Here is the best tool I have found for cutting
Victorinox 12-Inch Wavy Edge BreadKnife, Rosewood Handle
by Forschner, Victorinox, Swiss Army Brands.
Use it for cutting Rockwool batts, too. They cut best with a long knife. Taping knives sold as usable with insulation batts and always inferior, are useless with Rockwool.
I found mine among quality knives at a locksmith shop, and have used it for six years with complete satisfaction, never dulling. It is twice as hefty as my previous choice, a standard bread knife. I like to cut against decking plywood when I can, not leaving a score with the dull knife tip. I measure length or width by tape measure, always to over-fill a space. End cuts are always by eye upon measure. A length cut is often guided by some slot or by an edge over-hang. Cut full-depth without compression, or angle across the batt; an initial accurate lineup is easy to maintain.
Anyone may readily buy one from Kerekes bakedeco.com, Item 40146, at price $56 just now.
This knife will not cut cotton batts, which are best put down whole. Even a sharp hand saw struggles with batt tie threads, making a mess. The irregular cotton batts, not compressible, demand periodic fill-in with easily-cut fiberglass batts.
Basement wall insulation, to be tucked over half-buried heating ducts, is sliced with large scissors, following a drywall square.