According to the U.S. Environmental Protection Agency (EPA), about $40 billion is spent annually in the United States to air-condition buildings – one-sixth of all electricity generated in a year. EPA statistics show that when used appropriately, a reflective roof can reduce cooling demand by 10 to 15 percent and increase cooling energy savings by as much as 50 percent.
Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratories (LBNL) in Berkeley, CA, say light-colored, highly reflective roofing materials also can have a significant impact on improving air quality and cooling “urban heat islands” – pockets of increased temperatures in downtown areas of cities caused by radiant heat from dark surfaces – thanks to their solar reflectivity.
Reflectivity, Not Absorption
Also called albedo, solar reflectivity is the amount of solar energy reflected away from a surface and usually is reported as a ratio. Researchers say one of the best measures for keeping solar heat out of buildings is simply to make the most of albedo by using reflective roofing materials.
Why? These materials simply reflect the sun’s radiant energy back toward the sky rather than allowing the building to absorb it.
According to research from LBNL, black surfaces – such as traditional roofing materials – can become up to 70 degrees F. (40 degrees C.) hotter than the most reflective white surfaces in the sun. If those dark surfaces are roofs, some of the heat collected by the roof is transferred inside, creating a need for more intensive cooling measures.
The EPA funded a study that provided detailed analysis of energy-saving potentials of light-colored roofs in 11 U.S. metropolitan areas. The Lawrence Berkeley Laboratories Heat Island Group simulated 10 residential and commercial building prototypes in each metro area.
Researchers considered both the savings in cooling and penalties in heating and estimated saving potentials of using light-colored roofs at about $175 million per year for the 11 test cities. Extrapolated national energy savings were about $750 million per year, according to LBNL data.
Additionally, The Heat Island Group has monitored buildings in Sacramento with lightly colored, more reflective roofs. Researchers found that these buildings used up to 40-percent less energy for cooling than buildings with darker roofs.
Scientists at the Florida Solar Energy Center (FSEC), a research institute of the University of Central Florida in Cocoa, FL, performed a similar study on residential dwellings.
The study proved dark gray roofs reflect a mere 8 percent of the heat associated with sunlight, while white shingle and terra cotta tile roofs reflect 25 and 34 percent, respectively. White metal and cement tile roofs provide the most dramatic results, reflecting 66 to 77 percent of the sun’s energy.
“It’s surprising that something as simple as choosing a different color can make such a difference,” says FSEC’s Danny Parker, who with his team of Jeffrey Sonne, John Sherwin, and Neil Moyer conducted the study for Florida Power and Light Company (FPL) with cooperation from the Habitat for Humanity of Lee County.
Researchers at LBNL have developed a solar reflectance index, which is the combined value of reflectivity and emissivity, the measure of a surface’s ability to emit its energy in the form of heat radiation. Most roofing materials, with the exception of bare metals, have a high emissivity.
This index helps gauge the effectiveness of various roofing products in different levels of sunlight. Typical hot roofing materials have reflectivity in the range of 0.03 to 0.20.
Fresh reflective materials are 0.65 to 0.85. They can age to 0.50 to 0.70. The Cool Roof Rating Council (CRRC) will have a labeling program before the end of this year.
One thing to keep in mind with respect to solar reflectance: It can change as a roof gets dirty and ages. Proper roof maintenance can keep reflectance values higher and more efficient, experts say.
What’s Right for You?
There are many factors that go into determining if a building is a good candidate for a cool roof. It all comes down to how that roof is integrated into a structure, where the structure is located, and how it used.
“Don’t look at a roof as a thin layer that keeps the water out of your building,” says Matt Tendler, AIA, project manager at the Midwest Sustainable Collaborative, a Milwaukee, WI-based, cross-disciplinary consulting and engineering firm. “Roofing has many implications: how it holds up the building, how dry it keeps the building, and how comfortable the space is under it. It’s a whole system, not just components, and every part of it affects another part.”
A good roof must last for decades, should require minimal maintenance, and be installed and maintained at a reasonable cost. Factors that can shorten the lifetime of a roof include ultraviolet radiation from the sun, freeze-thaw cycles, wind, rain, damage from foot traffic, biological growth, chemical reactions with air pollutants, thermal expansion stresses due to temperature changes, and poor installation. This list goes on.
Both Tendler and researchers at LBNL suggest that you should take these factors into account and do a cost analysis of cool roofing products with respect to how they might perform on your facility. “We do not recommend changing perfectly good ‘hot’ roofs and replacing them with ‘cool’ roofs,” cautions Hashem Akbari, a staff scientist and leader of the Heat Island Group at Lawrence Berkeley National Laboratories. “Every commercial roof has to have a major re-roofing every 10 to 15 years. When the time for re-roofing comes, one should consider cool options.”
“The simplest way to increase roof efficiency is to add more insulation to the R20 to R25 baseline,” Tendler says. “For most commercial buildings, it usually makes sense financially to up the insulation value to an R30 in most cases. Most people make the wrong assumption that if they design to code minimum, they’re up to the optimum in efficiency. That’s not the case.”
The next upgrade after beefing up a roof’s R-values may be to upgrade to a more reflective roof. It’s imperative that you specify a better product than what you might currently use.
If a cooler material is inferior from a cost or lifetime point of view compared to a warmer material, it’s not a good deal, the experts say. As Akbari points out, both options are available for almost all roofing products. For example, single-ply membranes come in both dark and white colors.
Cost is another consideration, and the entire roofing assembly and its location must be considered, Tendler notes. “Most roofs go through thermal cycling and wind abrasion. Many [roof] coatings don’t hold up to that. They do make sense in places like the southwest such as San Diego or Phoenix where they don’t get a lot of rain. They have to be used in the right application. In the wrong one, they don’t make sense.”
Right now, TPO is “taking the market by storm,” Tendler adds.
This low-cost, efficient product, first introduced in the early 1990s, is starting to gain wider acceptance over other, more traditional products. It is heat-weldable, solar-reflective, and meets fire codes without chlorine or halogenated fire retardants. Its solar reflectance is hard to surpass, and as a result, it’s an excellent choice for reducing cooling loads and urban heat islands, Tendler says.
One way to ensure that you’re specifying high-quality, reflective roofing products is to follow guidelines provided through the EnergyStar® program, a joint effort by the EPA and the U.S. Department of Energy.
“It’s a program that the EPA came up with to establish performance benchmarks in reflectivity and emissivity,” Tendler explains. “Based on age reflectance, you look at a product’s performance several years later after the roof gets dirty. It’s a way of distinguishing different claims and separating a truly cool roof from a non-cool roof.”
Robin Suttell, based in Cleveland, is contributing editor at Buildings magazine.