100mm Celotex rigid insulation board installation advice and tips?

[Rob Cheetham]

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

 

[Jones]

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.

 

[Woodwoodpecka]

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

 

[RobinBHM]

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

 

[Spectric]

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.

 

[clive griffiths]

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.

 

[Adam W.]

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

 

[baldkev]

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.

 

[Ollie78]

A breadknife works pretty good.

Ollie

 

[DBT85]

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.

 

[lostb0y]

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.

https://www.amazon.co.uk/dp/B07M6PK..._dp_E2H3DKPRJGXTXCDXA5MA?_encoding=UTF8&psc=1

 

[Mrs C]

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!

 

[NormanB]

Robin Clevitt is an advocate of ‘gappotape’.

 

[Stuart Moffat]

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!

 

[Jacob]

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.

 

[Rob Cheetham]

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?

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?

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

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?

 

[Woody2Shoes]

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.

 

[owen]

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

 

[Rob Cheetham]

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

 

[Jones]

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.