JLAB Posted January 11 Share Posted January 11 Hello, Looking for some advice with insulating a vaulted ceiling. The rafters are 100mm in thickness, with 2 vented roof tiles at the top. Is it as simple as insulating it with 100mm PIR board and then using insulated plaster board, or are there other layers that need building up, such as a vapour barrier etc? Many Thanks Joe Link to comment Share on other sites More sharing options...
Redbeard Posted January 11 Share Posted January 11 Ventilation is a potential issue. You need a 'dropped section' (small area of flat ceiling) at the top to form a 'plenum chamber' otherwise the only things getting ventilated would be the spaces between 2 rafter pairs. Look at manufacturer's bumf and see whether 2 vents is enough. And is there 'supply ventilation' at the bottom of the roof structure? (You need 'ins' and 'outs'). Apologies if I'm teaching egg-sucking! Common practice since 2010 (when insulation became a requirement of re-roofing) has been to leave a 25mm gap between insulation and the top of the rafters, and this (https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf) - gov't Best Practice guidance which seems to have sneaked in without many people knowing - says 50mm. Having 'trouble-shot' a number of moisture-ridden room-in-roof retrofits I favour 50mm, but no void of any size is much good if it does not have the 'feed' and 'exhaust' - the 'ins and the outs'. I prefer 'raw' PIR, finished with battens and pl'bd, to insulated plasterboard, as it allows you to tape the life out of the VCL. Plenty of example in BH pic history, I think, but pics from me if anyone needs them. Link to comment Share on other sites More sharing options...
JLAB Posted January 11 Author Share Posted January 11 @Redbeard Thanks for the response. There is plenty of supply ventilation at the bottom as that is where the pitched roof meets the flat roof which has 50mm ventilation gap. If you have photos that would be helpful. Thanks Link to comment Share on other sites More sharing options...
Redbeard Posted January 11 Share Posted January 11 OK. Will get to that later. Link to comment Share on other sites More sharing options...
Redbeard Posted January 12 Share Posted January 12 On the upper left you can see 25mm of PIR between the (75mm) rafters, leaving a 50mm ventilation path behind. Below the purlin a further 100mm has been added to the initial 25mm, and all the joints and perimeters of the boards are about to be taped as the vapour control layer. On the right-hand side is the final stage before fitting the plasterboard. After taping of the joints in the 100mm board 50 x 25mm timber battens are fitted horizontally, screwed through the ‘insulation sandwich’ to the rafters, and a final sheet of 25mm PIR is fitted between the battens, giving a U value of around 0.16W/m2K. Link to comment Share on other sites More sharing options...
JLAB Posted January 15 Author Share Posted January 15 Thanks, that's great help Link to comment Share on other sites More sharing options...
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