Sacramento researchers coated a retail store’s dark roof white in 2002. Its peak roof temperature fell by up to 42°C and air-conditioning energy use dropped 52%

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.


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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.

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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.

Its lighter, more reflective surface sends much more incoming solar radiation back away from the building. As a result, the roof does not heat up as dramatically, reducing the amount of heat that the air conditioner has to fight.

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The building's design helped amplify the savings

But the researchers found that the building itself also played an important role.

The Sacramento store had relatively thin roof insulation and operated its air-conditioning system for long periods each day. Those characteristics meant that reducing heat at the roof had a particularly noticeable effect on the building's cooling requirements.

The other buildings in the experiment also recorded reductions, but their savings were smaller.

That distinction is important. The researchers did not conclude that every commercial building would automatically cut its air-conditioning consumption by 52% simply by installing a reflective roof.

Instead, the Sacramento store's construction, insulation and operating schedule helped explain why its results were particularly dramatic.

The experiment helped inform California's cool-roof push

The research did not end with the three buildings. Using measurements from the California sites, researchers developed calibrated simulations to estimate how reflective roofs could perform across the state's different climate zones.

Their projections indicated that retail buildings similar to the Sacramento store could save roughly 6 to 15 kilowatt-hours per square metre of conditioned roof area each year by switching to a reflective roof surface.

Scaled across thousands of commercial buildings, even relatively modest savings per building could become substantial.

The findings subsequently became part of the broader body of research supporting California's efforts to promote reflective roofing and reduce cooling demand.