As an ol'fart who's been doing DIY insulation projects for over 40 years, Long ago a found the perfect knife for this and still use it today: a Machete! It's long blade makes for easy long straight cuts and cross cuts in one easy swipe. The rounded end rides easily over any cutting surface or subfloor plywood without snags, or tearing the grain of the board.
They're still sold and often come with a scabbard so you can hang it on your belt. Good for ladder work where you may need to trim some on the fly between overhead Joists or Rafters
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