In the summer of 2002, researchers in Sacramento tested a deceptively simple way to reduce the amount of heat entering a commercial building by changing the surface of its roof. The building was a retail flooring showroom with a dark roof that absorbed large amounts of sunlight. After researchers applied a reflective cool roof coating, the difference was dramatic.
The roof surface became as much as 42°C cooler, while the building's air-conditioning energy use fell by roughly 52% during the monitoring period. The experiment offered a striking real-world demonstration of how much a roof can influence a building's energy consumption.
Scientists measured what happened inside a real California building
The research was conducted by scientists associated with the Heat Island Group at Lawrence Berkeley National Laboratory. Rather than relying solely on computer models, researchers monitored three California commercial buildings after reflective roofing systems were installed.
The sites included a retail store in Sacramento, a school in San Marcos and a cold-storage facility in Reedley. The Sacramento building produced the most striking result.
Researchers compared the building's performance before and after its dark roof was replaced with a reflective surface, monitoring roof temperatures and electricity consumption during a hot California summer. The Sacramento store used 83 fewer kWh of air-conditioning energy per day.
According to the federal report documenting the experiment, monitoring between August 8 and September 30, 2002, showed that the Sacramento retail building used approximately 83 kilowatt-hours less electricity per day for air conditioning.
That represented a reduction of about 52% compared with the building's performance under its original dark roof. The roof itself also experienced a dramatic temperature change.
During periods of intense sunlight, the reflective coating reduced peak roof surface temperatures by approximately 33°C to 42°C.
On particularly hot afternoons, when outdoor temperatures climbed above 38°C, researchers also observed substantial reductions in peak electricity demand during the afternoon and evening.
Why did the reflective roof make such a difference?
The science behind the experiment is relatively simple. A conventional dark roof absorbs a large proportion of the sunlight that strikes its surface. That solar energy is converted into heat, making the roof extremely hot.
Some of that heat eventually moves toward the interior of the building, increasing the workload placed on its air-conditioning system. A reflective or cool roof works differently.
