How to Cut Fiberglass Insulation

January 16, 2021

Fiberglass insulation is so far the most preferred way to keep a home warm in the winter. There are other types of insulation, but many homeowners opt for fiberglass because it is affordable, readily available, and very dense.

However, the sponginess of this insulation can make it challenging for some people to cut it. Besides, the material is quite abrasive and needs sharp blades and safety gear such as glasses and a dust mask. All these can be found in an insulation shop where you buy your fiberglass insulation.

So, how do you cut your fiberglass insulation, ensuring efficiency and safety? These few basic methods will help you get started.

METHODS

Method 1: Measuring Insulation

1. Start by placing the insulation on a piece of plywood – Select a plywood piece with the same (more) measurements as your fiberglass panel. This will offer you more space to spread your fiberglass while cutting it. You don’t have to use too big plywood, just ensure the part of the fiberglass you are cutting is on top of the plywood.
2. Wear protective clothing, e.g., cut-resistant gloves – The harsh surface of fiberglass insulation can bruise your skin, so cover yourself before working on it. We recommend wearing a long-sleeved shirt and long pants. Then wear protective gloves that can protect you from the sharp tool you are using and the fiberglass itself.
3. Put on safety glass and a dust mask for protection – the typical N95 dusk mask will be fine, but you can opt for a full face mask if you want. You can wear safety goggles if you don’t have glasses.
4. Roll out the fiberglass insulation with the paper side down – usually, fiberglass insulation is packed in a big roll. To roll it out, lay the roll on the plywood edge and then push it across and off the plywood. Just roll the length you want to use.
5. Measure with a tape measure where to install the insulation – you need first to have the measurements of where you want to install the insulation. These are measurements you would take on the fiberglass.
6. Measure and mark the insulation using a permanent marker – transfer the measurements you took in step 5 to the insulation. Indicate where you will cut. Don’t measure the exact figure of the fiberglass; add at least 2.5cm on both sides of the fiberglass.

Method 2: Using a Utility Knife

Choose a sharp utility knife with a 7.6 -10.2- in blade – go to any insulation shop and get your utility knife. Any can work, but when it comes to fiberglass, it dulls the blades faster. As such, we recommend you use something disposable. Another option is going for industrial utility knives—these last longer.
Lay a wood board close to where you want to cut the insulation – use 2×4-in board, longer than the insulation. Most projects would require a 12-in board. Now lay it against the measurements you marked. This is meant to make the cut more accurate.
Compress the insulation by kneeling on board – while on the side where you want to begin the cut, kneel on the part of the board right over the insulation’s edge. Then, bend forward so that you can reach the other end of the edge. Using your free hand, press the board over the edge of the fiberglass insulation to compress it.
Hold the utility knife at an almost 45-degree angle over the mark- Start cutting at the opposite edge where you had pressed the board with the freehand. While kneeling on the board, pull the knife to the edge where your knee is.

Method 3: Cutting with a Carving Knife

Choose a 7-inch carving knife attached to a long cord – take a clean and sharp knife and plug it into an electrical power supply. Turn the knife on to check whether the blades are working.
Bring closer the insulation to the socket and stand it so you can access the spot to cut – you need a lot of space when using a carving knife as you would cut the insulation while it’s upright. Also, ensure there is enough space between the knife and your hand.
Hold your knife in a perpendicular position to the fiberglass – put the knife against the side of the insulation you want to cut and hold the blade firmly and straight. Make sure there is enough space to move the blade down the fiberglass.
Steadily hold the knife as you cut the fiberglass – Turn on the knife and press it against the fiberglass. Ensure the middle section of the knife is centered over the fiberglass. You don’t need to move the knife as the blades move forth and back on their own, just like a saw. Ensure the cutting line ends exactly on the other side, where the cut mark is.

Method 4: Trimming with Shears

Choose a clean and sharp pair of heavy-duty metal shears – provided your shears have sharp blades, you can cut the fiberglass with any shears. However, heavy-duty-metal shears are recommended because they are made for hardy work, with thick, stainless steel blades. Besides, they have spacious handles that you can use even when wearing gloves.
Compress the insulation by kneeling on the board – while on the side where you want to begin the cut, kneel on the part of the board right over the insulation’s edge. Then, bend forward so that you can reach the other end of the edge. Using your free hand, press the board over the edge of the fiberglass insulation to compress it. This will make it flat and, therefore, easier to cut.
Open the shears and position them near the cutting mark – one scissor should be underneath the fiberglass and the other over it. While pressing the insulation down with the wood board, start cutting; if there isn’t enough cutting space, lift the insulation a little. Just remember to maintain your shears as close to the wood board as possible when cutting for a more accurate cut.