Warm roof question

We are proposing to use the Colorcoat Urban steel roofing system from Tata Steel. The roof will be a shallow pitch (about 9.5 degrees).
The top 3 layers (top down) are
steel sheet breathable membrane 18mm OSB
We are proposing a warm roof solution, with the next layers being: 50mm PIR 150*50 rafters with 100mm PIR between Foil backed plasterboard
So: (1) Should this need any more vapour barriers anywhere (2) Should the PIR be at the top of the rafters with a void below (seems sensible).
Cheers
Dave R
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su
On Tuesday, September 25, 2012 11:27:15 AM UTC+1, David WE Roberts wrote:

Surely with a warm roof (no ventilation) solution you put the waterproof covering above insulation above vapour barrier (i.e. the foil on insulation) above rafters above ordinary plasterboard.
Robert
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Hmm... The shallowest pitch found on BS5502 class2 agricultural barns is 12.5 deg. You may need to form a *drip* at the gutter.

Why? The very last thing you need is water vapour able to reach the potentially cold outer skin.

Sounds plenty:-)

Don't know. Surely the Building Research Station must have got this *cold roof* warm air business sorted by now? I have yet to meet an architect who can give convincing reasons for his chosen placing of vapour barriers!
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<snip>
The permeable membrane between the steel and the OSB didn't make a lot of sense to me, either, but this is specified in the installation guide from the manufacturers. AFAICS you don't want water to migrate inwards, and you aren't going to get much migration outwards from the OSB if there is steel sheet on top. However, that is in the insatllation guide and Building Control have accepted the solution I described.
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