Warm roof thickness

[AdaminCymru]

Hello 

 

I have plans for a cold roof on a loft extension. I believe cold roofs can be problematic with heat retention in warmer months. The rafters that are currently spec’d are 200x47mm and total cold roof thickness 342mm.

 

I am wondering whether a warm roof could be a better option, but I do not want to compromise on head space. If my engineer agrees to say a 125x75mm rafter instead of 200x47mm, what is the minimum thickness warm roof we could achieve here?

 

any input greatly appreciated 

[Dave Jones]

you wont have the headroom, it need above your ceiling/wiring. osb deck, vapour barrier, celotex, osb, rubber/felt etc. 200mm needed min really. then below that your 6x2 truss, plasterboard etc etc

[DevilDamo]

Normally, you’d require 150mm on top.

[EdHat]

You'll need 140mm or 150mm depending weather it's refurbishment of an existing element or a new thermal element.

When I've needed to keep the height down I've been able to reduce the 140mm down to 90mm by using a multi-foil insulation over the joists first.

You could also look at Recticel Deck VQ which has a lambda of 0.008 and I believe you can meet the regs with 90mm but I expect it is expensive.