A new report by the Lawrence Berkeley National Laboratory says white roofs are the optimal choice, both economically and for cooling the globe.

Compared to traditional dark-colored roofs and green or “vegetated” roofs, white roofs offer greater ability to lower temperatures that lead to the urban heat island effect, and they do it at less cost.

The report, Economic Comparison of White, Green, and Black Flat Roofs In the United States, states flatly that building owners “concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe.”

The report’s authors analyzed 22 commercial flat roof projects in the U.S. and performed a 50-year life cycle cost analysis. They assumed a 20-year service life for black roofs and white roofs, and 40 years for green roofs. Compared to black roofs, the report says, white roofs save $25 per square metre and green roofs have an an additional cost of $71 per square metre.

In addition to concluding that white roofs are the most economical, the authors concluded that black roofs should be prohibited in some areas.

“We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities,” the report states. In particular, black roofs have been associated with higher mortality during urban heat waves for people who live on the top floor of buildings.

The call for a phase-out of black roofs has drawn sharp criticism from the EPDM Roofing Association (ERA), a trade group representing manufacturers of a rubber roofing product that’s available in both black and white colors. The group’s associate executive director, Ellen Thorp, said the recommendation to ban black roofs is based on faulty science.

“We question the validity of this study, since it is based on a sample size of only 22 roofs, and we are challenging the conclusions that the authors draw from the data,” she said.

Green Roofs Provide Other Benefits

While white roofs provide the greatest economic payoff, green roofs provide benefits unavailable from other options, such as controlling runoff and curbing air pollution.

A new Growing Green Guide titled Victoria’s Guide to Green Roofs, Walls, & Facades offers an explanation of the many benefits that result from green roofs, walls, and facades in urban settings. The guide states that “Australia is beginning to realise the potential of green roofs, walls and facades to assist in reducing the impacts of increased temperatures, intense rainfall, habitat loss and increased energy use in cities.”

The report estimates that there were approximately 87 green roofs in Victoria in 2011.

Green roofs can effectively collect and slow storm water. Depending on the project’s design, a green roof may also be able to store water until it can be used by the plants or it evaporates. These systems can often filter that water as well, releasing water with lower levels of particulates and pollutants.

Green roofs are also effective at reducing urban air pollution. According to the U.S. EPA report, Reducing Urban Heat Islands: Compendium of Strategies, a green roof can remove particulate matter (PM), nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), and ground level ozone (O3).

Researchers estimated that a 93 square metre green roof could remove about 18 kilograms of PM from the air while also reducing CO2 and producing oxygen. That 18 kilograms of PM is roughly equivalent to the output of 15 cars in one year of typical driving. In addition to storm water management and air pollution mitigation, green roofs benefit cities by producing oxygen, providing wildlife habitat, and increasing human health and well being.

Though they cost more than white roofs, the cost premium of a green roof over its longer life span may not be a major issue. The Lawrence Berkeley report determined the extra cost of a green roof would be $3.20 per square metre.

“This annual difference is sufficiently small that the choice between a white and green roof should be based on preferences of the building owner,” the report said.

The report was authored by Julian Sproul, Benjamin H. Mandel, and Arthur H. Rosenfeld from the Lawrence Berkeley National Laboratory, and Man Pun Wan, assistant professor at the School of Mechanical & Aerospace Engineering at Nanyang Technological University in Singapore.