LBNL Report Number
Dark roofs raise the summertime air-conditioning demand of buildings. For highly-absorptive roofs, the difference between the surface and ambient air temperatures can be as high as 90 °F, while for highly-reflective roofs with similar insulative properties, the difference is only about 20 °F. For this reason, cool roofs are effective in reducing cooling energy use. Several experiments on individual residential buildings in California and Florida show that coating roofs white reduces summertime average daily air-conditioning electricity use from 2-63%.
This demonstration project was carried out to address some of the practical issues regarding the implementation of reflective roofs in a few commercial buildings. The authors monitored air-conditioning electricity use, roof surface temperature, plenum, indoor, and outdoor air temperatures, and other environmental variables in three buildings in California: two medical office buildings in Gilroy and Davis and a retail store in San Jose.
Coating the roofs of these buildings with a reflective coating increased the roof albedo from an average of 0.20-0.60. The roof surface temperature on hot sunny summer afternoons fell from 175 °F-120 °F after the coating was applied. Summertime average daily air-conditioning electricity use was reduced by 18% (6.3 kWh/1000ft2) in the Davis building, 13% (3.6 kWh/1000ft2) in the Gilroy building, and 2% (0.4 kWh/1000ft2) in the San Jose store.
In each building, a kiosk was installed to display information from the project in order to educate and inform the general public about the environmental and energy-saving benefits of cool roofs. They were designed to explain cool-roof coating theory and to display real-time measurements of weather conditions, roof surface temperature, and air-conditioning electricity use.