How was it possible to build soundly in the past without a knowledge of vapour barriers and their function? External walls were mostly of monolithic construction, and water vapour could normally diffuse through them without causing condensation within the material. If condensation did occur, it was usually small in amount and the material was able to dry out quickly. Even internal insulation with materials subject to capillary action was unproblematic in this respect. Waterproof membranes first became necessary with the introduction of external walls in lightweight timber-stud or framed construction. In terms of building physics, traditional pitched roofs did not present a problem either. Their protective, scale-like covering allowed air and water vapour to pass through. Only with the full exploitation of the roof space for dwelling purposes was it necessary to line the roof skin with energy-saving thermal insulation and to prevent water vapour from the internal space penetrating into the insulating material. (Joachim Achtziger)