Changes in the heating and cooling energy use in buildings due to lowering the surface solar absorptance of roofs

Abstract

This report addresses how changing a flat roof's solar absorptance alters the energy required to heat and cool a building. The increase in a roof's surface temperature due to insolation increases the building's heat gain during the summer and reduces its heat loss during the winter. This study examines this counteracting influence on annual HVAC energy use. The report reviews pertinent background and presents computed changes in heating and cooling needs obtained using the computer code, DOE 2.1B. All computations were made corresponding to a reduction in a flat roof's solar absorptance from 0.8 to 0.3. They were made for two different buildings using TMY weather data for twenty cities within the United States; they were also made for a third building using weather data for five US cities. Computed annual changes in building heating and cooling energy use are presented in the form of bar charts for each location. Calculations were made for three different roof insulation levels. The change in annual energy use caused by the reduction in solar absorptance decreases with increased roof insulation. This effect is depicted graphically for representative cases. Incorporating realistic HVAC system performance and using a particular energy cost scenario based on use of natural gas for heating and cooling via an electrically driven unit, the best cost savings occurred for locations in the Southwest and were equivalent to approximately the costs of 1 Kw-hr of electrical energy per square foot of flat roof surface. 16 refs., 18 figs., 29 tabs.