For cutting rockwool insulation, retire your old bread knife and pick up a pair of these
"Amazon likes to show me things they think I'll like, and... Their algorithm worked with these.
These are Swedish-made Bahco insulation knife and saw. For years, I've used a cheap bread knife to curt Roxul, works pretty well, at least the best I'd found. But I heard a couple of guys talk about these and took a stab at it.
They work really, really, really well.
You can see my bread knife is no longer sitting here, it has sadly been replaced, I'll save it for peanut butter and jelly at lunchtime.
So they're not cheap, but from the reviews I've read on them, guys say that they last for a long time, so you're not just buying them and throwing them out.
The knife has a measuring gauge on it, granted it is in metric, but I can use these to see sizes when I'm in the attic I can hold this into a tight spot and get an idea of a measurement and transfer it over onto my batt and cut it. Cuts cleanly, quick. Nice.
The saw works really well for the comfort board; it cuts like a laser through that stuff.
They both cut really nicely; you end up with a factory cut without piles of shredded insulation all over the place.
—This tool tip comes from jobsite of Professional Remodeler's 2017 Model Remodel.