Simulated influence of roof reflectance on the building energy balance in two northern cities

Solar irradiation on the roof surfaces of buildings affects cooling and heating energy demand. In this study the effect of enhanced roof reflectance is investigated using computer simulations of a prototype large retail building with four roof insulation levels (R-4 through R-24) driven by hourly weather data for Minneapolis, MN. The dark roof surface temperatures in the summer reach more than 160 degrees F (71 degrees C); the white roof reaches about 100 degrees F (38 degrees C), leading to 6%-17% lower cooling energy consumption. Although the increased heating costs in the winter negate a portion of the cooling savings, the total energy cost savings of the white roof are still positive. Also, a reduction in the maximum hourly compressor power-responsible for the monthly billed electric peak demand-of 0.06 to 0.3 W/ft(2) (0.6 to 3 W/m(2)) can be consistently attributed to the high albedo roof. The demand cost savings may be in excess of the energy cost savings. The total annual savings a white roof can attain are in a range between $27/1,000 ft(2) ($0.3/m(2)) and $10/1,000 ft(2) ($0.1/m(2)) for R-4 and R-24 insulation. For Denver, CO., which has a warmer climate and is located farther south than Minneapolis, limited simulation results indicate total savings twice as high. With higher insulation levels, savings of a white roof compared to a black roof are lower but total cost savings of increased roof albedo can be higher than the cost savings of increased insulation. Also, the effects ofsnow cover and the operation of an outside air economizer on the energy savings of a reflective roof are investigated.