I recently reported from Pacoima on how “cool pavement,” a type of coating that reflects sunlight and heat, could help bring temperatures down in cities as the climate crisis pushes us into a hotter reality.
It prompted an LAist reader, Denise Davis, from Crenshaw Manor, to ask us this:
"Last fall Los Angeles was terribly hot and it's becoming miserable again. If Pacoima used paint on pavement, like streets and parking lots, is there reliable information on how practical or effective it might be to paint our homes with it?
It’s a good question, because roofs and outside walls absorb a lot of heat, especially if they’re a dark color and unshaded.
That heat ends up in our homes, driving up energy costs from air conditioning and fans, and even leading to heat stroke and other dangerous health situations, felt disproportionately by low and moderate-income folks.
So the short answer is — yes — there are such things as cool roof shingles and cool paint and they’ve been around (and studied) a lot longer than cool pavement. Research shows cool roofs can cut air conditioning use by on average 20% and lower indoor temperatures by as much as 10 degrees.
“A reflective roof and a reflective wall are very simple and reliable passive strategies that don't require you to do anything other than make the right choice when it comes time to re-roof or re-paint,” said Ronnen Levinson, a cool materials scientist at the Lawrence Berkeley National Laboratory.
“It will help you now and it'll be even more useful as the climate warms.”
Long before air conditioning, humans have used these passive methods to stay cool. One example? The white or lighter-colored buildings that characterize cities from the Mediterranean to the Middle East. Those lighter surfaces reflect solar radiation, rather than absorb it.
“Ever since people have lived in hot places, they've tried to do the sensible thing, which is keep the building cool by absorbing less sunlight at its surface,” said Levinson. “North American architecture hasn't always worked that way.”
Levinson said commonly used materials on homes here, such as traditional asphalt roof shingles, make air temperatures in cities hotter than areas outside — that’s the urban heat island effect.
So a cool roof could actually be one of the quickest and most effective ways to curb that regional heat, as well as cut energy costs, especially for low- and moderate-income homeowners who tend to live in older housing and in denser neighborhoods with more pavement, said Alex Turek with GRID Alternatives, a nonprofit that installs cool roofs and rooftop solar on low-income homes for free.
“There's such a hardscape that trees may not be an option for some homeowners,” he said. “Cool roofs can definitely provide similar benefits to trees.”
By 2005, research by Levinson and his predecessors helped push California to establish new building codes requiring flat commercial roofs to be painted white to alleviate that impact. Those roofs are now ubiquitous across the state.
Fast forward to 2015, when L.A. became the first major city to require all new and replaced residential buildings to have “cool roofs.”
A cool roof is simply a highly reflective roof. For example, a roof with a clean, smooth, bright white surface can reflect about 85% of sunlight, whereas a dark asphalt shingle roof can absorb as much as 95% of sunlight. Now, that could be good in a cold climate, but in hotter climates it’s not so great.
The two main components that determine how “cool” a roof is are solar reflectance (the amount of sunlight that’s reflected) and thermal emittance (the ability of that material to radiate heat).
These are measured on a scale from 0 to 1, where 1 is 100% reflective or emissive. The higher the decimals (eg 0.7 or 0.8), the cooler the roof. That’s called the solar reflectance index.
The nonprofit Cool Roof Rating Council allows you to search for different roof or wall materials based on these measurements.
While commercial roofs are usually white and made of silicone, cool roofs don’t always have to be white or light. There are "cool color" roof materials, including coatings and shingles, that use darker-colored pigments that are still highly reflective in the invisible half of the solar spectrum.
While cool roofs can provide a lot of relief in the summer, they may actually lead to more heating in the winter. However, research has shown that increase is marginal and the cost savings of a cool roof in the summer also offset that issue. Furthermore, the technology of “smart roofs” and improvements in materials are rapidly offsetting this issue.
They can help extend the life of the roof because they relieve heat stress and wear on its surface, said Andres Lopez, marketing director at Central Roofing, a family owned company that installs cool roofs on commercial buildings across Southern California. “It just has too many benefits to pass up,” Lopez said.
A cool roof also cools the air outside the building, which slows the formation of smog, said Levinson.
At scale across commercial and residential buildings, they can actually help cool the entire planet.
"On a clear day, 75% to 80% of sunlight that is reflected from a horizontal surface like a pavement or a low slope roof will get all the way to outer space without being absorbed by the atmosphere, so that allows us to reduce warming of the planet as a whole,” Levinson said.
If you don't want or need to re-roof: Apply a cool roof coating to your existing roof.
If you do need to re-roof: Install cool roof shingles.
