Flat roof - Quasi design - ADVICE PLEASE!

[Builder9999]

Hello all, my first post. In the process of building a new flat roof dormer, using neither a cold or warm roof solution... but something in between which is unvented. Will this work and not suffer condensation issues?
Look forward to hearing your ideas/thoughts... View media item 50087

 

[^woody^]

If you leave an unvented void in a roof, then there is risk of condensation

 

[noseall]

You seem to have the warm deck principle sorted only you would fit 100mm of pir insulation above the joists as opposed to just 40mm.

The risk is that the cold will still be able to transfer down the joist due to inadequate insulation above the joists.

It is fairly typical however to have a secondary slab of insulation (25mm or so) pushed up tight against the underside or warm side of the deck.

 

[Builder9999]

Thank you for the replies, which I think I understand.
As I am looking at 120mm of PIR required overall, what about if I go for 60mm above and 60mm below? Or would I need 70/50 (above/below) to prevent cold transfering down the joists?
Many thanks!

 

[noseall]

.....er 100mm above is my final offer.