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When Chris Ellis of Brewster, Mass., was looking for the right tool to cut rigid-foam insulation, he tried a utility knife, a kitchen knife, and a saw. None of those options worked especially well. So Chris created something that works beautifully. As Chuck Miller demonstrates in this video tip, a 2-in. putty knife with a sharpened blade cuts through the foam like butter.
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After watching this vid a little while ago I used the sharpened putty knife to do my basement in November going through a few dozen 2" sheets of extruded foam and the approach works very well. I'd cut on the concrete floor using a thin piece of plywood as the straight edge. I was surprised to find such a clear "grain" in the foam; it can be sliced up very easily length-wise but across the foam it's always trickier.
Anyway, the sharper the knife the better. I used a cheap harbor freight cutoff wheel tool to grind down one edge into a pretty sharp blade. Also, to avoid ripping the top when cutting across, score a thin trench first, then deeper on each pass.
I cut several sheets with a circular saw before this and the sharp putty knife is cleaner (no dust), quieter, and much faster, not to mention more accurate, too. I cannot imagine a better way to do it, thanks!