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THis is the cold roof principle. You insulate and seal at the ceiling level and ventilate above. This prevents heat loss and slows migration of vaour into the roof space, importantly it prevents condensation on the roof structure.I wish that were true. UK builders weirdly like all the lofts to be "properly ventilated" so even our new homes are about as efficient as German homes from the 1970s. As soon as I can afford it I'm getting a Passivhaus built. Airtight with controlled ventilation, bugger all cost to heat/cool, and no draughts.
The warm roof principle is to have a total vapour barrier, and insulation and then the final water proof skin all above the roof structure.
Both work, but not when done badly.