Cutting Celotex insulating foam

Hi All
I'm insulating the roof of my garage conversion with Celotex, between and below the rafters but I'm having a bit of a job cutting the Celotex accurately.
The manufacturers recommend a trimming knife or a fine tooth saw, does anyone have any experience of cutting this stuff.
I'm using 50mm thick between the rafters so in order to cut it with a craft knife I'm using one of the snap off blade types with a blade fully extended, this reaches nearly right through the Celotex but because the blade is quite thin it is incline to wander from the straight line in the depth of the material. I tried a sharp kitchen knife which is better but because the blade is thicker the Celotex grips it making it difficult to pull through.
I'd rather not use a saw because of the toxic dust.
Is there a technique to cutting this stuff that I need to learn?
Any advice gratefully received
Thanks Roger
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craft
extended,
the
through.
A serrated steak knife is usually a good cutter for this stuff. The steak knife doesn't create to much dust, but it's always wise to wear some kind of protection against breathing any kind of dust particles, especially in confined spaces. It's also a good idea to have a vacuum cleaner running in the background, even an upright lying on its back, to catch anything that escapes from around the cut.
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<Snip>
An occasional smear of margarine or wax on the blade makes for easier cutting and less dust.
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On Tue, 16 Nov 2004 19:26:46 -0000, "Roger"

Yes, buy a woodsaw. Works fine here.
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An old serrated bread knife works well, as long as the blade isn't too bendy.
Rick
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On Tue, 16 Nov 2004 19:26:46 -0000, Roger wrote:

25mm bonded to 9mm plasterboard snaps very easyly. Not tried it on 50mm unbonded but I suspect a good deepish cut from a craft knife then putting the cut above a batten and pushing down would have the desired effect. A bit like cutting glass.

Is it toxic or just an irritant? Can't be worse than the dust that comes off fibre glass. As has alreday been said a vacum running will stop dust levels getting too high, very good at keeping dust levels to light mist instead of thick fog when sanding filler/plaster down...
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On Tue, 16 Nov 2004 21:15:38 +0000 (GMT), "Dave Liquorice"

Sadly not quite... The thicker boards have glass fibre threads in them for strength. You can snap to an extent but unless you cut almost through then no. The threads will pull out lumps and make a mess of the edges.

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On Tue, 16 Nov 2004 23:18:30 +0000, Andy Hall wrote:

My new thing for today, won't get caught now. B-)
Can you not open the cut over the batten and slice the threads or are they random through the body rather than descrete layers of open weave matting?
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On Wed, 17 Nov 2004 14:15:17 +0000 (GMT), "Dave Liquorice"

They seem to be pretty random fibres. Not very thick, but enough to mean that snapping doesn't work all that well if you're looking for a clean edge.
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Andy Hall wrote:

Must admit to not finding any threads in the PIR foam I used (made by Ecotherm)... must be something that varies with manufacturer. Having said that, snapping would still not work that well due the material not breaking cleanly anyway.
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On Tue, 16 Nov 2004 19:26:46 -0000, "Roger"

I used a cheap old portable table saw bought from somebody locally and then thrown away. It was used outside and together with a shop vacuum to keep the dust down, but I used a mask as well. This is a fast and accurate way.
Alternatively a cheap circular saw with dust hose hook up would work.
For smaller pieces and trrimming shapes, I used an old serrated bread knife with a sawing action.
If you can find a secondhand table saw for about 30 then the time saved will make it worthwhile. I have the teeshirt.
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Roger wrote:

I did most of mine with a jigsaw (agressive blade, pendulum action on full). Quite easy to do one handed as well. A knife can be useful for final trimming at times. The dust created is not that bad (don't think it is actully toxic anyway - although it will irritate if you get it in your eyes).
The foil backed stuff will knacker blades given time, but a pack of five jigsaw blades is cheap enough....
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Roger wrote:

Actually I used many things, frm a woodsaw, to a bread knife.
The dust is highhly iritating, but not toxic I think. Wear a mask.

Fine toothed saw or ultra sharp kitchen knife.

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We use a jigsaw fitted with an old blade that has had the teeth ground off to form a knife edge. I think you can actually buy them too.
--
mark

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I've just had delivered 9 x 8x4 sheets of the Celotex delivered but its 90mm thick, sounds like I'm doing to have fun with it ! Definitely a saw and mask job I think.
Peter
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Buy a new woodsaw. Biggest problem with thick sheets (I use 120mm a lot) is keeping the cut parallel. Sometimes have to revert to a bit of scrapeing away afterwards.
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Roger

How about those electric carving knifes?!?
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Thanks for some great advice everyone, very useful as usual. What a great newsgroup this is.
Rgds Roger
Roger

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On Tue, 16 Nov 2004 19:26:46 -0000, "Roger"

I cut it roughly, put one side in against the rafter so the other is too big, and then using a bread knife, cut it using the other rafter as a guide. Wack it home with a quick slap.
Rick
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