On April 14, 2020, the American Society of Heating, Refrigerating and Air-conditioning Engineers Position Document (ASHRAE PD) was released to offer guidance to mitigate disease transmission and as a reference for building readiness for post-pandemic return to operation. While the ASHRAE PD advises increased ventilation is not capable of addressing all aspects of infection control, changes to building operations, including the mechanical systems, can reduce airborne exposures. Though the ASHRAE PD does not make a definitive recommendation on indoor temperature and humidity set points for the purpose of controlling infectious aerosol transmission, it does offer immunobiologists correlated mid-range humidity levels with improved immunity against respiratory infections and unfavorable survival rates for microorganisms when the relative humidity (RH) is between 40 and 60 percent. Further, the research associated with the ASHRAE PD have shown a correlating increase in infections when the interior environmental conditions fall below the 40 percent RH.
If altering the operating conditions in an existing building is being considered, the enclosure design and construction, such as the walls, roof, fenestration, and below-grade assemblies, must be understood as well as their respective air, thermal, and vapor control strategies. Air, thermal, and vapor control fundamentals have congruences and differentiations but can work together to prevent dewpoint conditions, the temperature at which humidity in the air will begin to condense. Should humidified air encounter a surface below dewpoint, condensation can result, with an increased potential for premature deterioration of building components, corrosion, or mold formation. Buildings in colder climates are particularly susceptible to these concerns.
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