hi I don't understand my building reg drawings on Celotex. I assume that I place 50mm celotex between my 100mm rafters and then leave a 50mm gap above. This way they are flush with rafters. Then he specs 12.5mm foil back plasterboard that I lay straight on to rafters sitting flush with cellotex. But the drawings mention batterning out. Have attached as it's confusing please help.
According to the specs it says another 70mm sheet of celotex to be added under the rafters, and because its over 50mm recommends 25mm battening. Haven't got a celotex account, but looking at Kingspan, there is a 82.5mm laminated plasterboard.
Hi thanks I still don't understand that as batterning 25mm on rafter another 70mm sheet id have to battern out massively wouldn't I? I thought maybe the 25mm was an air gap between the 50mm celotex and the plasterboard maybe? Or 70mm below roof which would mean the 50mm celotex would stick out from 100mm rafters by 20mm and I battern the bottom of the rafters by 20-25mm?
Doesn't say anywhere on the celotex website about leaving a 25mm gap. May have to ring Celotex technical helpline. Another 70mm sheet has been specified though.
PIR boards, of which Celotex is one, get stuck between the rafters and finish flush to the bottom, inside face of the rafters - the gap is left in the hidden inside space. Some people bang a nail or 2 in at the stop point of the rafters, though if you cut the board right they friction fit into place. You then finish by using an insulated plasterboard with an in built VCL fitted to the inside face of the rafters. If no VCL, you need to install one prior to fitting the insulated plaster board.
Hi thanks that does make sense now so the gap doesn't have to be 25 exact as I've a load of smaller size roof battern left over from dormer cheeks I could use. Is there a particular fixing of the celotex board on the unde side? And then sub subsequent plasterboard? Or just long plasterboard screws ?
In the drawings it does not spec size for in between collar and Ashlar. If rafters are a total of 120 should they be the same? Collars and Ashlar are 4x2" . Thanks
Ok rang celotex. They said that the 70mm GA4000 would go directly over the underside of the rafters, joints can be taped using foil tape (celotex), this would suffice as the VCL or alternatively some people place a sheet of polythene of to act as the VCL, you could then avoid using foil backed plasterboard. They do an insulated plasterboard, but is only 60mm thick. The purpose of the battens is for fixing the plasterboard sheets.
Thanks for your help makes sense. Is there any reason why I could not use pl400 say 72.5mm dry line board as the under sheet to avoid batterning? Also would I have to do the same with Ashlar and collar infills? Thanks
I think I would be looking at using insulated plasterboard too, to avoid having to batten out. Did the architect provide any sectional drawings? It may be a good idea to contact BC to advise on change of specs too if going for the insulated plasterboard. Only going from my plan but I have been advised to use kingspan K18 82.5mm on top over studs with 100mm KS insulation.
Yes I contacted building control and he came back with 80mm underneath then battern then plasterboard in terms of what he'd expect to see. He didn't mention collar and Ashlar insulation though. I guess the weight of the dry line board at an angle rather than on the side walls is an issue hence using batterned fixed through the 80mm. My BC drawings are not very good to be honest. I don't really want to lower the ceiling any more by putting boards on other than plasterboard on them. The rafter insulation thickness equals 130 in total so was thinking of 130 solid board inbetween them?
There are other products that can be used for insulation such as super quilt or multi layer insulation to save on head room. Looks much easier to fit using staple gun.
Have emailed them to see what combination of PIR and quilt will be based on my spec. Will let you know! Thanks.
you have the 50mm correct and it's fitting correct. But once this has been done you with have a relatively flush ceiling of celotex and rafters. You then add another layer of 70mm Celotex that covers both the bottom of the rafters and the 50mm Celotex between them. the battening then goes over the celotex to provide a cavity between the thick layer of celotex and the plasterboard. To stop any condensation and damp ruining the plasterboard. this is why they suggest foil packed plasterboard and the two air gaps. 50mm between top of celotex and tiles and felt. Batten gap between bottom of celotex and plaster board. if the airflow is correct you won't need foil back plasterboard. sometimes you can getaway with less celotex it depends on how strict the building control officer is. Of course this will have a negative effect on your heat loss and heating bills. Always best to over engineer in my opinion.
The cavity created by the battens is an integral part of the thermal resistance calculation that yields the "U" value. My experience is that BCs often wrongly query the amount of Celotex (or Quintherm or Recticell or.....) and a printout from the on line calculator is required to justify what is proposed. BC are unlikely to allow a reduction for no good reason but equally are usually very sensible when there are good reasons for NOT being able to provide the proper thickness The battens are also a whole lot easier to hold in place while you search out the hidden rafter with a long screw!!
To keep the maximum roof height I've just been in Celotex u values calculator. The pitched roof should be .18. If I battern out the rafter by 25mm I could have 70pir board the underlay with 60mmthetmal PIR which adds12.5 totalling to 82.5 below actual rafters. This would mean no air gap between boards but I have a 50mm air gap above them. This creates the u value which I assume is what BC are interested in? Would this work? I'm also waiting to hear back from another recommendation on super quilt as to what thickness this would come up at. Thanks for help.
You need a dialogue with BC. It's not the total thickness that counts: the rafters are not such a good insulator as the Celotex so you get odd effects like different answers for 400 & 600 crs. Some authorities accept multi foils others are less happy and some have BBA certs and others don't. You need battens to space these materials out so they're not always as thin as you might expect at the end of the day.
The easiest way really is using insulated plasterboard to the rafters, and foil faced PIR between. No need for battens. Some good diagrams here http://www.kingspaninsulation.co.uk...ature/Therma-Literature/Thermapitch-TP10.aspx
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