# 100mm Celotex rigid insulation board installation advice and tips?



## Rob Cheetham (6 Dec 2021)

Hi all. I have just finished installing 100mm rockwool into my walls and am now going onto doing the 100mm celotex board inside the roof rafters? My first couple off cuts for small spaces have not gone brilliantly. Just a few questions really.

* What is the best tool to use to cut it. Wasnt impressed with handsaw tbh. Is it possible to cut half way with part stanley knife/snap of knife then snap like plasterboard?
* How to cut and measure for a tight snug fit? There are slight differences here and there in measurements from top to bottom of rafter spaces (only minor)
* For cutting space for spotlights how deep do I cut out the recess. Im guessing i shouldnt cut all the way through. Plus i assume i should leave room for wire and conncetion.
* Some rafters have wiring clipped to the sides. How do I fit a piece of board tight if the wire is obstructing it?

Any help would be greatly appreciated as I have to be more precise with this than the rockwool and cant be affording to make many if any mistakes.

TIA


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## Jones (6 Dec 2021)

I find a hand saw ok but dusty, cut a slight bevel and if necessary shave down with a bread knife. You should not really use recessed lights with flammable foam insulation if you do maintain the clearances suggested in the light instructions which may severely compromise the insulation effectiveness.


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## Woodwoodpecka (6 Dec 2021)

I‘ve just finished doing the ceiling and walls on my garage/workshop using 50mm kingspan and found that using a handsaw after clamping a level to the kingspan worked best for me. Definitely need a mask though cos it looks like it’s been snowing after a while! Also used 40 grit sandpaper to help taper the edges inwards and also reduce/shape the edges to make them fit as there were differences all over the place with my stud work. There we’re still a few gaps though so filled them with expandable foam and then taped over the edges with aluminium tape to finish it off. Looks like the inside of a space ship now


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## RobinBHM (6 Dec 2021)

Measure each rafter void carefully, mark your celetex including any taper. Mark about 2mm or so smaller.

when cutting, do your cut with a slight taper - so the Celetex ends up smaller on one face.

once you’ve cut one or 2 you will get the feel and get a nice fit - tight enough so they need a push to go in and won’t fall out but not too tight so they break getting them in.

use a disposable hardened point saw - wax the blade frequent. Alternatively buy a proper insulation saw.

you could cut the boards 8mm small - by scoring both sides about 30mm deep with a Stanley blade, then snap the board. To get a nice fit, use a couple of screws to hold in place, then use expanding foam.

or do the job much more easily by using Actis insulation instead….no mess and 3x faster, similar U value


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## Spectric (6 Dec 2021)

You can use a celotex saw, made by Bahco Bahco PC-22-INS 22-inch Insulation Saw : Amazon.co.uk: DIY & Tools or there is a smaller folding version Bahco Folding Insulation Saw

You can also use a tracksaw but 100mm is a little thick, works on smaller sizes though and even a bandsaw but messy. 

Remember any wiring within insulation will require different calculations and wire sizes, in effect it derates a cables capacity and requires a smaller protective device.


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## clive griffiths (6 Dec 2021)

Hi Rob.
A few years ago I did a new roof on our house ,I put 150 mm in the roof and used a track saw with extraction ,cut from two sides and finish with a handsaw ,perfect. in your case 100 mm just turn the board over,
The lights I use a core drill ,with a 100 mm just drill through and glue a capping over the hole.
Clive.


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## Adam W. (6 Dec 2021)

Sharpen a long carving knife and cut it with that, if you want to avoid the dust.


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## baldkev (6 Dec 2021)

One sites, i sometimes use a circular saw, sometimes my tablesaw if its repeat widths. Its incredibly easy and accurate to cut 352mm runs and use them to space your studs / rafters / joists as you go ( for 400mm centres )
I dont angle the sides otherwise the insulation isn't working at its best. Any gaps, even small should be foamed and ideally silver foam tape applied. A surform plane is great for removing material.


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## Ollie78 (7 Dec 2021)

A breadknife works pretty good. 

Ollie


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## DBT85 (7 Dec 2021)

I was fortunate enough to use an evolution table saw to do all of mine as it was only 60mm thick. Way easier and faster than any other method I tried.


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## lostb0y (7 Dec 2021)

I tried the Bhaco insulation saw but found it was binding quite a bit as it doesn’t so much cut a channel in the material as part it so that it springs back and grips the blade. I then tried an insulation blade for my jigsaw and found that so much easier. They are approx 150mm long and formed like a bread knife. Go through it like butter and virtually no dust. I bought the festool blades from Amazon as they were the only ones I could get at short notice (£16 for 3) but there are cheaper ones from Bosch.






Festool 204345 Jigsaw Blade 3-Pieces, 155 mm Tooth Length : Amazon.co.uk: DIY & Tools


Shop Festool 204345 Jigsaw Blade 3-Pieces, 155 mm Tooth Length. Free delivery on eligible orders of £20 or more.



www.amazon.co.uk


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## Mrs C (7 Dec 2021)

I am half way through doing triple a garage roof using 140mm. The best ways that I found are using a tracksaw, then turn it over and score it with a knife if it hasn’t gone all the way through. Alternatively a jigsaw with a long blade works well. Dust control is essential. I found a handsaw with that thickness was impossible, but yours is thinner so you might be ok.

If your rafters are like ours you will never get a perfect fit. It’s better to be under and use foam than try to force it in. If you have got it really wrong (and by really it only needs to be a couple of mm under not to stay put), use little nails in the rafters to support it then use foam.

Invest in a foam gun where you have a better chance of getting the foam where you want it Instead of on your head. Also be prepared to layer the foam for larger holes as you are never quite sure how much it will expand.

if it’s any consolation you will be good at it by the time you get to the end!


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## NormanB (7 Dec 2021)

Robin Clevitt is an advocate of ‘gappotape’.


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## Stuart Moffat (7 Dec 2021)

This might generate some howls but it works really well. Before trying this method, I dad previously gone through trying handsaw and knife approaches that needed foam or other filler to plug small gaps. and during lockdown l needed to celotex my 20M x 5M workshop. I measured every slot precisely as although the. Stud centres were accurate, the noggin heights varied a bit. then the sacrilege. I then did a first pass cut with the ts55 trachsaw out in the open. You can do it on the ground or any surface as it only cuts a bit over half way through. I had previously bought a. Japanese celotex saw which was i didn’t.t get on with. But it was perfect to do the other half of the cut in seconds. A long sharp knife such as a carving knife would be just as good. You don’t need a sawing action, just pull it through as though it were a long Staley knife.
the track saw cut provides a perfect guide for the knife. The knife is narrower than the ts55 blade, so you have one side that fits just right ‘if you measured right, and the other side is a bit proud. Just push in the side that fits, and have an off cut of CLS to hand with a mallet to persuad the proud bit in. absolutely all my bits had to be tapped home with a mallet. It resulted in a totally passive building.

no festools were harmed in the making of it!


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## Jacob (7 Dec 2021)

yes to hand saw or big knife. Cut slightly oversize so it's a tight fit.
Power saw really bad with dust everywhere, and no faster.


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## Rob Cheetham (7 Dec 2021)

Jones said:


> I find a hand saw ok but dusty, cut a slight bevel and if necessary shave down with a bread knife. You should not really use recessed lights with flammable foam insulation if you do maintain the clearances suggested in the light instructions which may severely compromise the insulation effectiveness.



I have some rockwool left from the walls. If I was to cut say a 15/20cm square tile out of of the foam board and replace with rockwool for the recessed light parts woiuld this be better do you reckon or should I just follow the clearence instructions?



RobinBHM said:


> you could cut the boards 8mm small - by scoring both sides about 30mm deep with a Stanley blade, then snap the board. To get a nice fit, use a couple of screws to hold in place, then use expanding foam.



So your reckon that scorning 30mm on each side of 100mm foam board will be enough to snap it cleanly with a square edge. And with screws do you meam screwing in from the other side of the rafter. Should any screw grip the foam ok?



Spectric said:


> Remember any wiring within insulation will require different calculations and wire sizes, in effect it derates a cables capacity and requires a smaller protective device.



Could you explain this a bit more pleas not sure what you mean lol. Thanks



clive griffiths said:


> Hi Rob.
> A few years ago I did a new roof on our house ,I put 150 mm in the roof and used a track saw with extraction ,cut from two sides and finish with a handsaw ,perfect. in your case 100 mm just turn the board over,
> The lights I use a core drill ,with a 100 mm just drill through and glue a capping over the hole.
> Clive.



I have a tracksaw but to save on waste I say i need a piece 100cm high and 40cm wide then how do you run the tracksaw up to cut at just 100cm then come in of the other edge up to 40cm

With the core drill im guessing you just mean a hole drill. So drill all the way through then take a little bit of the cut piece and glue in at the top. Should I cut a channel to hold the wire and connection block too?


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## Woody2Shoes (7 Dec 2021)

Adam W. said:


> Sharpen a long carving knife and cut it with that, if you want to avoid the dust.


+1 for this method. I cut a piece to manageable oversize with an old hardpoint handsaw with small teeth, then I hold the piece up to the 'frame' I want to fit it into and use an old serrated bread knife at a slight angle, using the timber as a template to run the knife along the edge. You get a snug fit - any gaps can be filled with PU gun foam later.


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## owen (7 Dec 2021)

I've used a plastic surform before to tidy up rough edges, it works surprisingly well. Wear a mask and goggles though, it's horrible stuff


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## Rob Cheetham (7 Dec 2021)

owen said:


> I've used a plastic surform before to tidy up rough edges, it works surprisingly well. Wear a mask and goggles though, it's horrible stuff



just bought one from toolstation. and bought some bosch soft material jigsaw blades which are meant to be brilliant for PIR board. the surform will come in handy for sorting some doors ive got catching a little too so win win


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## Jones (7 Dec 2021)

Regarding clearance round recessed downlights there's a number of issues. The springs that hold the light in will need free space to move in.
The fitting even with a 5 watt LED may get hot enough to melt/ ignite insulation, be aware the insulation may shift later.
The light may get hot if covered with insulation and this will shorten it's life.
Wiring should generally be fitted above insulation so there's no risk of it overheating and its conveniently out of the way, if it's under the insulation then you may need to use thicker wire, in reality with LED lights at low wattage it's rarely necessary.
Insulation should be continous , ideally avoiding cold bridges, to work as designed . Any gaps will make a difference like having your flies undone on a cold day so try to cut to fit, and minimize use of gun foam.
With all that in mind personally I would use suface mount lighting to achieve better insulation,a safer result and an easier fit, though I know many would disagree.


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## baldkev (7 Dec 2021)

Jacob said:


> yes to hand saw or big knife. Cut slightly oversize so it's a tight fit.
> Power saw really bad with dust everywhere, and no faster.



I gotta pull you on that one, a circular saw with a guide ( i welded myself a t shape about 700mm long that fits in my saw guide holes ) is way quicker and more accurate. Its square all the way, no wander.
A table saw is even quicker again. I have a dw745 with site extractor ( yes, still some dust gets thrown about ) but its so fast and repeatable. If you are cutting over 70mm, you flip the sheet and run again, perfect cut.... i usually aim for 1mm under and its always a tight fit


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## Jacob (7 Dec 2021)

baldkev said:


> I gotta pull you on that one, a circular saw with a guide ( i welded myself a t shape about 700mm long that fits in my saw guide holes ) is way quicker and more accurate. Its square all the way, no wander.
> A table saw is even quicker again. I have a dw745 with site extractor ( yes, still some dust gets thrown about ) but its so fast and repeatable. If you are cutting over 70mm, you flip the sheet and run again, perfect cut.... i usually aim for 1mm under and its always a tight fit


Tried both but settle for the 26" handsaw mainly because of the dust, but in fact more convenient and no slower (like hot knife through butter etc) and plenty accurate enough to a felt tip line from straightedge.


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## Stuart Moffat (7 Dec 2021)

By the time you have drawn your felt tip line Jacob, the rest of us will have already made a guaranteed straight cut, and it won’t have small air leaks that need taping or foaming.


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## Jacob (7 Dec 2021)

Stuart Moffat said:


> By the time you have drawn your felt tip line Jacob, the rest of us will have already made a guaranteed straight cut, and it won’t have small air leaks that need taping or foaming.


Only if the spaces between joists/rafters are perfectly straight to start with, and you are close to your TS with a 4" depth of cut.
With a 26" hand saw its only two or three strokes through 48" of 4" celotex board. If you are working your way through attic rooms moving the TS would take up half the time as well as needing a dust extractor.
My TS would need four blokes to shift it and have to be dismantled to get it through doors. Then there'd be the dust extractor.....


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## Stuart Moffat (7 Dec 2021)

Then you have a very weird track saw! and you have rather ignored the OP‘s opening comment that he wasn’t impressed with a handsaw and was looking for something better.


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## baldkev (7 Dec 2021)

Jacob said:


> Only if the spaces between joists/rafters are perfectly straight to start with, and you are close to your TS with a 4" depth of cut.
> With a 26" hand saw its only two or three strokes through 48" of 4" celotex board. If you are working your way through attic rooms moving the TS would take up half the time as well as needing a dust extractor.
> My TS would need four blokes to shift it and have to be dismantled to get it through doors. Then there'd be the dust extractor.....



2 or 3 strokes? What do you cut with? A lightsabre? Id love to see a video of that....
But no need to move the ts once set up, i generally take the materials to the saw


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## Jacob (7 Dec 2021)

baldkev said:


> 2 or 3 strokes? What do you cut with? A lightsabre? Id love to see a video of that....
> But no need to move the ts once set up, i generally take the materials to the saw


Have tried cutting Celotex with a hand saw? It's really easy. Big 26" hand saw, not a back saw or a fret saw.
Chapel conversion. 3 floors. Taking material to the saw would have involved hundreds of journeys to and from the saw, not to mention the noise, dust, size of sheets. Taking saw to material really easy.


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## baldkev (7 Dec 2021)

Jacob said:


> Have tried cutting Celotex with a hand saw? It's really easy. Big 26" hand saw



I have, many times, but never once did i manage to rip a sheet of celotex in 3 strokes.... it does ( for me ) take quite a few strokes and a bit of care, to cut a slice of celotex straight and square..... but now ive moved on and use my site tablesaw. I applaud your use of handtools etc, but there are other ways too and some are quicker, some are more accurate and some are quicker and more accurate...... for a site carpenter now, you have to move quickly but still with good accuracy to earn money.

Gone are the days of taking years to build houses with handtools and lots of blokes. Even during my 24/25 years, things have advanced dramatically. Even down to regularised 4x2s etc speeding stuff up. Lots of change, embrace it


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## Tazio (8 Dec 2021)

Try the festool insulation saw brilliant bit of kit does require building a cutting bench though to get the best out of it !


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## Jacob (8 Dec 2021)

baldkev said:


> I have, many times, but never once did i manage to rip a sheet of celotex in 3 strokes.... it does ( for me ) take quite a few strokes and a bit of care, to cut a slice of celotex straight and square..... but now ive moved on and use my site tablesaw. I applaud your use of handtools etc, but there are other ways too and some are quicker, some are more accurate and some are quicker and more accurate...... for a site carpenter now, you have to move quickly but still with good accuracy to earn money.
> 
> Gone are the days of taking years to build houses with handtools and lots of blokes. Even during my 24/25 years, things have advanced dramatically. Even down to regularised 4x2s etc speeding stuff up. Lots of change, embrace it


Taking 8x4 sheets to a table saw would take a big saw and ideally two blokes. Carrying the pieces around the building would need a little team especially if there were stairs and passages. Much easier to take the saw to the material, over a couple of saw horses. Much easier to saw with a long hand saw (for the reach and depth) than manoeuvering a much heavier track saw and the necessary dust extraction kit, for each cut. Can you get 4" tracksaws?


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## owen (8 Dec 2021)

Jacob said:


> Taking 8x4 sheets to a table saw would take a big saw and ideally two blokes. Carrying the pieces around the building would need a little team especially if there were stairs and passages. Much easier to take the saw to the material, over a couple of saw horses. Much easier to saw with a long hand saw (for the reach and depth) than manoeuvering a much heavier track saw and the necessary dust extraction kit, for each cut. Can you get 4" tracksaws?



A tracksaw helps, if you cut the first 50mm with a tracksaw then cut right through with a handsaw you do get a straighter cut. The labourers normally get this job with us, so trying to get them to cut in a straight line is hard work. We find the extra time spent using the tracksaw is worth it, compared to the time spent trying to straighten cuts afterwards.it also saves a lot of dust flying around.


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## Jacob (8 Dec 2021)

owen said:


> A tracksaw helps, if you cut the first 50mm with a tracksaw then cut right through with a handsaw you do get a straighter cut. The labourers normally get this job with us, so trying to get them to cut in a straight line is hard work. We find the extra time spent using the tracksaw is worth it, compared to the time spent trying to straighten cuts afterwards.it also saves a lot of dust flying around.


Seems a lot of bother, cutting twice a heavy electric saw and a hand saw. More practice with the hand saw for a straighter cut? Certainly be a lot faster.
I seem to recall cutting to the line or slightly over but undercutting a very slight bevel, so the piece would then be a tight fit wedged between rafters


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## owen (8 Dec 2021)

As for moving it around, you cut on the stack of boards so you're actually moving materials less.


Jacob said:


> Seems a lot of bother, cutting twice a heavy electric saw and a hand saw. More practice with the hand saw for a straighter cut? Certainly be a lot faster.
> I seem to recall cutting to the line or slightly over but undercutting a very slight bevel, so the piece would then be a tight fit wedged between rafters



Tracksaws are not heavy, don't know where youve got that from. Cutting a bevel makes it even more work because then you have to flip the board around for the next cut and cutting a straight bevel cut in this stuff isn't very easy. Honestly, if we had a room of 50mm kingspan to do in-between studwork, I could cut all of it with a tracksaw in the time it would take you to do half with a handsaw. I do the long cuts with the tracksaw, normally 355 on 400mm center studs, cut a load of pieces that size one after another then take the pieces to the wall and cut them to length with a handsaw. I've done it both ways, lots of times, and tracksaw always wins for me.


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## owen (8 Dec 2021)

Another thing you're forgetting is marking out, by the time you mark either end, drop a straight edge on and mark a line to cut, you may aswell just mark either end drop the track on and make the cut


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## Jacob (8 Dec 2021)

Just found a bit of 3" kingspan in the workshop. Couldn't resist temptation so did a trial cut with a 26" hand saw. It cuts 4" on the backstroke and 12" on the fore stroke. So that'd be 3 full strokes through a 48" board. In about 4 seconds. Pretty much what I remembered. 
No noise, very little airborne dust but a lot of crumbs.
Can be as straight as you want - some may need a little practice!
I've got a 28" rip saw which I guess would be even faster.


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## owen (8 Dec 2021)

Jacob said:


> Just found a bit of 3" kingspan in the workshop. Couldn't resist temptation so did a trial cut with a 26" hand saw. It cuts 4" on the backstroke and 12" on the fore stroke. So that'd be 3 full strokes through a 48" board. In about 4 seconds. Pretty much what I remembered.
> No noise, very little airborne dust but a lot of crumbs.
> Can be as straight as you want - some may need a little practice!
> I've got a 28" rip saw which I guess would be even faster.



I've just cut a piece of 50mm kingspan, cuts in approx 1/2 a second with a tracksaw and no dust at all. You're stuck in the stone age.


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## Jacob (8 Dec 2021)

owen said:


> I've just cut a piece of 50mm kingspan, cuts in approx 1/2 a second with a tracksaw and no dust at all. You're stuck in the stone age.


You are stuck in the expensive unnecessary gadget age! 
Have you got a few "systainers" too? They used to be called "boxes" when ar worra lad.


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## owen (8 Dec 2021)

Jacob said:


> You are stuck in the expensive unnecessary gadget age!
> Have you got a few "systainers" too? They used to be called "boxes" when ar worra lad.



 Sure, if I didn't have my tracksaw, the amount of work I'd get done everyday would more than half. I do have one systainer, it's the one the saw got delivered in and hasn't been put back in since .


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## Lefley (8 Dec 2021)

Mrs C said:


> I am half way through doing triple a garage roof using 140mm. The best ways that I found are using a tracksaw, then turn it over and score it with a knife if it hasn’t gone all the way through. Alternatively a jigsaw with a long blade works well. Dust control is essential. I found a handsaw with that thickness was impossible, but yours is thinner so you might be ok.
> 
> If your rafters are like ours you will never get a perfect fit. It’s better to be under and use foam than try to force it in. If you have got it really wrong (and by really it only needs to be a couple of mm under not to stay put), use little nails in the rafters to support it then use foam.
> 
> ...


We do a lot of net zero houses in Canada. 300-400 mm foam insulation screwed to the roof after cross strapped with 1x4 ( inch) then metal roofing. We only use a handsaw to cut. And it’s bloody hard putting a 375 to 475 mm screw through insulation and hitting a 1 1/2 ( inch) wide truss,


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## Denty (8 Dec 2021)

NormanB said:


> Robin Clevitt is an advocate of ‘gappotape’.


Clevett doesn’t have to pay for it thats why he uses it…..
just use a long Bosch jigsaw blade.
any gaps foam them trim it off once dried and silver tape it.


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## Rob Cheetham (8 Dec 2021)

Denty said:


> Clevett doesn’t have to pay for it thats why he uses it…..
> just use a long Bosch jigsaw blade.
> any gaps foam them trim it off once dried and silver tape it.



Just got them soft material bosch blades of amazon. They are massive. I have read reviews thought that they can wonder on the bottom edge. One review said to angle grind one a bit shorter so it just protrudes the bottom of the 100mm board. Im not sure but thinkingI will defo use a straight edge clamped to the board to run the jigsaw along. May help keepit steady. Also got one of them surform block planes to shave off little bits to make fit. 

What are your thoughts on them?


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## NormanB (9 Dec 2021)

Denty - Yes, but Clevitt does quality work.


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## JobandKnock (9 Dec 2021)

Rob Cheetham said:


> Im not sure but thinkingI will defo use a straight edge clamped to the board to run the jigsaw along. May help keepit steady.


Tried jigsaws a while back when we had some wierd recycled wood soiund insulation from Germany - the blades wander too weasily and they make quite a bit of mess. There's a little trick we show the apprentices to cut the stuff with a hand saw when they are starting - drop a 4 x 2 onto the face of the material and use it as a guide to cut to about 2in deep. Flip it up on its' edge then use the deeper guide to saw the whole way through. A bit slower than making a single cut, but can be done in situ somewhere like an attic and most importantly it delivers a *square cut*. For my own use I have a wavy blade insulation saw {sorry, Jacob) which cuts with less mess and less wander than a standard hand saw does. On thinner materials I tend to use an insulation knife rather than a saw (looks like a bread knife, but isn't) which is also handy for doing trimming cuts around pipes, etc. Can't say that I could justify lugging power tools, benches, etc around unless I was doing a heck of a lot of insulation.



Rob Cheetham said:


> Also got one of them surform block planes to shave off little bits to make fit.


Have you actually _tried _a Surform on Cellotex/Kingspan? Unless the blade is brand new it won't cut and even then it probably won't cut that well. If you need to shave bits off a hand saw will do the task quickly and efficiently, but ideally you want to avoid this

I'm lothe to admit this, but Jacob is probably right when he says that hand tools make complete sense doing this job unless you are insulating a sports stadium... With insulation you are often working in confined spaces with limited ventilation so the very last thing you want to do is make loads of dust - and even hand sawing PUR insulation makes loads of airborne dust - so there are other reasons for not going "high tech" (other than the £600 odd bill for a Festool)


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## owen (9 Dec 2021)

JobandKnock said:


> Have you actually _tried _a Surform on Cellotex/Kingspan? Unless the blade is brand new it won't cut and even then it probably won't cut that well. If you need to shave bits off a hand saw will do the task quickly and efficiently, but ideally you want to avoid this



Have you actually tried a surform on it? They work a lot better than you'd imagine, far better than shaving bits off with a handsaw, handsaws tend to rip the silver foil face too.


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## Denty (9 Dec 2021)

NormanB said:


> Denty - Yes, but Clevitt does quality work.


Oh ok


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## Denty (9 Dec 2021)

Rob Cheetham said:


> Just got them soft material bosch blades of amazon. They are massive. I have read reviews thought that they can wonder on the bottom edge. One review said to angle grind one a bit shorter so it just protrudes the bottom of the 100mm board. Im not sure but thinkingI will defo use a straight edge clamped to the board to run the jigsaw along. May help keepit steady. Also got one of them surform block planes to shave off little bits to make fit.
> 
> What are your thoughts on them?


Never had a problem with them myself, yes we use a straight edge has well.
if you’re ok with a saw using a straight edge will get you the same result.
never used a surform but I would expect a lot of dust and if I’m honest I would only do it outside unless you have a very good face fit mask.


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## Spectric (9 Dec 2021)

Rob Cheetham said:


> Could you explain this a bit more pleas not sure what you mean lol. Thanks



With electrical wiring during the design you have to derate a cable if it is run within insulation, the opposite is true if it is surface mounted or run on a cable tray. In the regs you have tables that give you correction factors for several parameters, so you may end up with a 4mm cable and then due to corrections, ie it is within insulation then you may need 6mm.

Table 6B in the site guide will give derating values, best option is not to run cables within insulation.
,


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## Jacob (9 Dec 2021)

JobandKnock said:


> ,,,,,,,,,,,,. For my own use I have a wavy blade insulation saw {sorry, Jacob) which cuts with less mess and less wander than a standard hand saw does.


Didn't know they existed, looks a good idea but I found a handsaw perfectly OK


> .....
> Have you actually _tried _a Surform on Cellotex/Kingspan?


Yes, they don't work.


> If you need to shave bits off a hand saw will do the task quickly and efficiently, but ideally you want to avoid this


Everybody makes mistakes - not least because rafters or ceiling joists may not be as straight as thought. Hand saw again , or bread knife


> I'm lothe to admit this, but Jacob is probably right


Gets said surprisingly often 


> when he says that hand tools make complete sense doing this job unless you are insulating a sports stadium... With insulation you are often working in confined spaces with limited ventilation so the very last thing you want to do is make loads of dust - and even hand sawing PUR insulation makes loads of airborne dust - so there are other reasons for not going "high tech" (other than the £600 odd bill for a Festool)


Track saws obviously a good idea but not for everything. Overkill for insulation, like cutting weetabix with an axe!


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## Stuart Moffat (9 Dec 2021)

You are going to keep saying that tracksaws are no good for this until you have the last word I guess. But it doesn’t make it any truer than the first time you said it.


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## JobandKnock (9 Dec 2021)

owen said:


> Have you actually tried a surform on it? They work a lot better than you'd imagine, far better than shaving bits off with a handsaw, handsaws tend to rip the silver foil face too.


I rarely need to shave them to get a good fit, and Surforms create dust which when working in confined spaces is a major issue. And in any case a Surform is just another piece of (relatively useless) kit to drag around which will rarely get used - something which you become sensitive to when you spend your life dragging tool kits around buildings


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## owen (9 Dec 2021)

Surforms DO work, and work well on it for straightening out dodgy cuts.


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## owen (9 Dec 2021)

JobandKnock said:


> I rarely need to shave them to get a good fit, and Surforms create dust which when working in confined spaces is a major issue. And in any case a Surform is just another piece of (relatively useless) kit to drag around which will rarely get used - something which you become sensitive to when you spend your life dragging tool kits around buildings



 It's hardly a heavy bit of kit


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## JobandKnock (9 Dec 2021)

Jacob said:


> Gets said surprisingly often


It was written tongue in cheek - you know where I'm coming from, I'm sure



Jacob said:


> Track saws obviously a good idea but not for everything. Overkill for insulation, like cutting weetabix with an axe!


Yes. A lot of solutions here which seem to be theory without practical experience. When you are putting in insulation and fire stopping yoiu are often working in very confined spaces with poor lighting and minimal ventilation. That makes dust suppression a number one priority, so you wear an FFP3 mask. Some insulation is intensely irritating on the skin (e.g. older mineral woold) and most of it is a respiratory irritant (including PUR type materials) so you want to avoid shaving bits off, etc and generally making dust where you can as it will contiminate your clothing (and eventually your van), Cutting on a table is all well and good, but at least for some of the time would involve having two guys doing the job - wasteful of man power - or constantly climbing up and down all day long - which be becomes really tiring and a potential accident risk. I can see a use for the Festool saw in some trade environments where you are doing mass installation of, say inner stud wall insulation on a housing estate, but it just isn't worth it for many jobs


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## owen (9 Dec 2021)

I guess it's comes down to....

Tracksaws cut faster more accurately and are very much dust free, can be cut on the stack to avoid moving boards around. 

Handsaw cut slower, require boards marking out with a straight edge are very dusty, and require boards being repositioned for each cut. 

I know which works best for me, I cut hundreds of sheets each year not just enough for insulating a garden shed.


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## JobandKnock (9 Dec 2021)

owen said:


> It's hardly a heavy bit of kit


It's weight _*plus*_ volume which are the issues, which anyone on site based work should understand. TBH in site joinery work a Surform is almost useless for me and many others - the only time you see them is when it's necessary to board out (plasterboard) and even there Marshalltown make far better alternative. 

You have your way of doing things - it doesn't work for me, nor seemingly the many people who've worked with or for me over many years


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## Jacob (9 Dec 2021)

Stuart Moffat said:


> You are going to keep saying that tracksaws are no good for this until you have the last word I guess. But it doesn’t make it any truer than the first time you said it.


No it hasn't got any truer it's still just as true as the first time I said it. I didn't say "they are no good" I said it's a lot easier, quieter and less dusty with a hand saw.
And you only have to cut once! The idea that you cut half way with a track saw and then finish with a hand saw seems to be particularly pointless.
Then there's the dust extraction, extensions leads, humping them from room to room etc.


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## owen (9 Dec 2021)

JobandKnock said:


> It's weight _*plus*_ volume which are the issues, which anyone on site based work should understand. TBH in site joinery work a Surform is almost useless for me and many others - the only time you see them is when it's necessary to board out (plasterboard) and even there Marshalltown make far better alternative.
> 
> You have your way of doing things - it doesn't work for me, nor seemingly the many people who've worked with or for me over many years



I do understand, but my site work is based on property renovation of larger properties than new builds so I guess I spend more time in one place than most. Yes those are the two things I use a surform for, tidying up plasterboard and tidying up kingspan if needed. Sure I can work without it, but having it makes life easier for me atleast.


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## owen (9 Dec 2021)

Jacob said:


> No it hasn't got any truer it's still just as true as the first time I said it. I didn't say "they are no good" I said it's a lot easier, quieter and less dusty with a hand saw.
> And you only have to cut once! The idea that you cut half way with a track saw and then finish with a hand saw seems to be particularly pointless.
> Then there's the dust extraction, extensions leads, humping them from room to room etc.



Less dusty? You've obviously never ever ever used a tracksaw with dust extraction to cut insulation if you think they're less dusty.


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## Hanman-Tools (9 Dec 2021)

I have always used a wavy edged bread knife with a straight edge, very little mess and doing it all the time it is quite accurate.


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## Jacob (9 Dec 2021)

owen said:


> Less dusty? You've obviously never ever ever used a tracksaw with dust extraction to cut insulation if you think they're less dusty.


Oh all right  . To put it another way; you don't need dust extraction with a hand saw. Though frequent sweeping and hoovering would be good - you get a bit of dust but most of it is crumbs. Not having a motorised tool blasting the stuff about is helpful.
It's not a competition Owen you can do it how you like. I'm just saying how I used you do it and why.


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## owen (9 Dec 2021)

Jacob said:


> Oh all right  . To put it another way; you don't need dust extraction with a hand saw. Though frequent sweeping and hoovering would be good - you get a bit of dust but most of it is crumbs. Not having a motorised tool blasting the stuff about is helpful




Thats a better way of putting it, you don't need dust extraction with a handsaw because the dust just goes everywhere, with a tracksaw and dust extraction the dust is removed at source.


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## Jacob (9 Dec 2021)

owen said:


> Thats a better way of putting it, you don't need dust extraction with a handsaw because the dust just goes everywhere, with a tracksaw and dust extraction the dust is removed at source.


Right ho Owen if you say so.


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## Jake (10 Dec 2021)

Hanman-Tools said:


> I have always used a wavy edged bread knife with a straight edge, very little mess and doing it all the time it is quite accurate.



They lose usefulness at 100mm depth of insulation or so but that's the point of the Bahco type insulation saws, basically wavy-saw-knives, none (or at least a tiny fraction) of that awful dust. Bit more effort to cut, but so much less objectionable overall.


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## RobinBHM (10 Dec 2021)

tracksaws do work on celetex very well.

and I’d say quicker than a handsaw and less dusty.


I know quite a few builders that have swapped to using actis. insulation for in between studwork or joists.


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## highwood122 (10 Dec 2021)

bought about 3k s worth of 100mm celotex, ok for under floor screed but when i came to do in between rafters it had developed a 30mm to 50mm bow across the width, had to put retaining battens behind, cut to a loose fit,then screw battens across the front and foam in with foam gun. i have other makes of insulation that have stayed flat for years so i know its not the way i stored it.


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## pm1961 (29 Dec 2021)

Once you've gone "track"......... you'll never go back! 

I stick with 50mm and a normal tracksaw.... If it needs 100mm, then double up.... Not much difference in price and not much longer to do it, but perfectly square in one cut with "normal" tools already owned. 

Obviously not for everyone. Whatever works for you. I'm coming from a Self Build and DIY background where I get to choose what I use and how much time and money I want to spend doing it............. 

Anyway, about using a track saw for plasterboard..................


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