Heating energy penalties of cool roofs: the effect of snow accumulation on roofs

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 winter the sun angle is low, days are short, the sky is cloudy, and most the heating occurs early in the morning or in the 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-2.1E 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 the 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 DOE-2 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 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. The annual heating energy consumption of the building with dark and cool roof without considering the snow are 85 and 88 GJ/100 m(2), respectively (3 GJ/100 m(2) 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 GJ/100 m(2), respectively (1 GJ/100 m(2) 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 $ m(-2) in Montreal and in Toronto the saving for cool roof is 0.04 $ m(-2) (not accounting for the effect of peak demand savings and potential downsizing of the heating, ventilation, and air conditioning systems).