When the air is dry no difficulty will be experienced, but when a building is humidified water vapour is readily drawn along with the air, encountering cooler and cooler surfaces along the outward path of the air flow until condensation occurs. Air leaking through the construction and carrying water vapour with it may produce more serious condensation conditions than vapour diffusion. This does not mean that vapour barriers can be disregarded or that the concept of vapour diffusion is incorrect. Both air leakage and diffusion mechanisms must be kept in mind. Vapour barriers may be made to serve a dual function if they are continuous and are adequately sealed against air leakage at all joints.
Modern buildings are almost always of full-frame construction.
Cracks often develop between masonry wall panels and the surrounding frame because of movement and shrinkage. Spandrel zones above suspended ceilings are seldom back-plastered, which would help to seal the passages through which air leakage occurs. Brick or concrete unit masonry alone can be relatively permeable to air leakage. The addition of plaster, however, can reduce air leakage through it by a factor of as much as 100. Unplastered emergency stairwells and other rough-finished or unplastered portions of buildings can provide generous paths for outward air leakage.