Humidity Management

The third pillar of world class indoor air quality (IAQ) environments is humidity management and control. Uncontrolled moisture in buildings can contribute to unacceptable indoor air quality, occupant discomfort, and damage to the building structure and furnishings. One source of building moisture is water vapor contained in the indoor air. ASHRAE Standard 62-2001 recommends maintaining indoor relative humidity levels between 30 percent and 60 percent. Humidity levels less than 30 percent cause some people respiratory discomfort while humidity levels over 0 percent near surfaces for extended periods of time promote the growth of some forms of mold and fungi. Today microbiological contamination (mold and fungi) is a common cause of occupant complaints and IAQ problems in buildings.

Although there are many different species of fungi, they all share the same basic needs for survival:

• Organic matter (nourishment)
• Moisture (liquid water or relative humidity > 70 percent)
• Moderate temperatures (40F to 100 F)
• Source of spores

Controlling indoor moisture levels is one way to control propagation of fungi and dust mites inside buildings: in the carpets, wall coverings, and furnishings, as well as within the HVAC system itself.

Building shell leaks, spills, and condensation on cold surfaces are some of the most obvious forms of water intrusion into a building. However, moisture also enters buildings with humid outdoor air brought in for ventilation purposes. Uncontrolled infiltration of outdoor air due to inadequate building pressurization, permeation through porous building materials, and from respiration and perspiration of the building occupants all contribute moisture to the indoor environment. Humidity control is especially challenging in buildings with high occupant densities, such as schools and auditoriums, due to high ventilation rates.

Most buildings require some form of humidity control: dehumidification, humidification, or both. HVAC systems differ in their ability to control humidity. Most air-conditioning systems use a cooling coil to sensibly cool the supply air and remove moisture by lowering the air temperature below its dew point. (The latter causes much of the water vapor in the supply air to condense on the surface of the coil and be drained away.) Reducing the cooling coil's leaving air temperature below the dew point is key to the system's ability to properly dehumidify. The following humidity ranges have the indicated consequences:

• 0 - 20 percent: Causes respiratory discomfort in humans.
• 30 - 60 percent: ASHRAE recommended humidity range for humans
• 70 - 100 percent: Causes growth of molds and mildew.