Moisture Basics - Clues to Moisture Problem Mysteries

Claudette Reichel  |  6/25/2008 8:26:22 PM

Figure 1, Relative Humidity

Water vapor is one of the many gases that make up air. A little water vapor is good; too much is trouble. The goal is to maintain an indoor relative humidity (RH) in the 40% to 60% range, for comfort and health benefits (including deterring the growth of mold). For optimal dust mite control, maintain an RH below 50%.

  • Relative humidity is a measure of how much water vapor is in the air compared to how much it could hold at its current temperature. 100% RH means the air can’t hold any more water vapor at its current temperature.  (Figure 1, Relative Humidity)
  • Warm air can hold more water vapor than cold air. When warm, humid air is cooled, it can’t hold as much water vapor (RH rises), so the excess condenses into liquid water (when air reaches 100% RH). That’s why cold surfaces “sweat.” 
    •  Dew point is the temperature that will cause condensation (100% RH).
    • The solution to condensation (sweating surfaces) is reducing the relative humidity of the air (which lowers the dew point temperature) or in keeping surfaces warmer (above dew point).
  • Much of the energy used in air conditioning is to remove moisture from the air.
    • Activities of an average household add about 3 gallons of water per day to indoor air (as water vapor).
  • Water vapor moves through walls, ceilings and floors (the building envelope) two ways:
    • Carried by air that transports it through holes in the assembly.  A great deal of moisture can be carried by air infiltration through a small hole or gap. In most cases, air transported moisture is a much more substantial and important source of moisture movement through the building envelope than vapor diffusion.
    • By diffusion of water molecules through materials. Moisture tends to move from an area of higher to lower temperature and moisture content. The amount of moisture diffused is usually relatively small unless driven by high vapor pressure, such as rain-saturated brick veneer heated by the sun on a hot, humid day.
    • Vapor barriers or vapor retarders are materials used to stop or reduce water vapor diffusion. They should not be confused with air barriers; some air flow retarders are and some are not also vapor diffusion retarders.
      • Vapor retarders do not have to be sealed or installed completely free of holes to control diffusion sufficiently; 90% coverage provides 90% vapor diffusion reduction, which is typically more than enough (since diffusion is usually a minor source of moisture migration).
      • A material’s vapor permeability refers to how readily water vapor (not air) can diffuse through it. Materials with a perm rating of 0.1 or less are classified as vapor barriers and impermeable. A perm rating between 0.1 and 1 is classified as a vapor retarder. A perm rating between 1 and 10 is considered a semi-permeable vapor retarder. A rating higher than 10 is considered permeable to water vapor.

Water Vapor Permeability Classification

Perm Rating


< 0.1

Class 1

vapor barrier

> 0.1 and <1

Class 2

semi-impermeable vapor retarder

> 1 and < 10

Class 3

semi-permeable vapor retarder

> 10

Class 4


·Liquid water doesn’t always flow downward. It can defy gravity.

  • Wind, the water’s flow momentum and surface tension can direct it into horizontal and even uphill, building surface gaps.
  • Liquid water is also drawn into the pores and tiny cracks of porous materials by capillary force. This force causes materials (including wood, concrete, brick, etc.) to absorb or wick water in any direction, including upward. A capillary break is a nonabsorbent material or a space that interrupts the flow of water from one material to another. The deeper the water level (floods), the greater the force it exerts (on walls and foundations)
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