How-To
A Simple Way To Cut Rigid Foam Insulation
Teamed up with a straightedge, a sharpened putty knife cuts through foam like butter.
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 gem from the Fine Homebuilding Video Vault, a 3-in. or 4-in. putty knife with a sharpened blade cuts through the foam like butter.
For more information on working with rigid insulation, try these links:
- Choosing the Right Thickness of Exterior Foam
- Does Rigid Foam Insulation Trap Moisture in the Walls?
- Insulating Walls with No Sheathing
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Like it! We did come up with another idea when we had to cut a LOT of foam board. We sprayed both sides of a thin-blade of a circular saw with Pam or other cooking spray. You can stack the foam board to the depth your circular saw cuts, but will have to reapply cooking spray fairly often. A saw blade normally burns the foam and gunks up. Since we knew my wife would disapprove of robbing her cupboard, we got the cheapest we could find. It lasts a long time in the can, doesn't need refrigeration, so my supply shelf still has about a can and a half. We did have a room were we could sweep up the "foam dust" easily. I wouldn't try that with petroleum-based spray! I have a spare putty knife to sharpen and going to apply cooking spray to that. Thanks for the tip!
A reasonable solution.
When faced with the same challenge, I dedicated an inexpensive fish filet knife to the job. Thin blade, razor sharp, clean easy cutting. And it comes with a sheath, so no exposed, unprotected (and on a putty knife, unexpected) sharpened edge.
The best solution that I found was a 4" serrated knife blade for my jig saw. Cuts thru all kinds of rigid foam insulation without any mess (the 'snow' from a regular saw blade) and, at the right speed, it cuts (not burns) its way through the insulation. Very much like Festool's 493656 S 155/W.