At noon on a clear summer day in the United States, a flat (horizontal) surface receives about 1000 watts of sunlight per square meter. (This is about 100 watts, or the power of a bright incandescent light bulb, per square foot.) Traditional dark roofs strongly absorb this sunlight, heating both the building and the surrounding air. This increases energy use in air conditioned buildings, and makes non-air conditioned buildings less comfortable. Hot dark roofs also aggravate urban heat islands by warming the air flowing over the roof, and contribute to global warming by radiating heat into the atmosphere.
Substituting a cool roof for a warm roof reduces conduction of heat into the building, convection of heat into the outside air, and thermal radiation of heat into the atmosphere. This benefits our buildings, our cities, and our planet.
Cooler outside air. Cool roofs lower urban air temperatures by reducing the amount of heat transferred from roofs to the air, mitigating the urban heat island effect.
Fewer power plant emissions. Reflective roofs reduce cooling energy demand in air conditioned buildings, decreasing emissions of greenhouse gases and other air pollutants at power plants that burn fossil fuels.
Better air quality. Cool roofs decrease urban air temperatures and thus slow the formation of ground level ozone. Ozone, the primary component of smog, can aggravate respiratory illness and can act as a greenhouse gas.
Slowed climate change. Cool roofs decrease heat absorbed at the Earth’s surface and thus can lower surface temperatures. This decrease in surface temperatures reduces the flow of heat into the atmosphere, offsetting warming caused by greenhouse gases.
Energy and cost savings. During hot summer months, cool roofs reduce the need for cooling in air conditioned buildings, which saves energy and money.
Reduced electrical grid strain. The diminished demand for cooling energy will also moderate peak energy demand during heat waves and very hot summer afternoons, thereby decreasing the risk of power outages.
Improved indoor comfort. Cool roofs lower the indoor air temperature in buildings that do not have air conditioning, promoting productivity and occupant health.
read more original article Berkeley Lab