If a structure is to be durable, nothing is more important that preventing water entry – from rain above ground and hydrostatic pressure below ground. The rainier the climate, the more rain control is needed. Gravity, wind pressure, momentum, surface tension, and capillary action can all cause rain to penetrate into a building surface. Each has been traditionally prevented by the following design techniques:
When moisture condenses or is trapped within a wall, roof or floor assembly, it can cause structural damage as well as mold or mildew, major cause of indoor air quality problems.
It is important to recognize the transport of water vapor in air that leaks through cracks in the building envelope. The amount of moisture that can be carried through currents of air escaping through cracks and voids can pose moisture intrusion problems, and these cracks should be sealed. As warm air rises, it causes high pressure at the top of a building and low pressure at the bottom, resulting in what is called the stack effect. At these points of greater pressure differential, the attic and basement, it is especially crucial to seal air leaks and use air flow retarders. A good example of the stack effect that better illustrates these principles is a typical building elevator and its shaft. On your next elevator ride, you can sense the air flow pressures.
Under the most severe rain exposures, providing a pressure equalized (vented) space behind the exterior cladding, combined with a “drainage plane” behind that, can prevent all these modes of water entry. For low-precipitation areas, a adequate approach (with a long track record) is to provide a face-sealed exterior wall of high mass masonry or concrete, which allows rain to be stored in the wall assembly mass for later drying. The least forgiving system is a face-sealed approach with no rain-storage mass, such as exterior insulation finish systems (EIFS). This system should be used only in the driest climates, unless a water management system (a drainage plane) is included.
Asphalt-impregnated felt (or tar paper) have been traditionally used as drainage plane, but water-resistant sheathings, such as rigid insulation or foil-faced structural sheathing, can also serve the purpose. Window, door, and roof/wall intersections must be carefully detailed to ensure drainage plane continuity. Because the drainage plane is toward the outside of the wall assembly, impacts to indoor air quality from the tar paper or rigid board are typically only an issue for chemically sensitive people.