The Impacts of Humidity
By Justin Iovenitti, Energy Engineer
Poor moisture management is a big problem in many buildings throughout the US. Excessively high or low indoor relative humidity (RH) can lead to occupant discomfort, asthma flare ups, and respiratory infections. Buildings in humid climates, such as the mid-Atlantic, are especially troublesome, as mold can thrive if indoor RH is not controlled.
Low RH
In winter, a leaky building will allow very cold, dry outside air to infiltrate the enclosure. As this already dry air is warmed by the heating system, it becomes even drier, since the moisture-holding capacity of warm air is much greater than that of cold air. This results in uncomfortably dry indoor air.
Fortunately, there is a straightforward solution to this which requires no additional HVAC equipment or sophistication. Introducing or adding air sealing to leaky buildings to ensure air barrier continuity will raise the indoor RH levels to a comfortable range (generally 30-40% minimum RH). Reducing excessive mechanical ventilation rates and adding energy recovery on outside air systems also help to prevent dry indoor air in winter.
High RH
Critically, some projects may tick off all the requisite moisture management boxes and still experience high indoor humidity. This can even be true of high performance and Passive House buildings, where drastically reduced cooling loads reduce the runtime and drying capabilities of cooling systems. In response, New Ecology recommends that multifamily projects should provide standalone dehumidifiers (or rough-ins to facilitate future installation) in each dwelling unit. In fact, some states, including Delaware, have started to require standalone dehumidifiers for all projects seeking low income housing tax credits. The added first cost of a basic ENERGY STAR dehumidifier is minimal when compared with the financial impact of remediating a potential mold problem.
