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Can’t Take the Heat? “Cool Walls” Can Reduce Energy Costs, Pollution

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A nationwide study by Berkeley Lab details benefits of lighter-colored, solar-reflective walls

As you likely know, NCCA members and their customers along the prepainted metal value chain have been supporters of “cool roofs” for 20 years. Without question, this technology effectively reduces energy consumption during the hot months of the year. So, what’s next?

“Cool walls”! Wait, what? Cool walls? Cool-roof technology is intuitive; roofs face the sun, but walls are vertical structures. How much solar energy can they possibly be exposed to? Well, you’ve come to the right place to find the answer.[i]

A study by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) modeled several different types and ages of homes, retail stores, and office buildings in cities across California and the U.S. The study found that in many places, sunlight-reflecting “cool” exterior walls can save as much or more energy than sunlight-reflecting cool roofs and can help mitigate the urban “heat island” effect. The study analyzed more than 100,000 building simulations. The Heat Island Group at the Lab explores the urban heat island effect—a rise in outdoor urban air temperatures caused by the concentration of buildings, roads, and other structures that absorb heat—and ways to abate the problem.

Savings found across California and southern half of U.S.

In warm U.S. cities from Miami, Florida, to Albuquerque, New Mexico (see map: climate zones 1A to 4B), researchers found that cool walls could lead to annual heating, ventilation, and air conditioning (HVAC) energy cost savings up to 11% for stand-alone retail stores, 8.3% for single-family homes, and 4.6% for medium-sized office buildings. And for single-family homes across all California climates, the study found potential energy cost savings of 4% to 27%.



Map of U.S. climate zones. Zone 7 (not shown) includes most of Alaska. Zone 1 includes Hawaii, Guam, Puerto Rico, and the Virgin Islands. (Credit: Solar-Reflective “Cool” Walls: Benefits, Technologies, and Implementation, Appendix P/California Energy Commission)

The study accounted for both energy savings in the cooling season and elevated energy costs in the heating season.

The researchers found that cool walls offer comparable reductions in annual emissions of air pollutants such as nitrogen oxides and sulfur dioxide and greenhouse gases such as carbon dioxide. They also lower peak demand for electric power.

“Cool walls provide energy cost savings and emission reductions across California and the southern half of the United States. In these climates, cool walls can save as much or more energy than the same size cool roof,” said Ronnen Levinson, a Berkeley Lab staff scientist and leader of Berkeley Lab’s Heat Island Group who co-authored the study, which was published in the journal Energy & Buildings. Walls receive less intense sunlight than roofs, but are also less insulated, he explained.

And there’s the key. The walls of most homes and buildings have about four inches of space into which insulation can be installed. In a home’s attic space, feet of insulation could be used, along with other means of keeping heat from entering the attic space. In addition to IR reflective roofing products, a batten roof installation provides an air gap underneath the prepainted metal panel that allows hot air to flow between the panel and the sublayer, escaping into the air at the eave, thus preventing it much of it from entering the attic. You just don’t have this facility with walls.

Across most of the U.S., about 40% to 60% of all buildings were built before 1980, when building codes generally specified much less wall insulation than is required today. As a result, cool-wall savings in these older buildings that meet the older codes could be three to six times greater than the savings for new buildings, the study found.

Early steps toward cool-wall standards

There is now some momentum toward specifications and ratings for cool walls. The Cool Roof Rating Council, which was created in 1998 as a nonprofit educational organization to support research and rating systems for cool-roofing products and serve as a resource for increasing energy efficiency in buildings, is pursuing an expansion into ratings for cool walls. The organization voted last year to amend its bylaws to permit the rating of wall surface products. The latest study makes it clear that many places could benefit greatly from cool walls. The study was supported by the Electric Performance Investment Charge (EPIC) program of the California Energy Commission and also by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy.

While there is not yet a formal universal definition for cool walls, Levinson suggests that lower-performance cool walls should reflect at least 40% of solar energy, while higher-performance cool walls should reflect at least 60%. The study assumed that an average wall that is not cool reflects 25% of sunlight.

The coil-coating industry is well ahead of the curve on the issue of cool roofs and cool walls. Nearly 1,400 coil-coating products are listed in the CRRC rated products directory (coolroofs.org/directory), representing a wide variety of color and coating technologies. And NCCA’s involvement with CRRC continues. David Cocuzzi, NCCA Technical Director, who was chair of CRRC in the early 1990s and now serves as an ex office member of the CRRC, is the chair of CRRC’s Wall Rating Steering Committee, which is charged with developing a program similar to the reflective roofing program. Many of the components needed for a wall-rating program are already in place with the roof-rating program (e.g.,accredited testing laboratories, test farms, an interactive web site, etc.).


[i] NOTE: This blog post has borrowed extensively from an excellent article published by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). NCCA has received permission to use the material contained in that article, which can be found at https://newscenter.lbl.gov/2019/07/09/cool-walls-can-reduce-energy-costs-pollution/.

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