Water Vapor Generation in Residential Buildings: Comparing Probabilistic Simulations and Field Measurements

Indoor humidity levels in buildings affect the quality of indoor air, the health and comfort of the occupants, and the durability of the structure. Interior water vapor generation is an important consideration in building design analysis. In this paper we compare two methods for quantifying water vapor generation rates in residential buildings. The first involves Monte Carlo simulations that consider time varying interior sources using statistical survey data. Typical sources include humans, pets, plants, showering, bathing, cooking, washing, etc. The second approach involves field measurements of indoor vapor pressure, outdoor vapor pressure, and air exchange rates for a sample of occupied dwellings, followed by calculations based on mass balance. Probability distributions of total daily water vapor generation as a function of the number of occupants are compared between the two methods, using simulations of single-family detached houses in Sweden and the United States, and field measurements in Finland. Water vapor generation rates generally increase with the number of occupants, though some anomalies are found in the Finnish sample, likely because of the limited number of houses. Probability distributions for houses with two or three occupants agree closely across the different countries, but houses with four or five occupants in Finland consistently have lower generation rates than those in Sweden and the US. Generation rates had broader distributions for US houses than for Swedish houses, which likely reflects differences in lifestyle. We discuss the advantages and limitations of both methods. Finally, we identify research that would advance the state of the art.