Condensation within walls and roofs of buildings in a cold climate is an inevitable consequence of warming and humidifying the air within them because the dew point of the air inside the building is, for several months, higher than the temperature of the outer surface of the building. The vapour barrier, often installed to prevent diffusion of humidified air into the walls, is not usually able to prevent transfer of moisture through air flow caused by the buoyancy of warm humid air in a tall building or by the pressure difference generated by the air conditioning equipment. The museum buildings of Washington DC, both old and new, testify to the difficulty of maintaining a humidity that will preserve the collections without damaging the building. Sensors installed within the structures have demonstrated the complexity of the patterns of air flow and of moisture transfer. Condensaton, ice formation, corrosion and rot in walls and roofs may show no symptoms but still cause serious damage over the years. For historic buildings that house historic collections the specification for the interior climate must be a compromise. It may be necessary to change the interior climate slowly through the seasons to minimise the difference between the inside and outside temperature and water vapour pressure.