Saving water may be as simple as changing the type of roof you have, two scientists in Berkeley discovered.
That might sound far-fetched, but their new findings about “cool” roofs recently published in the journal Nature Communications showed for the first time that it’s possible — and it could save California cities millions of gallons of water each year.
“This is a very intriguing study,” Stet Sanborn, associate principal for the green-engineering Integral Group branch in Oakland, said in an email. “It reinforces the interconnected web between water and energy, and I think the impact (of its findings) is significant and worth attention.”
“Cool” roofs are normal roofs built from or coated with a material that reflects heat rather than absorbs it, which often means they’re light in color.
It’s a lot like wearing a light-colored T-shirt versus wearing a dark one on a hot summer day: The lighter color reflects heat, keeping you cooler, whereas a dark T-shirt absorbs heat, making you feel hot, sweaty and uncomfortable.
That simple concept can cut business costs on air conditioning by 30 percent, says Amber Hoiska, who is the marketing director for Cool Roofing Systems, Inc. in San Jose and has been in the industry for 15 years. And those cuts, in turn, help reduce energy production that requires the release of carbon dioxide and detrimentally affects the environment.
Benefits such as these have been known for at least two decades and motivated the current Title 24 standards set by the California Energy Commission for constructed buildings.
But Pouya Vahmani and Andrew Jones, both scientists at Lawrence Berkeley National Laboratory in Berkeley and the authors of the new study, saw potential in this cooling effect on more than just cutting costs. They saw a potential impact on one of California’s most pressing issues that nobody else considered: water.
Using a computer model to simulate the next 15 years of dry-season temperatures and weather conditions down to 1-by-1-mile neighborhoods in 18 California counties, the scientists found the average temperature of cities built with “cool” roofs rather than traditional ones dropped by 3 to 4 degrees, which was enough to save cities like San Francisco anywhere from 4.1 to 7.3 percent of the gallons of water used per person per day for landscaping like lawns or home gardens.
Their finding stems from the effect that “cool” roofs have on what scientists call the “heat island effect,” a phenomenon in which cities tend to have temperatures 2 to 5 degrees hotter than only a few miles outside the city, the scientists explain. “Cool” roofs help bring that temperature difference back into balance by lowering the overall temperature of the city.
This alteration to the “heat island effect” was not surprising to Hoiska.
“People always think about the heat island effect with pavement,” Hoiska said, referring to how the materials and dark colors of pavement heat up the city. “But it’s the exact same concept with roofing, and that’s because roofing is usually made of asphalt.”
To make a roof waterproof, traditional roofs typically have a coat of material made from unreflective granules of dark-colored asphalt. But while waterproof, it means the roof heats up — a lot.
Regular roofs can reach temperatures in excess of 50 degrees hotter than the air only feet above it, according to the U.S. Department of Energy, creating sweltering conditions for the unlucky people inside the building. And if thousands of those roofs are close together in a city, it’s little surprise it helps create that “heat island.”
But the idea that “cool” roofs can have an impact on water usage, “That’s a new one to me,” Alex Bergeron, vice president of Teal City Roofing Inc. in San Jose, said as he let out a chuckle.
Turning a roof into a “cool” one requires specially reflective asphalt granules or a coat of white paint, which effectively drops the difference between roof temperature and the air around it to between 5 and 10 degrees rather than the scorching 50 degrees difference before. Installing “cool” structures such as these roofs, then, decreases heat build-up and, in turn, means lawns and gardens need less water because less of it will evaporate.
Bergeron acknowledged that he has seen these cooling effects, but he remained skeptical about its impact on water. “I don’t think a single roof with asphalt containing reflective granules will do much,” he said.
Which raises an assumption within the study that Peter Gleick, president of the Oakland-based Pacific Institute that focuses on developing water conservation policies, noted in an email. “What (the scientists) are saying is that if ‘cool’ roofs are implemented widely … then there will be a reduction in temperature in urban areas,” Gleick said.
To him and Hoiska, that seems plausible. “Put 25 buildings that have these roofs together, and yeah, you absolutely would expect to have that sort of impact,” Hoiska said.
Jones and Vahmani were also not surprised by the overall result of decreased water consumption, which they expected from the study’s outset. “What was surprising was the significant amount of water being saved,” Vahmani said. “Our study gives cities another reason to consider widespread implementation of ‘cool’ roofs,” especially after California’s record-breaking drought and the growing threat of warmer climate, both of which have added pressure on engineers to find new ways to conserve water and were the impetus for the study in the first place.
“In the wake of this recent drought in California and the first mandatory urban water consumption reduction that was imposed, we thought that it was important to look at new measures to reduce water consumption in urban areas,” Vahmani said.
But is coating or replacing your normal roof with a “cool” one, which Bergeson says for a residential owner may cost anywhere from $6,000 to $15,000, really the best way to save water?
Alone, maybe not. “I’d say one of the technical challenges of actually seeing these water-saving benefits is that you also need smart irrigation behavior,” Jones said.
The greatest benefits come when you combine “cool” roofs and good watering behavior, he explained. “Cool” roofs will decrease the amount of heat regardless of its water benefits, and smart irrigation practices will always save water.
“Cool” roofs may not be the overall remedy to the state’s water problems, but they offer a potential strategy for a field that needs many new and different ideas.
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