Evaluation of the Energy Efficiency Effectiveness of Cool Roofs for Residential Applications

Abstract

This study determines the effects of cool roofs on a home’s energy use, specifically on heating and cooling energy end-uses. Representative cities were chosen for several ASHRAE US climate zones. A series of parametric simulations in EnergyPlus was carried out to assess the performance of cool roofs for selected prototypical residential building models using detailed simulation analysis. The simulation results are then correlated for each climate zone type to give an approximation of the best roof color per climate. The results are given based on total energy used as well as energy cost based on national average electricity and natural gas residential rates. This method allows builders and homeowners the choice between the most cost effective roofing type, and the most energy efficient in the case that they are not the same. Overall, it was found that in hot climates, it is more efficient to have a white roof, while a black roof benefits cooler climates. In mild and mixed climates, the effect of roof color was found rather are different for energy use and energy cost. Therefore the choice is determined by the owner’s requirements. In the cooler and milder climate zones, the analysis shows that the cost excess or savings is fairly small; usually under $10 difference per year. Hotter climates also have a relatively small effect, but more so than the cooler climates, with Phoenix especially showing a savings of $48.60 per year when a white roof is used over a black roof. Energy changes as low as only 4% in the as-built construction style, or as high as nearly 100% change in upgraded envelope cases were found. The study further finds that both the lack of an attic, and high efficiency envelopes increases the magnitude of the percent change in energy requirements.