Cutting Insulation

Energy Conservation 4:03 PM Add Comment Edit

Cutting batts of fiberglass insulation takes forever because the batts are so thick and fuzzy. To speed up the process and produce a clean, neat cut every time, lay the insulation on top of a scrap piece of plywood with its foil or kraft-paper vapor barrier facing up. Then cut a 2X4 at least 12 inches longer than the width of the insulation.
Place the 2X4 across the batt on the cut line. Compress the insulation by kneeling on one end of the 2X4 and pressing down on the other end with your palm. Then use a sharp utility knife to slice through the batt in a single stroke. Always wear gloves, long-sleeve shirt, long pants, eye protection, and a dust mask when working with fiberglass. And work carefully so the knife doesn't slip and cut your knee.

Over the last several days I've been braving the heat and cramped surrounds of our attic all in the name of energy efficiency and comfort. It's a little bit funny to me that I will venture into a veritable oven on some of the warmest days we've had this year to run duct work and lay insulation in order to ensure we can stay warm in the winter and cool in the summer. I guess my comfort in the short term is easily sacrificed for our long term enjoyment. It makes sense, but it doesn't mean I have to like it. Although in retrospect, it would have been better to do this in February.

If you've ever worked with insulation before, especially in a home with odd sized joist and stud cavities, you know just how cumbersome and difficult it can be to cut the insulation to size. Whether you're using something light and fluffy like fiberglass, or dense and heavy like Nu-Wool, we are all quickly reduced to neanderthal DIYers, hacking, clawing, and tearing at the insulation with utility knives and scissors to make our cut.
I've seen many different approaches for cutting insulation. Let me assure you, not all are created equal.

There's the good old "hack it to death" approach, where you slice the same basic area with a utility knife, over and over, until you emerge victorious with a lopsided and not very smooth cut.
There's the compress and cut method (which actually works quite well), where you use a board or straight edge to compress the material while cutting with your knife. The big problem here, you tend to need to touch the insulation more, and you must fluff it back up after you've cut it.
There's the "my hands can do this" method, where you figure out where you need the cut, or where your knife missed the cut, and you impatiently rip it apart in your own hands. This tends to leave a less than neat cut, for obvious reasons.
And there's the "when-all-else-fails-I'm-going-to-use-a-pair-of-Wendy's-scissors-and-I-don't-even-care-I'll-get-yelled-at" approach. I guess the frustration is too much when I reach this point. They still don't do a good job, but at least I'm not using my hands and I'm not going to accidentally stab myself in the leg because I get over zealous with these.

All of the frustration and poor cuts when dealing with insulation come courtesy of one thing -- use of the wrong cutting tool.
Several years ago, after we bought the recycled denim insulation we opted to use for our kitchen work, the company that made the insulation suggested we purchase an insulation cutting knife, or more specifically, an "Insul-Knife" from CEPCO. While $30 seemed a bit much for an odd and single purpose tool, given my "one new tool for every project" mantra, I obliged, and I sure am glad I did.
The recycled denim insulation is super thick and tough to cut. We tried scissors, knives, Jedi mind powers (which actually worked particularly poorly), but the only thing that really worked was the Insul-Knife.
Since the purchase I've used this large and oddly shaped knife/saw to cut insulation wherever we've installed it. Just look at how simply and smoothly it can cut standard fiberglass insulation.

First you measure your cut, place the knife, and begin sawing.
The sharp blade easily cuts through thick insulation, giving you a clean and straight line to your cut. You can't beat that.

The insulation I was cutting in these photos is a very thick R30 fiberglass, but this works for other types of insulation as well. I'm very happy I bought this tool, or I hadn't I'd still be hacking away at our insulation like I was auditioning for a role in the next season of Dexter. Instead, I have perfectly sized pieces of insulation with perfectly squared edges. You can't easily achieve this with Wendy's illegally used scissors, that's for sure.

And what happens if you use and abuse this Insul-Knife to the point that the blade begins to lose it's sharp edge? Well, luckily they sell a sharpener that will keep it cutting for many years. We've not gotten to the point where we've needed the sharpener, but it's nice to know it's there if we need it.