So, to restart my thinking I’ve gone back to basics.   
 

Here’s our flat roof. 

 

 

My first question is:  Is there enough room to get a cold roof sufficiently ventilated using those little upstand vents and the fact that the skylight mucks up cross flow?

 

I did a similar one as a warm roof on our last build 

Less issues with condensation 

36 minutes ago, nod said:

I did a similar one as a warm roof on our last build 

Less issues with condensation 

If I promise to be your bestest friend can I have a look at yours…..?

I used 9x2 joists boarded with ply Then a layer of multi foil Then fairings Toped with OSB 3

Plywood on the top of the blockwork 

and returned the fibreglass over to make a watertight seal 

First attempt at fibreglass 

150 kingspan between the joists 

Then boarded with 15 mil duplex and then skimmed it over with a couple of coats of multifinish 

Ta.  Is this an accurate cross section?

 

So presumably the foil was your air tight layer.

That ply between the foil and the PIR will be cold and attract condensation and won't be able to dry out again unless you have superbly low internal humidity.