Use insulated board or battens, insulation between then PB

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Just looking at cost of insulated plasterboard, surprised in the cost difference. Guess there is a time saving though on a ceiling it may make it more difficult.

120mm celotex in between 200mm rafters on low pitch vaulted ceiling. Choice is either 37.5mm insulated PB over top (25mm insulation) or 25mm cross batten the rafters and fit 25mm celotex between, tape over battens to celotex making the foil of the celotex the VCL (or staple a plastic sheet over everything) Doesn't totally eliminate cold bridging but it's a very viable option.

Heard it said the battering method can reduce later movement cracks in the plasterboard although whether it's true I don't know.

Best I've seen 37.5mm insulated PB sheet is £31.50 (inc vat). Insulation on it's own £11.99 (25mm) a sheet of pb 12 or 9mm about £7.50 + battens probably £2 worth a sheet? so £21.49 plus the foil tape probably adds £1 or plastic sheeting. So at worst a £9 difference, on a 34m2 ceiling it's £300.

What do you guys choose in these instances and for which reasons?

Thanks
 
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Why batten at all? I believe s common approach is to overboard the rafters with the 25mm and then simply slap the plasterboard over the top (well, bottom). It also means the 25mm celotex gives you a nice easy to tape surface for your VCL.

It's also supposed to be easier to manhandle the celotex and plasterboard separately than insulated pb.
 
Yes that is another option IT Minion. I added battens as told it minimised pb movement cracks late,r also thought might prompt agree/disagree comments from those on here who do this type of job more often. Guess it's harder to find/follow joists (or battens) to screw PB into if covered with a second layer of separate celotex but negates cost of battens and time to fit.
 
Well that was traumatic finding a supplier of PIR insulation, tavis perkins shut, my usual Jewsons local branch shut and others only supplying essential jobs, nobody opening new accounts, online companies open one day closed the next, cheapest but giving concern to paying for something I'd never get Sparky friend had local Build merchant and it's gone on account for delivery next week through him.

Any one heard if cross battening then 25mm PIR between and plasterboard screwed to battens mean less risk of pb screw heads popping? thats apparently why my brothers builder is doing, reckons the flax in PIR can pop heads?
 
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I've done 100mm on a ceiling, 6" plasterboard screws, 12.5mm plasterboard - no problems after 2 years. Obviously stagger all joints and make sure celotex is butted up tightly. Previously done loft conversion with 50mm, no problems. I doubt you'll have any issues with 25mm.
 
Battening concept seemed reasonable in theory but the cold bridge is still there at timber to timber points, I'll go without battens and carefully mark all the joist centres as I go so when boarding I miss fewer of them!
 
Yes skipping the insulated PB for sure Woody. The batten thing I mentioned was 25mm cross battening inside, then fit 25mm celotex between them, then board, screwing PB to battens rather than through celotex to rafters, on the suggestion that fixing to battens had less flex than celotex so less likely to pop screw heads, I thought popped heads were when PB wasn't pushed up tight when screwing (steady, no jokes there please)
 
It's also supposed to be easier to manhandle the celotex and plasterboard separately than insulated pb.
Not supposed - it is easier, trust me. And more efficient and less prone to cracking.

The benefit is that the underdraw layer of Celotex fixed across the battens, need not have the edges land on any rafters/joists. With this in mind, you can plan the ceiling so that the joints of the plasterboard and the Celotex beneath are fully bonded and never coincide.
It also means that you can be slightly short of a joist with the plasterboard edge and still screw into said joist by skewing the fixings, because the underlayer of insulation will support it. You will need decent length fixings though.

Separates for me, win all round.
 
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If yr rafters are fairly even then screwing through the celotex is fine (bit less torque than plasterboard). If you hit problems, penny washers. If you hit more problems, penny washers and big square washers.
Don't try and be cheap (like me) and make your own penny washers out of 2p pieces- unless you get the hole dead centre the washer will cut in as badly as a bare screw!
 
Washers the size of modern pennies wouldn't be any use & I wasn't about to set about drilling holes in my pre decimal penny collection :)
 

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