Utilizing a cool roof is an efficient way to reduce the cooling energy use of a building. Cool roofs, however, may increase heating energy use in winter. In cold climates, during the winter the sun angle is lower, days are shorter, sky is cloudy, and most heating occur during early morning or evening hours when the solar intensity is low. In addition, the roof may be covered with snow for most of the heating season. All these lead to a lower (than what is commonly thought) winter time heating penalties for cool roofs.
We used DOE-2.E to simulate energy consumption in an office building in four cold climate cities in North America: Anchorage (AK), Milwaukee (WI), Montreal (QC), and Toronto (ON). The effect of sun angle, clouds, daytime duration, and heating schedules can be modelled with existing capabilities of DOE-2. Snow on the roof provides an additional layer of insulation and increases the solar reflectance of the roof. To simulate the effect of snow, we defined a function consisting of U-value and absorptivity of the roof on a daily basis to simulate four different types of snow on the roof. We used an average of six years meteorological data from National Oceanic and Atmospheric Administration (NOAA) and Environment Canada to estimate the snow thickness on the roof. Results show that the heating penalties of cool roof are significantly lower (than what is commonly thought) considering snow on the roof. Annual heating energy consumption of the building with dark and cool roof without considering the snow are 85 and 88 MJ/100 m2, respectively (3 MJ/100 m2 penalty for cool roof) in Anchorage whereas, the annual heating energy for the dark and cool roof considering the effect of Late-Winter Packed snow are 83 and 84 MJ/100 m2, respectively (1 MJ/100 m2 penalty for the cool roof). For a typical office building with electricity as cooling fuel and natural gas as heating fuel, cool roofs save $0.08/ m2 in Montreal and in Toronto the saving for cool roof is $0.04/ m2 (not accounting for the effect of peak demand savings and potential downsizing of the HVAC systems).