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Cutting fiberglass insulation

Discussion in 'General Knife Discussion' started by FlaMtnBkr, May 11, 2010.

  1. FlaMtnBkr


    Oct 20, 2004
    Has anyone cut fiberglass insulation recently? I just cut a section off a 2' wide roll with my M4 blade and it sure did a number on the edge. It was hair popping sharp and would tree top hairs and by the end of the cut would barely cut the fiberglass and now it won't shave the first hair.

    If anyone is interested in testing edge retention it seems like cutting insulation might be a great test because I have never had something dull a knife so quick.
  2. tjsulli790


    Jan 13, 2009
    Fiberglass insulation is some terribly abrasive stuff. I know it always comes up in those "What is worst for your blade" type threads.

    They make knives just for duct board, cheap and thin is the way to go. I wouldn't take a quality knife anywhere near that stuff.
  3. FlaMtnBkr


    Oct 20, 2004
    It was the paper backed stuff for putting in walls. I was amazed at how clean it cut the glass when I first started but then towards the end it was basically just ripping it.

    5 minutes on my diamond loaded strops and it is back to popping hairs off above the skin.
  4. tjsulli790


    Jan 13, 2009
    Yea it's still fiberglass insulation. If you want to use a nice knife to cut the stuff that's fine. I worked for A/C company for several years and found thin serrated knives work best for long term use. They still cut well even when dull, which your knife definitely will be. Unless you plan on sharpening it 5 times before a job is completed.
  5. I used a 'duct knife' (cheap, double-edged, bought at Home Depot), to cut some pink fiberglass insulation a while back. Had the same result, essentially dull within 2 or 3 cuts. I wondered at the time, whether it was just because the knife was a cheapie, or if the 'glass' was the main culprit. Now I know for sure. If it dulls M4 too, then I don't imagine much of anything will stand up to it.
  6. freedoom


    Jan 31, 2010
    How about a nice serrated hawk bill like a spyderco tasman salt or spyderhawk salt?
  7. FlaMtnBkr


    Oct 20, 2004
    I don't cut much insulation and just happened to need a piece so I used the knife I always have on me which is my M4 Benchmade Rift. In the past when I had to hang a lot of insulation I just used a razor knife but don't remember how quickly it dulled the razor blade. I'm sure a serrated knife would cut longer but not sure how it would handle the 'fluffy' insulation as it might grab it and tear more than cut.

    I'm curious as to how some other steels would fair against it but don't really need to cut more and don't want to just cut it to throw it away and waste it. I'm always fixing and building stuff so I'm sure I will need it in the future and it's not real cheap stuff. I guess the next time I need to hang some I will try to remember to get out an assortment of knife steels to see if it has a similar effect on everything. I bet S90V would probably hold up the best of what I have. Still surprised that a cut less than 24 inches long dulled it so bad.
  8. lambertiana

    lambertiana Gold Member Gold Member

    Jul 7, 2000
    Fiberglass is made of...glass, which is significantly harder than the hardest steel knife blade out there. We all cringe when we see someone using a glass cutting board in the kitchen, and fiberglass does the same thing to your edge. The individual fibers may give way and break, but the overall effect on the edge is bad.
  9. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007
    Yes, fiberglass will dull a edge in very short order. When I had to work with that stuff I used a razor and a diamond stone. It was faster for me to make a quick pass per side on the thin blade than to change it, also saved on blades.
  10. Brian Sargent

    Brian Sargent

    Jun 6, 2008
    I use metal shears.
  11. benh

    benh Gold Member Gold Member

    Sep 29, 2005
    The last time I dealt with kraft faced insulation, I used a pair of scissors to cut it. Worked pretty well, and the scissors stayed sharp enough to do the job.
  12. 65535


    Mar 14, 2007
    Maybe this would be a good application for ceramic blades.
  13. presz

    presz Gold Member Gold Member

    Nov 12, 2005
    Mineral wool insulation is harder on an edge. I use an old kitchen to cut it. And I keep the sharpener close by. I comes in 2'x4'x2" sheets and has to be cut down to 8". Lots of fun and veryyy itchy, way worse than fiberglass.

    Rockwool Insulation is made from recycled blast furnace slag — a by-product of other industries and Basalt or Trap Rock A sustainable naturally occurring mineral.

    We use it at the top of masonry walls to make them fire and smoke proof. When covered with fire spray.
  14. *jijoel*


    Jan 3, 2009
    The brown and black handled knives are my insulation knives.
    The black is a fiskars and is the best with the serrated edge.
    The brown "isover" (brand name of insulation over here) doesn't work really well.

    I often use stanley knives with the replaceable blades.

    I would not use one of my expensive edc knives but do think my se military would do a good job.
    Last edited: May 13, 2010
  15. Mongo


    Nov 5, 2001
    The last time I did any cutting of insulation, I used my Bark River Tusk... which is their fine A2 IIRC. It did a great job, even after if felt dull. Sharpened up quite easily too. But... if I had to do it again, likely to use a basic utility box cutter with the replaceable blades.
  16. Daniel Dorn

    Daniel Dorn Gold Member Gold Member

    Apr 21, 1999
    I used my M2 Benchmade for some cutting, and it held up reasonably well, but definitely needed a sharpen. After that, I ended up using the serrated blade on my Swiss Tool multitool. It worked very nicely, and cut for a long time.

    The trick was to compress the insulation against the floor (still unfinished, so no problem with marring it) and then cut. I just kept a pocket diamond sharpener for touch-ups throughout the day.

  17. Josh K

    Josh K

    Sep 29, 2008
    Sounds like we need some S150V / S125V here. :D
  18. darthsoaker

    darthsoaker Basic Member Basic Member

    May 2, 2006

  19. thebrain


    Dec 12, 2007
    I just had to cut some blown in type fiberglass bales in half today at work ,I cut as deep as the blade was long and after 3 bales I had dulled my endura 4 to slightly less sharp than the edge of a nickle and it would easily shave before .
  20. Bhamster


    Mar 27, 2004
    This might sound all pansy ass but when you are up in a hot attic and you want to get out of there, one of those $15 electric carving knives (or 5 bucks at most thrift stores) that so many moms used when I was growing up in the 70s, work really well. Get a thrift store one, people bought them and used them once a year for holiday turkey. They are also what a lot of people who have to cut thick non rigid foams use as well. And they come in such wonderful colors, avocado green, rust orange or black. Yeah the cords are a pain I know. Sorry for the non-sharp response to all you shaving sharp folks.

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