How to Cut Fiberglass Insulation Fast

While fiberglass insulation is usually cut with a utility knife, as demonstrated in our Cutting Insulation video, one of our viewers came up with a novel idea which worked great and gets the job done in a hurry. Here it is:

“We were insulating our bonus room with R-38 bats in the ceiling, and about two-thirds of the way through the job I wished out loud that I had a longer knife to cut the 12” thick bats. Then it occurred to me: MY ELECTRIC CARVING KNIFE!!!

“It’s incredible how easily it works and can even cut the kraft paper, though I usually use scissors for that.”

LaCrosse, Virginia


  1. Here I am again!! Is there a more compact version of the R-38 that has the same value?? I know there is for R-30 and itis called C-30

    • Hi Heather,
      Several companies either do or used to make high density (also called high performance) R-38C insulation (10¼” thick vs. 12″ for regular insulation). I don’t know about availability.
      Good luck with your project!

  2. Why does a roll of 5.5 inch thick fiberglass insulation have 2 R-values?
    One roll is faced the other is unfaced.
    The paper facing on one roll does not add to the R value, at least from
    what I learned many years ago.
    TX for any info on this.


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