There are three main “moisture transport vehicles” that create avenues and allow water into your home.
1. Bulk Water Movement
The most common moisture intrusion issues I see in Twin Cities’ homes are a result of “bulk moisture movement”, also known as liquid flow (rain, snow, ground water). This type of intrusion is typically immediate, easily discovered and also the most damaging. Three conditions are required to allow bulk moisture flow into a building: 1) a source of water 2) an opening or failure in the drainage plane of the building and 3) a driving force such as gravity or pressure.
2. Capillary Action
Capillary action refers to the ability of water to travel up against the pull of gravity through a porous material. It works because the friction of the moisture against the wall surface (adhesion to the wall) actually exceeds the cohesive forces between the water molecules. Although not as serious as bulk water movement, capillary forces are both destructive and secretive, since they often work and accumulate in commonly unobserved areas. They can potentially cause significant damage over a period of time without a homeowner’s knowledge.
3. Air Transported Moisture and Diffused Air Flow
Air-transported moisture (water vapor hitchhiking on air) can leak into, or out of buildings. Both uncontrolled pressure sources (wind) and controlled sources (fans and air handlers) can move significant amounts of moist air past a home’s exterior through openings and improperly flashed transitions. A similar process known as “diffused air flow” forces small amounts of moisture in the form of water vapor to pass directly through a building’s envelope. The amount of diffused air flow (vapor) that occurs in a home is determined by two things: the driving force that pushes it and the permeability of the building material the vapor is passing through.