Long-term hygrothermal performance of white and black roofs in North American climates

When solar radiation hits a roof surface, a part of solar energy is reflected and part is absorbed. The absorbed part of solar energy results in an increase of the surface temperature of the roof. Cool reflective (white) roofs use bright surfaces to reflect a significant portion of the incident short-wave solar radiation, which lowers the surface temperature compared to conventional (black) roofs with bituminous membrane. As such, white roofs help reduce the urban heat island effect during the summer. The question is “do white roofs lead to moisture-related problems in northern and southern climates?” To help answer this question, numerical simulations were conducted to compare the hygrothermal performance of a single kind of white and black roofs under different outdoor and indoor conditions. The outdoor conditions are obtained from the weather database of the National Research Council of Canada, Institute for Research in Construction (NRC–IRC). The indoor conditions are taken based on the European standard (EN 15026) and ASHRAE recommendations for conditioned space. The type of roofs considered in this study is Modified-Bitumen (MOD-BIT) roofing systems. The numerical simulations were conducted for the outdoor climate of Toronto (ON), Montreal (QC), St John’s (NL), Saskatoon (SK), Seattle (WA), Wilmington (NC) and Phoenix (AZ). Results showed that for the outdoor climates of St John’s and Saskatoon, the white roofs could lead to longer-term moisture-related problems. However, for the outdoor climates of Toronto, Montreal, Seattle, Wilmington and Phoenix, buildings with white roofs were shown to have a low risk of experiencing moisture damage. Also, buildings with white roofs in these locations were predicted to show a net yearly energy savings compared to buildings with black roofs.