Hygrothermal behaviour of flat cool and standard roofs on residential and commercial buildings in North America

Installing roofs with high solar reflectance and high thermal emittance, known as “Cool roofs,” are becoming increasingly popular because of their cooling energy saving potentials, cost effectiveness and sustainability. Lower surface temperature of cool roofs may affect the hygrothermal performance of roofing systems and hence their performance should be characterized in different climates.We simulated the performance of several roofing systems including: typical, smart, and self-drying roofs for residential and commercial buildings. In addition, we proposed vented roofs with smart vapor retarders in very cold climate regions across North America. We also developed an algorithm to investigate the effect of snow on hygrothermal behaviour of dark and white roofs.Results showed that office buildings never experience moisture accumulation problems during the simulation period (5 years). In residential buildings, white typical roofing compositions with conventional vapor retarders experienced moisture accumulation problems in very cold cities such as Anchorage, Edmonton and St. John's. Using smart vapor retarder (smart roofs) or self-drying roofs helped to decrease risk of moisture accumulation. In these climates, adding a ventilated air space along with using smart vapor retarder eliminated the risk of moisture accumulation and prevented excessive OSB (oriented strand board) moisture content. The risk of mold growth was significantly lowered in vented smart roofs. Simulating the effect of snow on the roofs for Anchorage, Montreal and Chicago showed that the hygrothermal performances of white roofs improved with snow accumulation on the roof.

► The hygrothermal performance of cool and dark roofs is almost the same in hot and moderate climates.
► Proper design of roofs in cold climates is essential to avoid moisture problems in both cool and dark roofs.
► In cold climates, snow accumulation of the roof slightly improves the moisture behaviour of roofing assembly.