Common sense tells you to reach for a white shirt in the morning when you know a scorching day of heat is coming.
Scientists at Lawrence Berkeley National Laboratory say similar thinking should go into what kind of paint goes up on the exterior walls of buildings in warm climates.
The research team at Berkeley Lab found in a recent study that in much of the U.S., sunlight-reflecting “cool walls” could save as much or more energy than reflective cool roofs. Reflective walls can prevent solar heat gain indoors during the summer, shaving electricity consumption in the building and lowering peak demand on the grid.
There's no formal universal definition of what constitutes a “cool wall," said Ronnen Levinson, study co-author and staff scientist at Berkeley Lab. In general, the term refers to exterior walls with above-average solar reflectance stemming from the use of either light-colored paints or “cool colors,” paints with pigments enabling darker colors to reflect sunlight.
Based on an analysis of more than 100,000 building simulations, the Berkeley Lab researchers found that in climate zones encompassing the West Coast and southern half of the U.S., cool walls could lead to annual HVAC energy costs savings of up to 11 percent for standalone retail stores, 8.3 percent for single-family homes, and 4.6 percent for medium-sized office buildings.
Across all California climate zones, modeling showed potential energy cost savings of 4 percent to 27 percent for single-family homes.
A report published by the California Energy Commission in April estimated that adoption of solar-reflective cool walls across California’s single-family home building stock would yield around $500 million in annual energy cost savings.
The problem is that shoppers can’t today walk into a home store and select a paint, stucco or cladding certified by a third party to be “cool.”
In an interview with Greentech Media, Berkeley Lab’s Levinson said, “Today, ‘cool’ wall paints are not marked as such, but a light-colored paint is likely to be cool.”
“A few years from now, I hope you will see a label on each product that tells you the fraction of sunlight that it reflects," he said. "This will help you find a cool paint, stucco or cladding for your building.”
Biggest savings potential in older buildings
Cool walls offer the greatest potential energy savings in older buildings, researchers say.
“Repainting the exterior walls of pre-1980 buildings — whether homes or office buildings or stores — with cool paint offers the greatest benefit because they have the least insulation,” Levinson said. Before 1980, building codes did not require substantial wall insulation.
Researchers found the potential cool wall energy savings in pre-1980 buildings could be three to six times greater than for new buildings. Around half of U.S. buildings were constructed before 1980.
Building owners don’t have to opt only for light-colored paint when seeking cool wall benefits.
“Perhaps you want a wall that is not light in color, say, red or green or blue,” said Levinson. “You can use color pigments with exceptionally high near-infrared reflectance that helps the painted surface stay cool in the sun.”
It so happens there are common pigments that are conducive to these “cool colors.”
“Iron oxide red, chromium oxide green, phthalo blue — many pigments are naturally cool,” said Levinson.
Some manufacturers are already making paints that use cool colors. Tex-Cote, for instance, produces cool-colored wall coatings that the company claims can reduce exterior wall surface temperatures by up to 40 degrees Fahrenheit compared to traditional paints.
Berkeley Lab’s work on cool walls is an outgrowth of its decades-long research into cool roofs. Cool roofs, which became a required measure in 2005 for low-slope commercial buildings and in 2008 for some single-family homes under California’s Title 24 building energy-efficiency standards, are now commonplace.
“Today, most painted-metal roofing products use cool colors,” said Levinson. “They switched to cool-color technology because there wasn’t any substantial cost.” He added that cool colors are also commonly used in tile roofing and asphalt roofing shingles.
Expect cool walls to follow a similar path to regulatory approval and commercialization.
"Cool wall" certification coming
Before cool roofs became a required measure under Title 24, builders could earn credit toward compliance with the code, and potentially qualify for utility rebates, if they installed a sunlight-reflecting roof.
Levinson suggested that by the time of the 2022 update to Title 24, builders could earn credit toward code compliance with cool walls, and utilities could offer rebates for the installations as well.
Third-party certification of cool walls is on the way, too. Members of the Cool Roof Rating Council, which has rated the solar reflectance of roofing products since 1998, recently voted to allow the group to expand into ratings for cool walls.
A logical next step, Levinson said, would be cool wall demonstration projects sponsored either by the California state government or the state’s utilities.