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Joining roof construction

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Our front pitched roof is a cold roof. 140mm between, 40mm over and 50mm air gap to ceiling height. 


The ceiling joists run from dormer wall to the pitched side. 

Dormer roof is a warm roof construction. 


Question is above the ceiling what would you use to fill between ceiling joists and where do you stop? Just short of where the warm roof starts? 


If cold air gets beneath the warm roof will this make it ineffective?


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No, you should not have cold air coming in under your warm roof.


Your cold roof has internal and between rafter insulation and outside airflow above.  The outer layers will be open to outside air but inside will be nice and warm.


Your warm roof will have insulation on the roof deck, so the whole thing is warm.  It needs no ventilation.  Again, no outside air should get in at all.


On the interface you will need continuous insulation, while still allowing the airflow over the outside of the cold roof.


Perhaps post a drawing.

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Thank you the issue I have is on the warm roof side there is a small pitch coming down to it from the ridge (approx 500mm) before the warm roof starts. This itself won't need insulating as its above ceiling level or do I put insulation between ceiling and the bottom of the flat roof to create a barrier as such? 


Where the warm roof starts how do I prevent this air passing into the cold space and visa versa? 


I know the design is poor but I am committed in now and need to find a solution. 


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