I have cut about a million sheets of all three kinds of rigid foam boards with a knife and a saw. My personal conclusion is that the knife technique is only to be used if there isn't a suitable saw available. The cut is much cleaner. I just put a really big drop cloth down. There are some hand saws that cut with less dust than others. Last week I was working in a basement where they happened to have a hand saw sitting right next to me when I needed to cut a board. The thing made very little dust at all and the particles were pretty big. I think it was a rip saw. Just go gently. Maybe if I had all the time in the world I'd score both sides and break it but it takes too long and the chances of making a rough edge are too great.
Just my 2 cents.
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