Vapour barrier even when roof is ventilated. Dampfsperre auch beim belufteten dach.

Shows that efficient moisture removal in a ventilated roof via ventilation openings is a function of roof length. Beyond a certain critical length all moisture diffusing in from below will be deposited in the roof. The critical roof length will be greater the more vapour tight the section beneath the roof space is. If a vapour barrier is necessary, as in an unventilated roof, it maybe that an unventilated roof is preferable in the first place.