Different sets of weather data exists for varying purposes such as yearly energy use calculations, dimensioning HVAC and solar energy systems and so forth. In the past decade the hygrothermal simulation models have become everyday tools for building envelope designers, yet there is still no consensus about how to select the weather data for the design calculations. ASHRAE project RP-1325 analyzed several existing weather selection methods such as the one in ASHRAE standard 160 (Moisture Design Reference Years: the 10th-percentile warmest and 10th-percentile coldest years from a 30-year weather analysis) and compared their performance by simulating two wall systems – one light weight wall and one massive CMU block wall – with commonly used simulation models. The analysis showed that the existing methods were not very successful in selecting the right weather data and therefore a new method was developed. The new method is construction independent and uses only average yearly weather data as input and predicts an RHT-index for each year based on these. The orientation effects are taken into account by calculating the solar radiation and the wind driven rain loads on the wall. The north orientation was found to produce the highest values for the damage function and was chosen as the design orientation. The coefficients of the equation were optimized by using weather and simulation data for eight cities and these were subsequently validated with another four cities. The new method was found successful in selecting the worst years in all analyzed locations and to be consistent in its predictions. This new method was selected as the final method to pick the weather years for hygrothermal designs. The selected year is the third one out of 30 years in the ranking as provided by the new equation based method. This selected weather year is assumed to occur approximately once in every 10 years.

Weather years for hygrothermal analyses were selected for 100 locations in the US and seven locations in Canada. These weather files are available via a piece of software that was developed to help print the weather data in different formats as used by different simulation models.