When designing exterior wall systems for residential buildings, interior environmental loads are typically considered as constants, if considered at all. More advanced design and modeling tools may incorporate dynamic interior environmental conditions, i.e., scheduled hourly fluctuations in temperature and relative humidity, into the analysis of wall performance. IEA Annex 14 and 24 recommendations are one alternative for specifying daily interior moisture product rates for use in dynamic hygrothermal performance modeling studies. The use of interior environmental moisture production rates from the ASTM Handbook of Moisture Control or the proposed ASHRAE standard 160P on “Design Criteria for Moisture Control in Buildings” may produce substantially higher interior loads than those observed by measurements. There are two issues that must be examined: the use of appropriate interior calculation methods and the use of appropriate interior loads. In other words, are the IEA Annex 24, ASHRAE, or ASTM Handbook assumed rates of moisture production appropriate for all locations or should they vary?

In this paper, the authors report measured interior temperature and relative humidity in several units of multifamily housing and compare the measured moisture production rates to the rates suggested by other design sources. In general, measured moisture production rates in Seattle were found to be lower than the handbook design rates. Using measured interior moisture production rates versus calculated ones may produce distinct differences in hygrothermal performance modeling analysis of exterior wall systems. The modeled performance differences resulted in significantly different wall component recommendations depending on the interior moisture production rate chosen. The authors conclude that using a single handbook design moisture production rate and suggested interior calculation load for hygrothermal performance modeling analysis is not appropriate in all cases but, rather, that the standards should require hygrothermal analysis using at least two of three possible ranges of interior moisture production, i.e., low, medium, or high. The authors suggest that the choice of which range of moisture production rates to use should be based on the designer’s investigation of how the building will be occupied and the type of HVAC system used.