Why construct a warm flat roof and a cold pitched roof? Why not have them both as warm roofs?
Moving on from that, why have an area of air below the warm roof insualtion when it can be filled with insulation? This will allow the reduction of thickness of Celotex under the deck/tiles by using quilt to compensate for less Celotex, or will create a much better insualted roof by using quilt to supplement Celotex.
That's what I was alluding to in my first post, in the construction of a completely insulated roof structure.
Then there are no issues in venting, part venting, designing an interface between the two or even the potential for air leakage by having any sort of void - vented or not.
It just seems so simple.
It would solve my problems fairly easily if it is sound. It is not that different to what the designer has drawn. Just eliminating the air gap with insulation. Effectively the ceiling level insulation and any top up fill just becomes a continuous extension of the warm roof insulation between rafters (like under rafter board insulation would if it was mounted).
Wonder if anyone has modelled / proven / constructed or tested this. Would this condensation risk analysis do anything. Tyvek web site says they will do one and supro is specified. I'd have to get the air gap removed and convince the designer. It'd be nice to be confident, especially when it comes to building it (no post build issues), Building regs inspectors querying it, etc etc. False pitches aren't that uncommon are they...