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Cutting XPS foam - Best tool?

kodak's picture

Has anyone discovered the best tool for cutting thicker (1 in. plus) extruded foam. An electric saw would produce a lot of harmful dust.

I've run into the same dilemna as "gonegolfin" post 107456.1 -DIY spray foam. Rear wall flange of bathtub was 3 inches outside the wall and just simply tiled over, by whoever originally installed it (70 year old house). Tiles, grout, etc. predictably failed. Didn't discover it till I was in the basement while a teenage daughter showered and it was raining in the basement ! Have stripped out everything to the studs - wet insulation, black mold - horrible !

 I'm now trying to decide whether to hire a foam contractor, or follow some of the suggestions to gongolfin, re., use great stuff to glue in XPS foam.

  I have 2 products gathering dust in the basement; 1 in. dow blue xps and 2 inch Homasote R plus with foil on both faces. Anyone have an educated opinion on which to use (or not) ? Thanks

(post #115228, reply #1 of 17)

Uh, hand saw? Drywall cutout saw? Razor knife?

(post #115228, reply #2 of 17)

Would one of the long insulation knives (the kind used with fiberglass bats) work?

(post #115228, reply #3 of 17)

I just waterproofed and insulated a portion of my foundation. Used 2" rigid pink foam and some white XPS foam.

I cut the foam with a cheap razor knife; the kind with the long segmented blade that you break off the blades as they wear out. With this 3+ inch blade fully extended, a few passes was all it took to get a reasonably clean cut with little mess.

You could cut along a straight edge for more accuracy.

(post #115228, reply #4 of 17)

To cut down XPS and polyiso foam panels, I've tried various tools such as a razor knife, a sharpened putty knife, a jigsaw, a chef's knife, etc. In most situations, I've found the very best tool is a circular saw and guide rail system, hooked up to a shop vacuum to collect the dust. I use the Festool TS55 saw and rails (sometimes resorting to a razor knife to finish the last fraction of an inch on deep cuts).

This produces a precise cut with a nice clean edge, even on bevel cuts for a roof-to-wall junction. With this method I can cut panels closely enough to friction-fit between studs or blocking, using firm hand pressure or a rubber mallet to install. Another approach is to aim for a quarter inch gap, or so, then fill it all around with Great Stuff (the low-expanding version for doors and windows). In either case, you can seal joints with metal foil tape.

Trust me, if you are going to use any electric saw for more than a few panels, it is worth the expense to set up good dust collection. You would NOT want to be breathing polyiso dust or getting it in your eyes. XPS does not seem to be as irritating to your body, but it's still a mess to clean up.

For a few panels, of course, this is overkill and a hassel to set up. Just follow Southbay's advice and use a knife, whatever you have.

Edited 7/30/2008 6:25 pm ET by tom21769

(post #115228, reply #5 of 17)

dewalt portable table saw hooked up to shop vac, works like a charm. nothing faster.

(post #115228, reply #10 of 17)

That's how I do it.

(post #115228, reply #6 of 17)

>> I have 2 products gathering dust in the basement; 1 in. dow blue xps and 2 inch Homasote R plus with foil on both faces. Anyone have an educated opinion on which to use (or not) ? Thanks

As far as I know, foil-faced polyisocyanurate (such as Dow "Tuff-R") offers the best R-Value per inch (~ 6.5) of any foam panel. So, without knowing how you want to use it, that probably would be my first choice. I'm not familiar with Homasote "R plus". Maybe it is similar to Homosote "Ultra R"? That apparently is another polyiso product, like the Dow Tuff-R sold at Home Depot.

The blue XPS (or pink alternative) seems to be the panel of choice for under a slab (as in a radiant floor system). Which is not to say it's bad in other places.

There's an overview of the properties of polyiso, polystyrene (xps etc), and other insulation materials at:

(post #115228, reply #7 of 17)

Most everyone I know cuts it with woodworking power tools without a problem. I wouldn't do it inside because of the mess. You can use a knife or a handsaw though accuracy will suffer. But maybe that doesn't matter.

(post #115228, reply #8 of 17)

Circular and a sawzall. I've tried the razor knife and it's a waste of time. Clogs up and leaves ugly cuts. Your hand will cramp with even a sharp knife after multiple cuts. I just chalk lines as I would with plywood and go at it. I hate to admit it, but I've probably inhaled sh#t a lot worse than XPS dust many, many times. Wear a mask and squint.

(post #115228, reply #9 of 17)

I've used a serrated kitchen knife with success - It's one of those "as seen on TV" knives guaranteed to cut through tin cans and tomatoes.  :)

(post #115228, reply #11 of 17)

I've used a handsaw w/ success ... cutting 45 deg angle across the 24 inch width. Like anything ... you need to acquire a little skill, but I figure if I can do it, anyone can.

But no one has suggested something I've seen ... a hobbyists hot wire ... an electric wire designed for slicing polystyrene ... been around for decades ... although I've no clue where to find them. I've heard/seen them also used w/ e.g. SIPS panels for 'routing' grooves for wires/plumbing, etc.


There ain't NO free lunch. Not no how, not no where!

(post #115228, reply #12 of 17)


 I had to cut a lot of foam panels when I built my SIP house. I tried them all, melting it with a hot wire, utility knife, sawsall, tablesaw, skillsaw, snapping it at the marks etc..

 The least messy, fastest  was  the tablesaw but since that was in the shop and at times I needed something cut right on the location I used the sawsall most of the time.. the advantage of that was it was faster.. Instead of carefull measurements I'd hold it up, mark it with my finger, and saw away. 

(post #115228, reply #13 of 17)

I just did a room with this foam.

I used a japanese handsaw (well, a knock-off of one). The mess is minimal because the amount of side set is minimal, and it doesn't get stuck because it only cuts on the pull stroke. And it's fast.

(post #115228, reply #14 of 17)

A CO 2 laser in the power range 1 - 1.5kW would be perfect.

Or a sawzal with a knife blade or even a regular wood blade if no cutting laser is available.

(post #115228, reply #15 of 17)

We cut a lot of 2" pink foam for conditioned crawlspaces. Serrated knife leaves the least clean-up... but I will try the snap-off blades next time.

If clean-up isn't an issue, table saw is the most accurate.

Now you see this one-eyed midget
Shouting the word "NOW"
And you say, "For what reason?"
And he says, "How?"
And you say, "What does this mean?"
And he screams back, "You're a cow
Give me some milk
Or else go home"

The Village Woodworks, Inc

Chapel Hill, NC


We'll have a kid Or maybe we'll rent one He's got to be straight We don't want a bent one He'll drink his baby brew From a big brass cup Someday he may be president If things loosen up

(post #115228, reply #16 of 17)

I use a utility knife with no problem.

If you're using 2", draw or snap a line on both sides. Score both with a sharp utility knife, and snap it off. I use a sheetrock saw to cut notches, if needed.

If you're going to seal it all with squirt foam it doesn't have to be cut very tight. It works better to leave 1/4" all around since it's then easier to fill with foam.

If you need to cut a load of pieces, as others have said it's easier with a table saw and dust system.

(post #115228, reply #17 of 17)

Electric carving knife - - picked mine up at the Salvation Army store.