Flat Roof insulation and airtightness detailing

[knobblycats]

I've been going through the Architectural Technicians' detailing for the flat roof.

 

The current design shows the top chord of the joist flying over the external wall to form the roof overhang.

 

I'm concerned that this would be a cold bridge - as well as a pig of a detail to make airtight. Here's the drawing that was provided:

 

 

Does anyone know if I'm justified in my concern of this being a cold bridge or am I being over-cautious?

 

In the case of it being a significant cold bridge, I've had a go at drawing up an alternative detail:

 

 

The idea here is to encapsulate the top chord in insulation and then continue the airtight barrier over and under to join with the external wall. In my mind, this would reduce the cold bridging and simplify the air tightness detail. 

 

Does that seem like a sensible way forward? Does anyone have any other thoughts or solutions?

[jamiehamy]

Your alternative looks nice - but don't underestimate the depth of the top chord of your posi joist - it'll be 100mm or more, so quite a gap. You can either chop up spare insulation and foam it in, or cut out slots in your internal/external insulation and 'slot' in the joiats and foam with airtight foam, wit the top flush to the top of the joist - and foam as you put on the board.