Insulation thickness on cold flat and pitched roof

[chez4836]

Hi, I am building an extension with a part flat roof and also pitched roof construction. The joists are 150mm x 50mm - 400mm centres on pitched roof and 200mm x 50mm on flat roof. Want to have insulation between rafters. Due to the fact that I need an air gap between insulation and breather membrane [cold roof], can I calculate what thickness Kingspan/Celotex I need. I was hoping for 120mm, then have an air gap of 30mm. Not too sure if this reaches the 0.2 U value though. Flat roof is 2Mx4M, pitched roof is 1M x 4M. Flat roof covered with 18mm ply, normal concreete tiles on pitched roof. Happy to put layer of insulation quilt under rafters before I put up 12.5mm plasterboard. Thanks in advance for any help - new to this site.. Chez4836

 

[Chukka63]

Are you using kingspan on the pitched roof for a reason and not quilt on the ceiling joists??

Current thinking is 50mm airgap, 100mm between the rafters,and 40mm under the rafters.

thats on both roof types.

dont want/cant have a warm flat roof??

 

[chez4836]

Thanks Chukka63, had not thought about quilt - thought Kingspan easier. The roof is unfortunately tiled and felted, so warm roof now not possible [I think]. Therefore could go with the 100mm Kingspan, then quilt underneath, also need vapour barrier, does this go on last before plasterboard? Can I screw the plasterboard directly to the rafters through the quilt or does this compromise the uValue? Cheers