How to Cut Fiberglass Insulation Rolls and Batts

Cutting Fiberglass Insulation Requires a Sharp Utility Knife, a Straight Edge, and a Smooth Cutting Surface

By Mark J. Donovan

Fiberglass insulation rolls and batting are used to insulate walls, floors, and attic spaces. Fiberglass insulation is the most common type of house insulation. Installing fiberglass insulation is something most homeowners can tackle, however it is important to use the right tools. Cutting fiberglass insulation, for example, requires a utility knife with plenty of extra sharp blades. You also need a straight edge, such as a carpenters square or a 2 foot length of 2×4, to help compress the fiberglass insulation when cutting it.

Cutting Fiberglass Insulation for Standard Stud Bay Widths

To cut fiberglass insulation first measure the length of the wall stud bay.

Then roll out the insulation onto a smooth cutting surface area such as a subfloor. Using a measuring tape, and the utility knife and straight edge, cut a length of fiberglass insulation approximately one inch longer than your measurement.

When cutting fiberglass insulation, place the straight edge across the length of fiberglass insulation roll or batt where you’ve measured, and put your knee on it to compress it firmly against the subfloor.

Then use your utility knife to cut through the insulation. Note that you may need to make a few passes with the utility knife to cut through the insulation cleanly.

If you are installing faced fiberglass insulation, cut it so that the face side is down against the subfloor. This way you’ll be less likely to tear the facing paper as you cut it.

Once you have cut the fiberglass insulation to length simply push it into place in the stud bay.

How to cut and Install Fiberglass Insulation

Make sure to push it firmly up against the back wall of the stud bay and then pull the outer edges of the insulation to the edges of the wall studs. If you are installing faced insulation, the facing should face the interior of the room. Staple the faced insulation tabs to the wall studs on the inside edges.

Cutting Fiberglass Insulation for Narrow Bay Widths

To cut unfaced fiberglass insulation for narrow stud bays it is often better to cut it in place, after it has already been installed in the narrow stud bays.

If cutting faced fiberglass insulation for narrow stud bays, it is best to cut it as you would normal stud width bays.

However, you’ll need to cut it to both the proper height and width. When cutting faced fiberglass insulation to the required width, simply press down on the insulation with your straight edge, make a cut, and then move the straight edge forward and cut again. Continue this process for the length of the insulation piece.

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