Dear Readers, you may want to skip this post (unless you are planning on tackling a large insulation project in the near future), as it does not contain actual house progress.
Apparently, no human being has ever cut a substantial amount of 3″ rigid foam insulation or, at least, none have lived to write about their endeavor on the internet. I have fruitlessly spent weeks researching the best way to cut rigid foam. And, by “researching,” I think you all know I mean Googling…
If you search “how to cut thick foam,” you may come across the electric carving knife. I have tried this. It works much like a weak Sawzall. The blade is not sturdy enough and tends to bend, so you’re cutting at an angle, and then it freezes up about a foot into the cut. It also creates a lot of pink “snow.” I think this method would work best for cutting upholstery foam.
Should you search “saws for cutting rigid foam,” you will probably find the CenterFire Foam Blade from Bullet Tools. This looks like something a magician would use to saw someone in half. Dad tried a regular table saw on the foam and found it hard to control. We thought this blade looked like an accident waiting to happen. It’s possible that this was the answer all along. Theoretically it makes sense but I don’t want to risk losing a limb. Bullet Tools also makes a straight blade that fits in a utility knife holder or a Sawzall but the blade itself isn’t deep enough for the 3″ foam. A few people suggest using a heavy-duty snap-off utility knife. I do use one of these to shave off the edges if they’re uneven but I haven’t found one strong enough to make the main cuts. Festool makes a foam-cutting jigsaw blade, which I did end up purchasing. It makes a clean cut with no mess but the blade is too flexible and ends up bending, so you’re cutting on a slant.
Traditional saws will also come up in the above search results and don’t think I didn’t try them! Again, the table saw grabbed the foam and made it hard to control. A reciprocating saw works but makes a terrible mess of foam snow and cuts on a slant (though, this could be the operator’s fault…). A handsaw works but, also, makes a mess and I had a hard time keeping my cuts consistent. A drywall saw works a little better, as I have a bit more control over it, and I do use this occasionally, but it makes a snowy mess and rough edges. I haven’t tried a circular saw. That may work if you have a way to keep the foam secure as you cut and can keep two hands on the saw.
Some people recommend a hot knife or wire but I found mixed reviews on this and didn’t want to spend so much money on something that may or may not work.
A sharpened putty knife is the most common search result and it is my method of choice but it has taken its toll on my hands. The cut is fairly straight and it doesn’t leave a pile of snow but it takes a lot of muscle to push it through the foam and pull it back out. Actually, I’m not using the knife as it was suggested in the search results because the suggested method is more for thinner foam and would take forever. I score a line down the foam with the side of the putty knife against a T-square and then wedge the straight edge of the knife straight down into the line, rocking it side to side until I hear it pop through the bottom (a very satisfying sound, by the way) and then wiggle it back out, and continue like this along the entire scored line. After cutting 200+ pieces of foam this way, I’m now wearing a wrist brace and haven’t had feeling in my thumb for 4 days. The floor and upper walls are insulated and all the pieces are cut for the ceiling. I just need to finish up the walls and then I’ll see my doctor. Hang in there, thumb, we’ll make it!
All of these methods go a lot smoother if you use both hands… Alas, I didn’t have a tripod to hold my phone while I recorded.