Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants

Cool roofs-roofs that stay cool in the sun by minimizing solar absorption and maximizing thermal emission-lessen the flow of heat from the roof into the building, reducing the need for space cooling energy in conditioned buildings. Cool roofs may also increase the need for heating energy in cold climates. For a commercial building, the decrease in annual cooling load is typically much greater than the increase in annual heating load. This study combines building energy simulations, local energy prices, local electricity emission factors, and local estimates of building density to characterize local, state average, and national average cooling energy savings, heating energy penalties, energy cost savings, and emission reductions per unit conditioned roof area. The annual heating and cooling energy uses of four commercial building prototypes-new office (1980+), old office (pre-1980), new retail (1980+), and old retail (pre-1980)-were simulated in 236 US cities. Substituting a weathered cool white roof (solar reflectance 0.55) for a weathered conventional gray roof (solar reflectance 0.20) yielded annually a cooling energy saving per unit conditioned roof area ranging from 3.30 kWh/m(2) in Alaska to 7.69 kWh/m(2) in Arizona (5.02 kWh/m(2) nationwide); a heating energy penalty ranging from 0.003 therm/m(2) in Hawaii to 0.14 therm/m(2) in Wyoming (0.065 therm/m(2) nationwide); and an energy cost saving ranging from $0.126/m(2) in West Virginia to $1.14/m(2) in Arizona ($0.356/m(2) nationwide). It also offered annually a CO(2) reduction ranging from 1.07 kg/m(2) in Alaska to 4.97 kg/m(2) in Hawaii (3.02 kg/m(2) nationwide); an NO(x) reduction ranging from 1.70 g/m(2) in New York to 11.7 g/m(2) in Hawaii (4.81 g/m(2) nationwide); an SO(2) reduction ranging from 1.79 g/m(2) in California to 26.1 g/m(2) in Alabama (12.4 g/m(2) nationwide); and an Hg reduction ranging from 1.08 mu g/m(2) in Alaska to 105 mu g/m(2) in Alabama (61.2 mu g/m(2) nationwide). Retrofitting 80% of the 2.58 billion square meters of commercial building conditioned roof area in the USA would yield an annual cooling energy saving of 10.4 TWh; an annual heating energy penalty of 133 million therms; and an annual energy cost saving of $735 million. It would also offer an annual CO(2) reduction of 6.23 Mt, offsetting the annual CO(2) emissions of 1.20 million typical cars or 25.4 typical peak power plants; an annual NO(x) reduction of 9.93 kt, offsetting the annual NO(x) emissions of 0.57 million cars or 65.7 peak power plants; an annual SO(2) reduction of 25.6 kt, offsetting the annual SO(2) emissions of 815 peak power plants; and an annual Hg reduction of 126 kg.