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. 

Do you have a preferred method for cutting insulation? Or is insulation something you like to leave up to the pros? It would warm our hearts if you shared your insulation successes and failures.

Did you enjoy reading this post? Want to learn more about our first-hand experiences with other tools, devices or items used throughout our renovation? If so, check out our complete list of product reviews in our Toolbox Tuesday section

Note: We weren't paid for this review. We simply want to share good products when we see them, and hope that learning from our mistakes can help save you time, money and frustration.

Comments 8


6/5/2012 at 10:23 AM
Insulation is totally a hot topic for me right now.. (nerd alert)! I really like what I've read about recycled denim. We opted to go with spray foam, which had to be left to the pros.
I've long had an internal struggle about using the spray foam. On one hand, I love it. It gets in all of the nooks, does a great job, and can even be used (in a closed cell product) on damp basement walls. On the other hand, I hate that it closes in the area (no more opening walls to easily run new wires or plumbing), and that you have to hire pros for it.

But just recently, Steve over at www.ouroldrowhouse.com did a DIY spray foam application and blogged about it. Here's his post www.ouroldrowhouse.com/2012/05/15/at-long-last-insulating-the-porch-ceiling/ lots of great info!
6/5/2012 at 10:33 AM
With the heat we had last week, it must be miserable up there! Not to mention, you need to be on the constant lookout for other summertime attic surprises like bats, wasp nests, and other pests.

I like the insulation saw. I might have to look into investing in one of those.
6/5/2012 at 1:21 PM
When I redid the insulation in our eaves, I went with the compress and cut method. We used the "batts in bag" type because it cuts down on the handling of the fiberglass. With a new, sharp blade, it was pretty easy to slice through with the cut and compress method.

Threadbndr (Karla)
6/5/2012 at 2:11 PM
Dude, hands off the scissors! Walt and I very seldom had major fights, but I went to town on him once, poor guy. He was using my (expensive) Gingher quilting scissors (which should never touch anything but fabric) to cut bags of mulch open.

It's funny, in retrospect, how peeved we got at each other. But I bought him a pair of heavy utility shears and put a huge PINK ribbon on my quilting tools.

BTW, any tool that was pink, had bright pink nailpolish on the handle or a pink ribbon tied to it was MOM's/honey's tool. Do not touch or move! I take MUCH better care of my tools than any of the men in the house and was tired of getting them back all beat up (or worse, not getting them back at all). I could never find a screwdriver until I bought the pink ones LOL.

For the record, I'm a 'compress and cut with a utility knife' gal. Long sleeves and gloves and a mask required......
6/7/2012 at 4:19 PM
I also use the compress and cut method with insulation, although that saw looks pretty awesome. I'm in the middle of adding attic foil (radiant barrier to the underside of my rafters over the insulation and baffles. Our house didn't have ventilation and insulation in the attic so I'm thankful it was a mild winter. Also the weather has been unusually cool which makes insulating 2000sqft tolerable. Hopefully with everything we're doing we'll notice a huge difference in our heating bills. I would definitely check out attic foil's website its pretty cheap and easy to install and if its half as good as they claim you'll notice a huge difference.
6/29/2012 at 11:09 AM
Not to take anything away from what you've done - Kudos for using an insulation knife. But you'll get similar results from a cheap serrated knife from the dollar store. And even if you have to hold to the "right tool for the job" fetish, there are better knives, such as:
graduated blade allows you to make quick measurements and cuts when you are fitting. Brilliant.
12/3/2016 at 4:02 PM

I can disabuse you of the notion of insulating the attic in February. I'm doing so in December and while it's not completely uncomfortable, being the attic and thus where the heat rises to, it is cold. And if you are doing anything tricksie- I'm doing a combo of rigid foam glued up surrounded by canned spray foam where I need to keep the rafters clean, batt insulation in the crawl spaces and spray foam elsewhere, the cold makes using glues and sprays difficult because most won't stick properly if too cold. So I'm also washing off the ancient grime, which also gets cold. Had I actually gotten around to it in September, life would be much warmer now, and the project going faster.

As to the insulating chaos, the original plan was spray foam but it's disastrously messy, taking lots of prep time to avoid overspray. Had I been working in a shell that could be sprayed by a pro in one go, I would have, but needing to work room by room makes it DIY. And I'm not loving working with the spray foam which is also quite expensive.

Which is why I'm here reading your advice on cutting fiberglass batts, because I just went thru all your steps but for using the fabric scissors to cut the stainless steel wool I'm using around the edges to prevent pest infestation.

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