Minimum Possible warm roof insulation. Conservation area

[al501]

We are building a a modest 3.5 meter deep rear extention extention. The SE has specified 8 inch joists plus furrings. 18 mm ply
40 mm optim r
25 mm thermaroof tr27 (inc 6 mm ply)

Problem I have is that 40 mm optim R is very expensive and I'm also not convinced that the boards will be robust enough.

any idea what the minimum level of insulation I need for a warm roof ? We are in a convervation area and cannot raise the flat roof height .

Is it possible to do a hybrid roof with lets say 75mm between rafters and 75 mm on top without needing to ventilate ?

any help appreciated

 

[^woody^]

Is it possible to do a hybrid roof with lets say 75mm between rafters and 75 mm on top without needing to ventilate ?

Yes.

Tell your architect to design a roof to meet your needs - maximim ceiling height and economical to build, and planning requirements (maximum roof height).

The problem with hybrid roofs, is that many designers don't know how to argue their case with building control, and some building control inspectors don't really understand the concept.