Setting Realistic Design Indoor Conditions for Residential Buildings by Vapor Pressure Difference

Indoor relative humidity (RH) is commonly used to characterize the indoor environment for heat-air-moisture (HAM) simulations, chamber studies, analysis of monitoring data, or test hut studies of buildings without recognition that indoor RH and condensation potential depend on concurrent outdoor temperature and RH. This can lead to the use of unrealistic boundary conditions for HAM simulations and test programs, which may result in misleading conclusions. In buildings operating without mechanical dehumidification, the indoor air moisture level (vapor pressure) is directly related to the outdoor vapor pressure, moisture sources in the space, and the level of ventilation. Mathematics suggests that one can expect buildings with similar operation, occupancy, and construction, but affected by different weather conditions, to have a similar difference between indoor and outdoor vapor pressures.

This paper provides a foundation for selecting appropriate and realistic boundary conditions for the design of residential buildings that are based on vapor pressure difference with the aim to eliminate any significant bias for a particular climate.

Author: Patrick Roppel, Mark Lawton, William C. Brown