Understanding Moisture and its Flow in Homes for Energy Efficiency

Reviewed and Revised on 11/12/2013

Water vapor is one of the many gases that makes up the air we breathe. A little water vapor is good; too much is trouble.

An important term to understand moisture issues in a home is Relative Humidity (RH). Relative humidity is a measure of how much water vapor is in the air compared to how much it could hold at a given temperature. 100% RH means the air can’t hold any more water vapor at its current temperature.The goal, for energy efficiency in a home, should be to maintain an indoor RH in the 40% to 60% range, for comfort and health benefits (including deterring the growth of mold). For optimal dust mite control, RH should be below 50%.

Image:Relative humidity.jpg

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. That’s why cold surfaces “sweat.” Dew point is the temperature that will cause condensation or when the RH is 100%. The solution to condensation (sweating surfaces) is to reduce the relative humidity of the air (which lowers the dew point temperature) or keep 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 can move through walls, ceilings, and floors (the building envelope) of a home in following two ways:

  • Air Infiltration: Moisture can be carried by air that transports it through holes or gaps in the built structure of a home. A great deal of moisture can be carried by such infiltration. Generally speaking,air transported moisture is a much more substantial and important source of moisture movement through a building envelope than through the second method of vapor diffusion.
  • Vapor diffusion: This occurs 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 Classifications  
< 0.1 Class 1 vapor barrier
> 0.1 and <1 Class 2 semi-impermeable vapor
> 1 and < 10 Class 3 semi-permeable vapor retarder
> 10 Class 4 permeable

 Few cautionary tips:

  • 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).