Cool walls save energy and fight summer heat, study says
David Wagman | July 17, 2019Summertime heat is on full blast all around the country, and energy demand to run home air conditioners and commercial HVAC systems is ramping up.
With that as a backdrop, a study by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory modeled several different types and ages of homes, retail stores and office buildings in cities across the U.S. It found that sunlight-reflecting “cool” exterior walls can save as much or more energy as sunlight-reflecting cool roofs.
Row houses in Washington, D.C.Cool walls are generally considered to have a higher-than-average solar reflectance. That means they reflect a higher fraction of incoming sunlight than average exterior walls. Cool walls also have high thermal emittance, which means they can efficiently release absorbed heat.
Walls receive less intense sunlight than roofs, but also tend to be less insulated. Across most of the U.S., roughly 40% to 60% of all buildings were built before 1980, when building codes generally specified less wall insulation than is required today. As a result, cool-wall savings in older buildings could be three to six times greater than for new buildings, the study said.
The researchers said that repainting the exterior walls of pre-1980 buildings with cool paint is not difficult to do, as many light-colored cool paints can be found in local home supply stores.
Although no formal, universal definition exists for cool walls, the Berkeley Lab scientists suggest that lower-performance cool walls should reflect at least 40% of solar energy. Higher-performance cool walls should reflect at least 60%. The study assumed that an average wall that is not considered cool reflects 25% of sunlight.
Light-colored walls are coolest, the study said. But pigments that reflect the invisible half of sunlight and pigments that fluoresce can make cool paints in a wide range of colors. Shiny, bare-metal walls tend not to be cool because they are slow to release absorbed heat, the study said.
Potential energy savings
In warm U.S. cities from Florida to New Mexico, researchers found that cool walls could lead to annual HVAC energy cost savings of up to 11% for stand-alone retail stores, 8.3% for single-family homes and 4.6% for medium-sized office buildings.
The study accounted for both energy savings in the cooling season and elevated energy costs in the heating season.
The Cool Roof Rating Council was created in 1998 as a nonprofit educational organization to support research and rating systems for cool roofing products, and to serve as a resource for increasing energy efficiency in buildings. It is pursuing an expansion into ratings for cool walls. The organization voted in June to amend its bylaws to permit the rating of wall surface products.