Can you tell that it’s winter by your scratchy throat or by the static shock you receive when you stroll across the office carpeting and touch your desk? Does that fuzzy green stuff growing in the corner of your office herald the arrival of summer? If so, you may have a humidity problem in your building.

Humidity control can be a daunting—and sometimes overlooked—task for building owners and managers. Improper humidity control can cause discomfort to building occupants (sore throats and static shocks) as well as degenerative effects to the building itself (premature cracking and shrinking of floors, plaster, furniture, windows, doors, and woodwork). In addition, high humidity levels can result in the growth of microorganisms, ultimately causing problems with indoor air quality.

Humidity Terms

Relative humidity refers to the percentage of water vapor in the air, versus the maximum amount it can hold at a given temperature. The measure of relative humidity varies with temperature, and the lower the air temperature, the less moisture it can hold. Air with a relative humidity reading of 50% at 72^o F has a different moisture content than air with a relative humidity reading of 50% at 65^o F.

Absolute humidity—also known as humidity ratio—is a more consistent method of measuring humidity because it is independent of air temperature; rather, it is a measurement of the amount of water vapor in a given unit of dry air.

Dewpoint temperature is also used to define humidity and is also independent of air temperature. It is the temperature at which air at a particular humidity ratio becomes saturated.

Low Humidity

In the northeastern United States, low humidity conditions typically occur during the winter months. As the outdoor air temperature falls, the ability of the air to hold moisture decreases. When the outdoor temperature falls below 0^o F, the indoor humidity can fall to as low as 5% at 72^o F. By comparison, the Sahara Desert averages nearly 25% relative humidity annually! Low indoor relative humidity can cause dry skin, itching eyes, and respiratory irritation. Static electricity is also more prevalent, increasing the potential for damage to costly electronic equipment.

Low humidity can be caused by a number of factors, including the introduction of dry outdoor air through ventilation systems or by building infiltration, vapor pressure diffusion to the outside through building materials and breaches in the vapor barrier, and the absorption of water vapor directly into building materials.

For a typical office building, the humidification load is based upon the amount of dry outdoor air entering the space and the amount of moisture migrating out through the building envelope. The outdoor air load consists of two sources: outdoor air entering the building through the ventilation system, and outdoor air infiltrating through cracks in doors and windows. The integrity of a building’s vapor barrier is the single most important factor in determining the amount of moisture migrating out of the building. A small breach in a building’s vapor barrier can contribute to a large leakage of water vapor to the outside.

Humidification systems are needed to correct low relative humidity problems. Several types of systems are available to building owners:

• Direct Steam Injection
• Steam to Steam
• Electric Steam
• Gas-Fired Steam
• Air-Water Atomizing
• Ultrasonic Atomizing

Humidity setpoints must be correct to prevent condensation on windows or metal surfaces in direct contact with outdoor temperatures. In a typical office building with double pane thermal windows, the maximum indoor relative humidity setpoint at 72^o F is 30%.

Older buildings with little or no insulation, poor vapor barriers, and single pane windows require special attention to prevent condensation problems. Indoor moisture generated by a humidification system can migrate through a poorly insulated wall, condensing inside the wall and causing mold and mildew damage. Single pane windows have poor insulating values and may allow condensation to form at indoor relative humidities as low as 25%.

High Humidity

High humidity conditions are more likely to occur during the summer months on hot, humid, or rainy days. These conditions are more prevalent in southeastern climates, but can also occur in the northeastern United States. During a typical peak summer day—when the outdoor temperature rises above 95^o F and the relative humidity is over 50%—the indoor relative humidity can increase to 80% or more without adequate dehumidification control. High indoor relative humidity can cause discomfort for employees and, more dangerously, can increase the potential for microbial growth and thus contribute to poor air quality. Building components (walls, floors, ceilings) and building furnishings (carpets, furniture, wall coverings) can provide ideal sites for fungi, bacteria, and dust mites to flourish and produce odors, allergens, and even toxins.

The main source of high relative humidity is the introduction of unconditioned outdoor air through ventilation systems or through building infiltration. Other sources include vapor pressure diffusion, vapor barrier breaches, evaporation from people, and liquid sources such as fountains, pools, cooking vessels, and showers.

In a typical office building, the air conditioning system keeps the indoor relative humidity at acceptable levels. When cooling the supply air, the air conditioning coil also removes moisture from the air. Since a thermostat can only sense temperature, it cannot directly control humidity. A common corrective action is to turn the thermostat down to provide more cooling and dehumidification; however, the lower temperature actually causes the relative humidity to rise, because the air has less ability to hold moisture at the lower temperature.

Constant volume reheat or variable air volume (VAV) air conditioning systems can be designed to keep the supply air at a constant dewpoint temperature (usually 55^o F), thus controlling space humidity levels while controlling space temperatures.

In most cases, high indoor relative humidity can be controlled through proper sizing and control of the air conditioning system. However, in some instances, control of space humidity by the air conditioning system alone is not enough. Two examples of this are an assembly area requiring large amounts of outdoor air ventilation and an industrial process application in which lower than normal humidity levels are required. In these cases, a dehumidification system may be required. Types of dehumidification systems available to building owners include:

• Refrigeration Cycle Dehumidifiers
• Portable Dehumidifiers
• Liquid Desiccant Humidifiers
• Dry Desiccant Dehumidifiers