How to build a super insulated flat roof

[FrogParty]

Any suggestions for building a well insulated flat roof, u-value < 0.08 like the pitched roof?

Filling the pitched roof loft with 500mm of mineral wool does the trick there, but it seems difficult to get anywhere near with a flat roof. Wanted to just stuff mineral wool into a flat roof truss, but that would be a cold deck and suffer condensation problems if incorrectly vented. Everywhere says cold decks are no longer recommended, but there is a practical fixing limit to how much rigid insulation board can be attached above the rafters. Warm decks I have seem just aren't that insulated.

German passive houses seem to have lots of flat roof, so how do they do it? Anyone?

 

[^woody^]

Use foil backed insulation on the top, in between joists and on the underside, or combinations to suit whatever depths you have available.

 

[Xenon]

120mm Will give you .18. We did fit 240mm the other week which got it to .10 i think, but only possible due to high parapet.