Effect of Selected Moisture Reference Year on the Durability Assessment of Wall Assemblies under Future Climates

Abstract

Hygrothermal simulations are commonly used to evaluate the moisture damage risk of building envelopes over the long-term. For such assessment to be accurate, a proper selection of representative climate data is required. A common method is the selection of a moisture reference year from a set of available long-term climate data. For instance, the IRC-led research consortium MEWS (Moisture Management of Exterior Wall Systems) developped the Moisture Index (MI) approach, which consists of a wetting and a drying function. Therefore, the reference year selection would be based on the MI ranking. ASHRAE 160 is adopting a procedure named "the severity index" for the selection of moisture reference year. Combining climate loads and durability criteria, this method allows to select more "severe" weather years, thus providing a more representative ranking of the weather data. The objective of this paper is twofold. First, to compare the selection of the moisture reference year based on two different approaches for both historical and future climate loads. Second, the effect of chosen representative years is evaluated and compared to long-term simulation periods (of 31-years) based on the durability of building assemblies. The methodology includes hygrothermal simulations of two different types of wall assemblies located in three different Canadian cities under a changing climate. In general, higher mold index values were obtained by the long-term simulation and MRYs using Isev. Comparing the results of different models under future climates, the three methods were in good agreement, except for a brick wall facing WDR in Ottawa and Vancouver. This might be due to the Isev correlations were developed based on a north-facing stucco wall. In addition, for a north-facing wall, an extremely low mold index was predicted for Vancouver, compared to WDR direction. Thus, considering a north-facing wall as a criterion for performance evaluation might misrepresent the reality in some locations. Hence, both WDR and North orientations should be considered. A further study will be carried out to investigate the performance evaluation of Isev method for different types of wall systems and orientations.

Accuracy of using Moisture Reference Years for assessing the Long-Term Moisture Performance of Wall Assemblies

“…The first step is to rank the years of a longterm series in terms of their moisture severity using either climate-based indices or performance-based indices, e.g., the Moisture Index (MI)4, Severity Index (Isev) [5], Climatic Index (CI) [6], or Mould Growth Index (MGI) [1]. Thereafter, the MRY is selected as the worst year [7,8] or a combination of years having different moisture severities [9,10]. Chetan et al [8] investigated the accuracy of using three methods to sort years in a given climate series for a brick-clad wall assembly in terms of their moisture severity for the purpose of selecting MRYs: MI, Isev, and CI.…”

“…Whereas, the Isev provided a relatively more accurate results than the other two methods. Sahyoun et al [7] also used the MI and Isev for selecting the MRYs. Hygrothermal performance of a brick-clad wall assembly and a stucco wall obtained from simulations using MRYs, which had the 97th percentile value of the indicators (e.g., MI and Isev), was compared with that obtained from 31-year simulations.…”