# Cutting foam board insulation Q????



## Danaus29 (Sep 12, 2005)

What would be the best thing to cut styrofoam 2 inch thick colsed cell insulation? Dh's thin bladed Japanese saw twists a bit and gets out of line. Could be the hand guiding it too, maybe. Anyway, is there anything that might work a bit better? I'm putting the foam board under the floor in the crawlspace. This floor is just way too cold.

Thanks in advance.


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## deerhunter5555 (Jun 21, 2007)

I know it sounds crazy but rip it down with a tablesaw. It will make a perfect cut...just make sure you set the saw outside or there will be a mess.

Erich


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## mtman (Sep 2, 2004)

i strike a line with the chalk box then use my skill saw fast and neet


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## ibcnya (Feb 16, 2004)

I put 2 inch pinkboard under the floor of my berm home and used one of those 1.99 plastic handled razor blades that you break off a blade when it gets dull. I went thtough a couple of them but they worked well.


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## mellowguy (Jul 8, 2009)

A utility/drywall/boxcutting knife works best. If you can't go through, just go 1/2" or 1" deep, and then snap the styrofoam the other way. It's a clean cut.


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## plowjockey (Aug 18, 2008)

I use one of those big aluminum "T" squares, made for cutting drywall. Makes for a nice straight, square cut. Do the cutting with the insulation setting on saw horses.

I too, use a large utility razor, with replacable blades. Simply make a straight cut into the insulation (using the T square) and then snap the insulaton down, as other's have described. If there is facing in the uncut side of the insulation after snapping, just run the knife along the inside edge.

Neat and quick.


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## Danaus29 (Sep 12, 2005)

Dh has one of those somewhere (big aluminum t square) but we haven't been able to find it for a couple years. I know we have one, we bought it when we built the shed. But I can draw a straight line (with the use of a straight edge, a long board substitutes nicely) and have the utility knife (part of a present from ds). I will try that tomorrow. If I had been able to cut straight I would have got half the crawlspace done instead of 2 sections. Not fun to fit and cut, fit and cut, even though I did measure correctly.

Thanks so much, I'll let you know how well it works for me.


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## artificer (Feb 26, 2007)

I've used the "scribe and snap" method, as well as tablesaw, jigsaw, and skillsaw methods. If you want perfectly cut, accurate dimensions, the table saw gives the best results, and is the fastest for repetitive cuts. A good jigsaw blade will give you more control for non-straight cuts.

On the other hand... our 1890's house is balloon framed, and the full dimension lumber isn't perfectly on center. What I did was intensionally cut the pieces undersized, and foamed in place with Great Stuff. The benefit is that the non-uniform areas get filled/sealed, and it bonds the foam in place.

I've tried to get a snug fit without the foam, but it alway seems to end up either too tight or too loose. For the ceiling, I drove some nails through the foam into the joists to hold it temporarily place until the foam sets.

The mail reason I like this method is that it gets close to the spray foam sealing and insulation, but MUCH less in cost.

Michael


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## Homesteadwi5 (Mar 16, 2008)

mellowguy said:


> A utility/drywall/boxcutting knife works best. If you can't go through, just go 1/2" or 1" deep, and then snap the styrofoam the other way. It's a clean cut.


Thats what we've always done too.Works great as long as the blade is fairly sharp.


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## FreightTrain (Nov 5, 2005)

a hot wire cutter. one can be easily made for pennies that operates on 12v.

here is one that uses a simple old train transformer... 
http://www.vatsaas.org/rtv/construction/hotwirecutter.asp

try searching yahoo for "homemade hot wire foam cutter"


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## The Paw (May 19, 2006)

deerhunter5555 said:


> I know it sounds crazy but rip it down with a tablesaw. It will make a perfect cut...just make sure you set the saw outside or there will be a mess.
> 
> Erich


ditto. The other methods work fine, but if you already have a tablesaw, this is fastest and most accurate. 

For tricky corners etc. I use a serrated steak knife to saw it...


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## Shawn (Apr 2, 2008)

OK, it may sound odd, but I have used a electric carving knife before to cut the foam. It works as well as a jig saw. We used to build sets for tv shows in college. Easy to handle and you can even carve shapes...


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## Hooligan (Jul 18, 2007)

I use a handsaw and a square.


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## NCLee (Aug 4, 2009)

Ditto serrated knives and electric carving knives. For big pieces I like to use a large bread knife, if I'm cutting by hand. 

Lee


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## Witterbound (Sep 4, 2007)

Why cut it? Why not just attach an entire sheet to the bottom of the floor joists?


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## fishhead (Jul 19, 2006)

I use a utility knife for the 1" foam I'm putting on my house right now. For 2" thick I would look for a knife that can be extended beyond 1". Foam cuts best with a sharp knife held at a low angle for the first pass.


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## crobar (Sep 20, 2006)

I know this sounds stupid, but I saw this on you-tube clip. Get 2"putty knife use a file to put a knife sharp edge on the side(not the flat end) of the putty kife blade, use like a knife. It cuts excellent and makes very accurate cuts. It will cut 2" foam. I tried utility knives, hot knife, various saws the putty knife works best. Keep the file handy & resharpen as needed.


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## packerfan (Aug 21, 2008)

Ditto serrated knives , I used plain bread knife, worked out good.


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## Wolf mom (Mar 8, 2005)

Awww - here I thought serated kitchen knives and electric knives to cut foam board was a woman thing. Just like repairing things with chewing gum & hair pins. Another bubble burst


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## Danaus29 (Sep 12, 2005)

Insulating the crawlspace, day 2. Well, I tried the utility knife. Worked ok until I got to one section that was 2 inches away from a premarked break. Part broke where it should, the other part was on the premarked break. Still a matter of measure a million times, cut once, then shave shave shave. The breaks don't come even and the floor joists are a bit warped and not quite centered. I thought I had more done than I did, not even one section totally finished. Well today I have 2 totally finished except a 3 inch gap in the middle, a half section finished and several end pieces done.

Why don't I just fit the full sheets under the joists? Because there is no insulation there at all, the outside isn't wrapped with a vapor barrier, and if I put it between the joists I can build up to 2 layers of foam board and cover that all with plastic wrapped fiberglass. I need to take pictures to show how it looks but this was the way I wanted to do it. 

When I went to put the first piece in place against the outer floor joist the wind coming through there was enough to blow the piece back out. I made that one a bit tighter and hammered it in place.


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## The Paw (May 19, 2006)

given what you describe, I would pre-cut the widths to one-quarter inch less than the space between the joists and use scrap bits as shims to wedge them in. Then I would run a bead of foam insulation from a spray can down the little gap.

You might still have to do some shaving around places where the joists bend more, but I bet it would be way faster....


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## Danaus29 (Sep 12, 2005)

It would be lots faster. I will try cutting them a smidge narrower. No can do with the spray foam right now, too cold. I did put some screws up to hold one board in place. That worked well as long as the holes were drilled before I put the foam board in place. Even if it's not a tight fit it would be better than the nothing we have now.

19 years in this house, you'd think we could have done this sooner. But the crawlspace has been accessible for only 3 years.


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## artificer (Feb 26, 2007)

Danaus29 said:


> It would be lots faster. I will try cutting them a smidge narrower. No can do with the spray foam right now, too cold. I did put some screws up to hold one board in place. That worked well as long as the holes were drilled before I put the foam board in place. Even if it's not a tight fit it would be better than the nothing we have now


How cold is too cold? If you warm the can to room temperature, you can dispense it in cold weather. It may take more time to cure, but the expansion of the foam should be fine.

If you are going to foam in place, I suggest cutting the foam board 3/8" -1/2" undersized. If you do that, you can get the Great Stuff nozzle into the crack, and seal the whole side of the rigid foam. If you can't get the nozzle in there, you end up with only a small bead of Great Stuff in the corner.

When I do my foaming, I run two beads of Great Stuff. First is as deep in the crack as I can. Once its fully expanded, I come back and add some more to fill the crack. Don't do a complete side at one time. The expanding foam will push the rigid foam to one side, and you will not have space for the nozzle on the second side. Foam wedges or just alternating side to side (or being really quick) works.

Michael


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## Danaus29 (Sep 12, 2005)

It was below freezing the past couple days. Just barely above freezing now. Too cold to work under there long. I should be down there now instead of lazing around here.


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## Danaus29 (Sep 12, 2005)

Crawlspace insulation project day 3. Out of 18 sections I now have 5 finished. Ds's floor is noticibly warmer close to the south wall. 11 end pieces are in. I still feel a draft in my room but not as much as before. The hardest area to reach is just about complete.


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