Roof insulation thickness / venting

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Hi, just about to fit velux, insulate pitched roof in picture. Being closed boarded and the fact that i removed rock wool that was completely filling the voids can i use 100mm celotex or 75mm and leave 25mm gap or 50mm and leave 50mm air flow gap?. Obviously with the later choices could also combine with insulated backed plasterboard.
 

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Surely with any of those choices you'd use insulated backed platerboard in order to eliminate cold bridging.
 
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regs would be 90mm between and 50mm under, which would mean backing out your rafters to maintain a 50mm air gap, but 50mm with 50mm under would make a significant difference to the rockwool you removed.
That being said leaving an air gap is irrelevant if there isn't actually any venting to the outside
 
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Many thanks. There is air flowing in through soffit but a roofer said pointless unless vented at the top as well so it flows. I worry alot so will allow for ventilation, batten out the rafters to take 100mm celotex with 50mm airflow, then add further 50mm celotex on top and counter batten to take the plasterboard as easier than screwing through 50mm celotex into the rafter. This will then achieve U value above building regs. Right?
 
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Doggit

You should be using 130mm to cover current regs, usually made up of 100 celotex, and then 42.5mm insulated plasterboard to stop the cold bridging. As there's no airflow at the top of the slope, I'd just put a 25mm batten up at the roofline, then a 25mm batten on the bottom of the joists. Put the 100mm celotex between the joists, and then put the insulated plasterboard over the top of the joists, in the opposite direction. If you counter batten over the celotex to take the plasterboard, then you're leaving an airgap underneath that could collect moisture.
 
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90mm between and 50 over meets regs for domestic refurb 100 between and 42.5mm laminated board would be borderline
 
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There's never any clear cut answers when searching how to implement projects - many people have different ways of doing things. In my mind 150mm of solid insulation ie Celotex, kingspan etc is above building regs. There is no cold bridge as the 50mm (2400mm x 1200mm) sheets of celotex will lay across the rafters completely covering them - adding the batten allows for easier fix of plasterboard and and ideal channel top run any services. Thanks for all your help guys and whatever I decide to do will be much better than what was there previously, may I also note that even with the old rockwool (below regs.) it was still really toasty and stayed warm.
 

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