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Posts: 261
Reply with quote  #1 
I saw this great tip on the Green Building Advisor site about cutting rigid foam insulation.  It originally appeared in Fine Homebuilding (there is a short video which I was unable to copy, but it can be viewed there - search for 'cutting rigid insulation').

Basically, you sharpen the SIDE of a 2 or 3 inch taping knife with a whetstone or fine-toothed file.  Since the blade is thin and flat, it cuts right through without binding.  The 2-3 inch width of the blade gives you a good visual indication that the cut is staying square to the stock.  I haven't tried it yet, but it sure looks better than all the methods I have tried.  I've tried all manner of kitchen knives (they all bind), a utility knife (not deep enough, although I haven't tried one of the models with snap-off blade sections), hand saws (crumbs everywhere) even an electric carving knife (didn't 'cut it' and it overheated after a couple minutes).  Lately, I've been cutting part way with a utility knife, then finishing with a steak knife.  The taping knife looks like it makes a cleaner, straighter cut than anything else.


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Posts: 2,897
Reply with quote  #2 
Good idea!  I HAVE used a snap-off utility knife and it works pretty well, though the blade is flexible and may not cut true.  I usually cut maybe 2/3 of the way through and then break the foam.
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 80
Reply with quote  #3 
Are you talking polyiso or polystyrene? For polystyrene you can make yourself a hot wire knife. Get some nichrome wire and a 6/12 volt battery charger. Mount the wire to some sort of wooden holder, attach the charger to it, test at six volts. If that doesn't work or cuts too slowly use the 12 volt setting. The voltage needed usually depends on the length of wire. For either type of foam you can always use a circular saw if you don't mind the mess.

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Posts: 128
Reply with quote  #4 
I made a hot wire foam cutter using a G guitar string and a wall wart black box power source.

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