Smart meters and other high-tech energy efficiency gadgets may grab all the headlines, but researchers have shown that saving energy can be as simple as painting over a dark roof with a lighter color. Now researchers at Lawrence Berkeley National Laboratory in California have added another twist to the “cool roof” trick: cool walls.
Researchers have already assembled ample evidence that painting a lighter color over a dark roof can save a significant amount of energy for buildings located in warmer, sunnier climates. Since most buildings have far more wall area than roof area, the next logical step to look for energy savings would be in those vertical surfaces.
The new Berkeley Lab study is titled Solar-Reflective “Cool” Walls: Benefits, Technologies, and Implementation. The researchers examined the potential for light-colored walls to save energy in different types of buildings in California and elsewhere in the U.S., including standalone retail stores and office buildings as well as homes.
The best results occurred in cities located in the southern U.S. In those areas, the study demonstrated an annual HVAC savings of 11 percent for standalone retail stores. Medium-sized office buildings registered a 4.6 percent savings.
Residential properties also fared well. The study found an average energy efficiency gain of 8.3 percent in the southern U.S. Savings ranged from 4 percent to 27 percent across all of California.
In another recent study, researchers found that a fresh coat of “cool” paint on exterior walls provides about as much energy savings as a cool roof upgrade.
That result may seem counterintuitive. After all, vertical walls generally do not receive as much direct sunlight as rooftops.
The difference is that walls in U.S. buildings are typically not as well insulated as roofs. That’s especially the case for older buildings.
Before 1980, U.S. building codes called for less insulation. The result is that even a modest improvement in older walls can have a significant impact.
In fact, the study found that property owners with pre-1980 buildings on their hands stand to realize energy savings of three to six times more than owners of newer buildings.
Creating a cooler community
The study shows that cool walls are one of the low-hanging fruits of the energy efficiency field, ripe for the picking with relatively little effort.
The savings goes beyond direct bottom line benefits. Another recent study found that cool walls can enable building owners to claim credit for helping to reduce the “heat island” effect in cities (heat islands occur where dark surfaces absorb heat and release it slowly).
Since cool walls (and roofs) go to work during peak daylight energy consumption hours, they also enable building owners to take credit for helping to shave peak demand.
The ability to shave down demand can have enormous consequences. Typically, policymakers rely on building new power plants to meet peak demand. Cool walls and other energy efficiency improvements can help avoid those costs.
Forward-looking policy makers are beginning to rely more on renewable energy, smart grid technology and energy storage to meet peak demand. Building owners can accelerate that trend with cool walls and other energy efficiency upgrades.
Building owners with rooftop solar and energy storage systems can also enhance the impact of those investments by making their next paint job a cool one.
The Berkeley Lab team also suggests how building owners can maximize the energy efficiency impact of their next paint job.
Standalone buildings in hot, sunny parts of the U.S. (and many parts of California) are the best candidates for a high impact, cool wall upgrade.
Buildings that receive shade may still realize some savings, depending on how much they receive.
As for choosing a paint color, the study confirms a common sense approach: light colors are the most efficient at reflecting sunlight.
However, the study also indicates that building owners do not need to settle for plain white:
“Light-colored walls are coolest, but pigments that reflect the invisible half of sunlight and pigments that fluoresce can be used to make cool paints in a wide range of colors.”
Metal roofs are popular in parts of the southern U.S., but the researchers caution that shiny, unpainted metal surfaces are not necessarily good candidates for cool walls. Unpainted metal absorbs heat and in turn is slow to release it.
On the other hand, painted metal roofs can provide for significant energy savings.
Cool is in the eye of the beholder
As of now, there are no formal standards for labeling cool wall paint, though the Berkeley Lab team suggests that a typical cool wall should reflect about 40 percent of sunlight.
The lab also indicates that claims about high-performance paint should be reserved for those reflecting about 60 percent. In contrast, non-cool walls reflect only 25 percent.
Help is on the way. The nonprofit Cool Roof Rating Council is looking into taking wall paint under a rating system that will help take some of the guesswork out of choosing the coolest paint.
In the meantime, the study indicates that there can be hidden energy savings behind dark walls. Building owners who are considering a fresh coat of paint can give their property an energy efficiency upgrade simply by choosing a lighter color.
Image credit: Christian Perner/Unsplash
Tina writes frequently for TriplePundit and other websites, with a focus on military, government and corporate sustainability, clean tech research and emerging energy technologies. She is a former Deputy Director of Public Affairs of the New York City Department of Environmental Protection, and author of books and articles on recycling and other conservation themes. She is currently Deputy Director of Public Information for the County of Union, New Jersey. Views expressed here are her own and do not necessarily reflect agency policy.