The average American spends up to 85% of each day indoors. Whether at home, work, or school, our spending the majority of our time inside places tremendous importance on maintaining a comfortable indoor environment. Consequently, building heating, ventilation, and air-conditioning (HVAC) systems must be designed to promote occupant health and comfort.
Several factors can influence occupant comfort levels including temperature, humidity, air speed, clothing level, and metabolic rate. While temperature tends to be the most obvious factor, humidity also plays a major role in determining the perception of an environment. The saying that ‘you can’t please everyone all of the time’ certainly holds true for perceived human comfort. Unique physiological and psychological differences prevent us all from being satisfied with the same environmental conditions. In fact, industry design practices traditionally aim to provide comfort deemed satisfactory to 80% of the target population. Controlling both the temperature and humidity levels of a space can allow an HVAC designer to meet the acceptable thermal comfort requirements of a broader audience.
While there is no comprehensive ‘golden rule’ for humidity control, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has published design criteria for occupant thermal comfort levels in Standard 55 entitled Thermal Environmental Conditions for Human Occupancy. ASHRAE Standard 55 states that HVAC systems designed to control humidity levels are required to maintain conditions below a certain upper humidity limit. Staying below this upper limit is necessary primarily for the preservation of building infrastructure and for occupant health. No specific lower limit for humidity is published, though adverse health and comfort effects are indeed experienced at very low humidity levels, including excessively dry skin, mucus membrane irritation, dry eyes, and excessive static electricity discharges.
“…relative humidity levels maintained between 30-60% will provide satisfactory humidity comfort levels for most occupants.”
ASHRAE Standard 55 further indicates that relative humidity levels maintained between 30-60% will provide satisfactory humidity comfort levels for most occupants. Below this range, the potential for bacterial and viral growth and the risk for respiratory infections are increased. Research has also shown that falling below this lower limit too far can lead to other increased health risks, compromised comfort, and decreased productivity. Certain specialized building types have even more specific humidity requirements. Hospitals and museums, for example, require tighter control of humidity levels to promote physical therapy and preserve valuable collections. HVAC system design should therefore be scrutinized more closely depending upon the building application.
Centralized humidification systems are the best choice for most non-residential applications, as they are more economical and easier to maintain. The most common central humidifiers are heated-vapor generators powered by electricity, steam, hot water, or gas that are installed at central air handling units. The heated water vapor is absorbed by the dry supply air stream, mixed, and then distributed within the building. For these systems, code-required outside ventilation air represents the largest portion of a building’s humidification load. In most cases, providing humidification only for ventilation air can adequately humidify an entire building during the characteristically dry winter heating season. In fact, increased indoor humidity levels during the winter season can allow temperature settings to be lowered and still result in the same perceived comfort level.
Maintaining a comfortable indoor environment is obviously important for building occupants, who spend the majority of their time inside. Humidification, having a demonstrated positive impact on comfort levels, productivity, and health, is an attractive option for many building HVAC systems.
– Samuel M. Snyder, PE, LEED AP
Sam is a Licensed Mechanical Engineer and LEED Accredited Professional. Please feel free to contact Sam for further details regarding the above information.