In rehabbing a 19th-century house, I had to custom cut a lot of fiberglass insulation. The ideal tool turned out to be a chef’s knife, or French knife, which has a 10-in, or 12-in, long blade that’s wider at the heel and tapers to a point. I tuned its edge up every now and then with a sharpening steel. And to keep the blade moving through the kraft paper, which tends to tar up an edge tool, I gave the chef’s knife occasional squirts of silicone spray.
Roger S. Apted, Milton, WI
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I use my machete - it's a 22" blade, and I drag it across. Works like a charm, and I sharpen it with a file every now and then. The edge then acts like a Ginsu knife -
I lay a small piece of 1/2" plywood across the insulation, to compress it, and then cut with a utility knife.
The knife trip is great for Roxwool though, I used a serated bread knife and it worked like a charm!
When I installed fiberglass insulation I ripped a narrow slot in a 1 x 6 and securely attached some 3' of wire to the backside. The other end of that wire was secured to a 4: piece of broomstick. In use the wire is fed through the slot, onto the floor. The batt is passes through over the wire on the backside if the 1X. Just stand on the 1X and pull the wire through the slot. It works a lot like an egg slicer. P.S. it also worked with upholstery foam.
Bread knife.
I use my old Japanese pull saw blades (dozuki).
hedge shears. snip snip you're done.