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Insulation batts are lovely and thick, which is a godsend for stabilizing ambient temperature, but they are rather unwieldy, making trimming them with any accuracy particularly tricky.
Go in without a sound plan and before you know it, your skin is sore, there’s fiberglass everywhere, and you’ve got an incredibly messy installation that won’t insulate the area half as well as you’d hoped.
On the other hand, if you go in with a plan, you can make what is quite a daunting task an absolute breeze, and that’s what I’m going to help you with today.
Stick with me and turn that DIY insulation project into a pro-grade installation!
The Best Way To Cut Insulation Rolls: A Step-By-Step Guide
There are a number of different cutting tools and methods for trimming fiberglass insulation batts, and I’ll fill you in on most of them later on, but we’re going to kick things off here with my favorite approach.
What You’ll Need
- A table saw
- A tape measure
- A set of cheap utility knives
- Utility gloves
- A construction-grade full-face respirator mask
- A large square of scrap plywood
- A second, smaller piece of scrap plywood
Step 1 — Making A Jig
The jig is the key to this method’s success. It’s going to facilitate perfect cuts of insulation time and time again — No exceptions!
To get the ball rolling, you need to find a scrap piece of plywood long enough to stretch across the entire width of your fiberglass insulation and then some.
There needs to be about 7 inches of excess plywood on one side of the insulation, roughly 4 inches excess on the other side, and the board should be about 10 inches wide.
To finish the jig, use your table saw to cut 6 inches across the width of the board roughly 7 inches up from one of the ends.
Then make a perfectly straight cut from the termination point of your first cut all the way down the length of the board.
You should end up with the full-width 7” block at one end, and a long 4” wide finger-like extension — It should look like a big, blocky P.
Step 2 — Suit Up
With your jig ready to go, you’ll be handling fiberglass shortly, so it’s time to don all your protective gear.
Put on your utility gloves and your full-face respirator mask, and it’s a good idea to wear an old long sleeve sweater too.
Fiberglass is incredibly rough on the skin, and the particulate matter launched into the air during a cut can irritate your lungs and make pre-existing respiratory issues a darn sight worse.
Step 3 — Unraveling Your Insulation
Take your large piece of scrap plywood, lay it down on the floor, then unravel a length of your insulation on top of said board.
The board needs to be large enough to hold the insulation and the full length of your jig.
Step 4 — Preparing Your Cut
Measure the length of insulation you need, then use a pen to mark this length on the roll of insulation.
Take your jig, lay the finger-like extension across the insulation so the inside of the extension intercepts the pen mark, and the edge of the fiberglass is aligned with the horizontal cut (the first cut you made) of the plywood board.
Next, bend down and place your knee on the 7” section of your jig and apply plenty of pressure.
This will simultaneously flatten the fiberglass - making it easy to cut with a utility knife - and perform the jig's primary duty of guiding the cut.
Step 5 — Making The Cut
Take one of your cheap utility knives, and starting from the farthest edge of the insulation, make one firm, fluid pass towards you and the jig, and voilà — You’ve just nailed a perfectly squared cut.
Now, I’m sure some of you are wondering why I’m encouraging you to buy and use cheap utility knives.
Well, fiberglass is as hard on a knife’s edge as it is on our skin and lungs.
It will dull even high quality blades within a few cuts, and being that you’ve likely got plenty of cuts to go, it’s best to use multiple disposable blades.
What Are My Other Options?
Of course, you don’t have to use utility knives, and you don't have to make a jig with an extra edge for alignment, and you can still get the job done reasonably well.
In fact, you may not have to use a jig at all.
Electric carving knives are remarkably good at cutting through the wooly profile of fiberglass insulation, and you won’t need a jig to get a clean cut.
All you have to do is use a straight edge as a guide to mark on your cutting line, hold the insulation on its side, then slice through following the line.
It’s a simple enough process, but it won’t be quite as accurate as the method detailed above, and as you have to carefully follow the cutting line, a full installation can take a while.
You could also do a pretty good job with a pair of scissors and a bit of plywood.
The plywood would rest across the insulation, providing both compression and a cutting guide, and then you just have to snip, snip, snip your way along the fiberglass.
This is my least favorite approach, as the lack of a jig makes lining up perfectly squared cuts impossible, cutting the fiberglass with scissors starts to ache your hands before long, and they make for an incredibly sluggish cut compared to a utility knife.
Insulation rolls may seem tricky to manage at first, but with a makeshift jig, a large plywood base, and a few cheap utility knives, making the perfect cut couldn’t be easier.
Settling for anything less than perfection won’t only result in a messy installation, but subpar performance, so, in my opinion, using a jig is the only truly responsible approach.