Cooler tile-roofed buildings with near-infrared-reflective non-white coatings

Abstract

Owners of homes with pitched roofs visible from ground level often prefer non-white roofing products for aesthetic considerations. Non-white, near-infrared-reflective architectural coatings can be applied in situ to pitched concrete or clay tile roofs to reduce tile temperature, building heat gain, and cooling power demand, while simultaneously improving the roof's appearance. Scale-model measurements of building temperature and heat-flux were combined with solar and cooling energy use data to estimate the effects of such cool-roof coatings in various California climates. Under typical conditions—e.g., 1kWm-2 summer afternoon insolation, R-11 attic insulation, no radiant barrier, and a 0.3 reduction in solar absorptance—absolute reductions in roof surface temperature, attic air temperature, and ceiling heat-flux are about 12, 6.2K, and 3.7Wm-2, respectively. For a typical 1500ft2(139m2) house with R-11 attic insulation and no radiant barrier, reducing roof absorptance by 0.3 yields whole-house peak power savings of 230W in Fresno, 210W in San Bernardino, and 210W in San Diego. The corresponding absolute and fractional cooling energy savings are 92kWhyr-1 (5%), 67kWhyr-1 (6%), and 8kWhyr-1 (1%), respectively. These savings are about half those previously reported for houses with non-tile roofs. With these assumptions, the statewide peak cooling power and annual cooling energy reductions would be 240MW and 63GWhyr-1, respectively. Statewide energy savings would reduce annual emissions from California power plants by 35kton CO2, 1.1ton NOx, and 0.86ton SOx. The economic value of cooling energy savings is well below the cost of coating a tile roof, but the simple payback times for using cool pigments in a roof tile coating are modest (5–7yr) in the warm climates of Fresno and San Bernardino.