board on the insulation. Line up the board's edge with either the pen marks or the small cut marks.

Kneel down on the board's edge nearest you to compress the insulation. Extend the blade of a utility knife about 3 to 4 inches upward so it's long enough to effectively cut into the insulation.

Place the blade on the mark farthest from you and tight against the board's edge. Apply pressure to push the knife blade all the way through the insulation's thickness. Drag the blade toward you, keeping your knee out of the way as you cut along the board's edge.

Place the cut insulation in its appropriate location. Check that it fits tightly against the ceiling rafters.