Cold Bridging in Flat RoofsSteven Dickinson
The metal will not get “colder”. It can only get as “cold” as the ambient temperature it is “contact” with. Sorry, about the three replies, but I wanted to address each point separately.
That I beam, is at the top of the roof, near an exterior wall, with no “top load” (of significance) and appears to be acting like a ledger, for the roof rafters and ceiling loads. But, it also appears to not be supporting the ceiling below (appears to be a gap between rafters and ceiling).
Either way, the point is, keep the barrier on the heated side of the “insulation” or you will be putting your dew point in the insulation in the this “detail”.
Oddly, the “heated side”, could, in fact, be the roof side and NOT in the inside of the house depending on temperatures of your drawing’s location (in the world and daily mean temperatures).
Also, more consideration should be given to venting all roofs.
In England, we put the vapour control layer the warm side of the roof we also don’t vent our warm roofs.