Water moves in and through a building in a number of ways. It creates dampness and odors, material decomposition and a host for biological mold, mildew and fungi issues.
Wind induced rain is driven primarily by gravity and pressure differences. External leaks will allow large amounts of water into building cavities and cause no end of problems. A system of interconnected flashings and weather-resistive barriers, directs the water down and off the building.
Capillary water moves under tension through porous building materials or narrow channels between building materials that act like tubes. The porous nature of many building materials, and the incredible cohesion and adhesion of water means that water can move against the force of gravity. The primary defenses against capillary water movement are capillary breaks in appropriate locations.
Air moisture is driven by a combination of holes through the building envelope and one of three driving forces: wind, stack effect, or mechanically induced pressure differences (fans) between the inside and outside of the building. Air leakage is managed with a continuous air barrier surrounding the building envelope.
Vapor diffusion is the movement of water as a gas. While building assemblies can get wet by all four forms of water movement, once water gets in, the main way it get’s out is by diffusion, so it pays to make sure that assemblies can dry through diffusion in one or more directions.
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