Historical and Projected Future Changes in Potential Moisture Damage in Building Envelopes across Canada

Buildings and infrastructure across Canada, and around the world, are exposed to the threat of climate change. To facilitate the design of climate resilient building envelopes, it is important to evaluate the risks to which they have been exposed from external climate in the past, and can potentially be exposed to in the future as a consequence of global climate change (Gaur et al. 2019). Moisture accumulation and retention in wall assemblies is one of the major reasons for the premature deterioration of the building envelopes from mould growth. Climate based indices have been developed to quantify the potential of moisture accumulation in buildings and have been used for instance to identify the most critical time-periods (or hygrothermal reference years) for building envelopes towards potential moisture accumulation in wall assemblies.

The Moisture Index (MI; Cornick et al. 2003), Climatic Index (CI; Zhou et al. 2016), wind driven rain index (WDRI) are amongst the indices that have been most widely used, and are investigated in this study. The MI consists of a wetting and drying function. Following National Building Code of Canada (NRCC 2015), the wetting function is defined in terms of annual total rainfall normalized to 1000mm whereas the drying function is defined as a function of the difference between saturation vapor pressure and the vapor pressure of the ambient air, normalized with reference to the drying simulated for Lytton, British Columbia. The CI also consists of a wetting and a drying function. The wetting is calculated as a function of wind-driven rain however in this case, the drying is defined as the difference between saturation vapor pressure at the wall surface and the water vapor ratio in the air calculated as a function of outdoor air temperature, humidity, wind speed, and solar radiation. Finally, the WDRI is calculated as a function of the exposed free wind-driving rain load on the building.

In this study, the historical (1986-2016) and potential future distributions of MI, CI, and WDRI are calculated for locations within the Canadian landmass. The climate data for the calculation of these indices over the historical time-period is taken from hourly database of Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2; Gelaro et al. 2010). Attached figure shows the CI values calculated for different parts of Canada using MERRA-2 datasets.

The potential future changes in the magnitudes of these indices are calculated between the historical time-period and future time-periods when 0.5 ºC, 1 ºC, 1.5 ºC, 2 ºC, 2.5 ºC, 3 ºC, 3.5 ºC of global warming (calculated with reference to the historical time-period) is expected to be reached in the future. To do so, the future climatic projections corresponding to different levels of global warming are extracted from the historical and future climate simulated by the Canadian Regional Climate Model, CanRCM4 (Scinocca et al. 2016). The effect of the internal variability of climate on the projected changes is also assessed by taking into consideration a large ensemble of future climate simulations comprised of 15 members generated by initializing the CanRCM4 model with different sets of cloud parametrization.

The results from this study will highlight the risk of potential moisture damage that building envelopes in Canada have been exposed to in the past, and can potentially be exposed to in the future. This information will help designers and policymakers in Canada to provide information pertinent to use in building codes and standards for the design for durability of building envelopes against historical and projected future effects of climate change.

References

Cornick S., Dalgliesh W.A. (2003), A Moisture Index to Characterize Climates for Building Envelope Design, Journal of Building Physics, 27 (2), 151-178.

Gaur A., Lacasse M., Armstrong M. (2019), Climate Data to Undertake Hygrothermal and Whole Building Simulations Under Projected Climate Change Influences for 11 Canadian Cities, data, 4, 72, doi:10.3390/data4020072.

Gelaro R., et al. (2017), The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2), J. Clim., doi: 10.1175/JCLI-D-16-0758.1.

National Research Council Canada (2015), National Building Code of Canada 2015, Retrieved from http://www.nrc-cnrc.gc.ca.

Scinocca J., Kharin V., Jiao Y., Qian M., Lazare M., Solheim L., Flato G., Biner S., Desgagne M., Dugas B. (2016), Coordinated global and regional climate modeling, J. Clim. 29, 17–35.

Zhou X., Derome D., Carmeliet J. (2016), Robust moisture reference year methodology for hygrothermal simulations, Building and Environment 110, 23-35.