The probability of an electrostatic discharge (ESD) is roughly six times greater at a relative humidity of 2% than at 7%. ESD events can cause costly failures in sophisticated electronic equipment. This study was designed to measure the water content of indoor air during a period when the outdoor air was quite dry, with emphasis on recording the fraction of time that the relative humidity was below 5%. We selected an unoccupied telecommunications building located in a region with extremely cold winters—Hinckley, Minnesota. From late 1995 through early 1996, the concentrations of water, carbon dioxide, and carbon monoxide were continuously measured both indoors and outdoors using a photoacoustic infrared technique; corresponding temperatures were also recorded. When the outdoor air was driest (outdoor temperature ~ -40 °C), the water content of the indoor air was approximately four times greater than outdoors despite the absence of obvious indoor sources. The results suggest that re-emission of sorbed water from indoor surfaces is an important source of water during dry outdoor periods. As a consequence of such re-emission, the measured indoor relative humidity (RH) seldom dropped below 7%. This result is in contrast to modeling results that predicted more than 700 hours/year below 5% RH; however, the modeling did not take into account moisture capacitance of indoor surfaces.

Paper from IAQ 1997 -- Design, Construction, and Operation of Healthy Buildings: Solutions to Global and Regional Concerns