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Research Looks at Cool Roofs, Heat Waves
A recent study by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory takes a look at the impact cool roofs could have in California, with the goal of shielding people from heat waves.
The study, “Interacting Implications of Climate Change, Population Dynamics and Urban Heat Mitigation for Future Exposure to Heat Extremes,” was recently published in the journal Environmental Research Letters.
“The researchers predict that heat waves are likely to become two to 10 times more frequent across the state by mid-century,” according to Berkeley’s press release.
“But if cool roofs were adopted throughout California’s most populous areas—the San Francisco Bay Area, Los Angeles, San Diego and Sacramento—by 2050 these reflective roofs could bring down heat wave exposures (defined as each time a person experiences a heat wave) by 35 million each year, compared to an estimated 80 million heat wave cases in 2050 with no increase in cool roof adoption.”
The Study
In the study, researchers had two goals, according to the lab: They wanted to predict heat wave occurrences across California’s 29 major counties between now and 2050, and they wanted to analyze the effectiveness of cool roofs in mitigating heat wave impact.
Researchers used regional climate conditions between 2001-15 as a starting point to simulate the climate under two scenarios (with and without cool roofs). Combining those conditions with high-resolution satellite images is what allowed them to look at buildings, roads and vegetation, which absorb and release heat.
Then, they used population estimates for 2050 to assess population exposure to heat waves.
“We wanted to gain a better picture of future climate change risks for California’s urban environments and adaptation options,” said Andrew Jones, a scientist in Berkeley Lab’s Climate and Ecosystem Sciences Division and co-author of the study. “Making such refined and realistic predictions can help urban planners and citizens prepare for heat events in an increasingly warming future.”
The study found heat waves with temperatures exceeding 95 F and lasting three consecutive days to become two to 10 times more frequent under the estimated scenarios. The researchers expect that there will be 80 million heat wave exposure cases in California each year, more than double what is currently seen.
They found that white-coated roofs or the installation of sun-reflecting could be a helpful answer.
To analyze the effectiveness of the cool roofs, the team repeated the simulations, but replaced all the existing building roofs with cool roofs.
They found that if every building in California sported cool roofs by 2050, it could bring down the annual number of heat wave exposures in California to 45 million from 80 million, according to the lab.
“Although a small percentage of California’s land is urban, I was surprised at how effective cool roofs could be in pushing back risks of heat extremes,” Jones said.
The researchers acknowledge, however, that the retrofitting of existing buildings needed to accomplish the every-building goal is a difficult task, so the next step is finding the minimum cool-roof coverage needed to accrue similar benefits.
Recent Studies
This research was released about a month after similar tests out of Berkeley, when scientists looked at the use of cool exterior walls, and how those could save building owners energy in comparison to cool roofs.
“Cool walls provide energy cost savings and emission reductions across California and the southern half of the United States,” said Ronnen Levinson, a Berkeley Lab staff scientist and leader of Berkeley Lab’s Heat Island Group who co-authored the study, published in the journal Energy & Buildings.
“In these climates, cool walls can save as much or more energy than the same size cool roof.”
That report analyzed more than 100,000 building simulations and modeled several different types and ages of homes, retail stores and office buildings in cities across California and the rest of the U.S.
According to Berkeley, about 40% to 60% of all buildings in the U.S. were built before 1980, when building codes generally specified much less wall insulation than today. As a result, cool-wall savings in these older buildings could be three to six times greater than those for new buildings, the study found.
