© 2010 ASHRAE. ABSTRACT Until recently, the most common use of reference year data has been related to energy calculation. Representative weather data for use in moisture design calculations is as critical as other parameters required for heat, air, moisture (HAM) simulations e.g., material data or indoor climate data. The objective of ASHRAE’s research project, RP-1325 (2010), was to develop a method for generating more representative design weather years for use in heat and moisture performance predictions of building enclo- sure systems. Existing methods for selecting moisture design reference years have been reviewed. A new method was developed that provides an improved approach for selecting the most critical years in terms of hygrothermal performance. During the devel- opment of the method, eight U.S. locations were investigated, with 30 years of hourly weather data for each location. Another four locations were used to validate the accuracy of the method. The new method was applied in selecting the hygrothermal weather year for European location. The performance of six different building enclosure components, 4 walls and 2 roofs, was investigated using existing hygro- thermal simulation models. The effect of weather data on the performance of building enclosure components was evaluated. The new method allows for selection of weather years that are among the most severe for the simulated structures. The results show that weather data is an essential component of design criteria. INTRODUCTION Advanced hygrothermal simulation models require both indoor and outdoor environmental conditions to calculate heat, air, and moisture transport across building enclosure components. More advanced transient models typically use hourly climate parameters such as temperature, relative humidity, solar radiation, wind speed and orientation, cloud index, and rain. Simplified steady-state models, such as the Dew-Point method or Glazer method (ASHRAE 2009a), use averaged winter and summer conditions to predict hygrother- mal response of a construction and do not provide correct building enclosure moisture design guidance. This is particu- larly true in enclosure systems with high thermal performance. In a moisture design process, the environmental data should impose a more severe stress than the average climate in order to provide a level of safety related to moisture performance and durability. Figure 1 shows differences in predicted mois- ture contents in exterior sheathing when simulated with 30 years of hourly weather data for the same location. Some years provide net moisture accumulation from initial conditions, and some years allow the wall to dry. Several attempts have been made in the past (Sanders 1996; Hagentoft and Harderup 1993; Geving 1997; Kara- giozis 2003; Cornick et al. 2003) to select representative mois- ture design years for different locations. The existing available approaches for generating repre- sentative weather data, such as the IEA-Annex 24 approach, the Carsten Rode method, the Geving approach, the π-factor method, the Moisture Index method, and the ANK/ORNL- method are reviewed below. The Annex 24 approach is a construction-dependent method based on determination of the 10% level of condensation Effect of Selected Weather Year for Hygrothermal Analyses Mikael Salonvaara Klaus Sedlbauer, PhD Andreas Holm, PhD Marcin Pazera, PhD Member ASHRAE Associate Member ASHRAE Mikael Salonvaara is field application specialist, Owens Corning (formerly senior building scientist at Huber Engineered Woods, LLC). Klaus Sedlbauer is director of the Fraunhofer Institute for Building Physics in Stuttgart and Holzkirchen, Germany. Andreas Holm is head of the Indoor Environment Group, Fraunhofer Institute for Building Physics, Holzkirchen, Germany. Marcin Pazera is staff II, Building Technology at Simpson, Gumpertz, and Heger, Rockville, MD.