In Tucson, a 28,000-square-foot city roof got a white elastomeric coating; cooling energy fell 50–65%, saving over 400 million Btu and nearly $4,000 a year

Every summer, air-conditioning bills start climbing as soon as temperatures top 100 degrees, and hot states are no exception. One of the simplest solutions lies right above your head: a cool roof coating. According to the United States Environmental Protection Agency, a city administration building in Tucson, Arizona, used one of these cool roof coatings on its own building, and the results show why they should be considered more than they are.

The Tucson cool roof that proved the concept

The building is a Tucson city hall facility known as the Thomas O. Price Service Center, a roughly 23,400-square-foot office building. In June 2001, the building's existing roof was a 28,000-square-foot aluminum roof that included portions of a darker, blackened copper surface, and it typically absorbed heat and radiated it back into the building. City officials applied a white elastomeric coating to the metal roof as a demonstration project, which the EPA monitored. The city used 50 to 65 percent less energy to cool the building, saving over 400 million British thermal units per year, according to the EPA's Heat Island Community Actions Database. This percentage reflects the cooling-energy savings measured for the roof and cooling system covered by the demonstration project, rather than the building's total energy use, which is part of why a 50 to 65 percent reduction in that portion of energy use lines up with a savings of about $4,000 a year rather than a far larger sum. This saved nearly $4,000 a year in energy costs for this single building. In addition to the roof, city officials also used the project to test other cooling methods, including different cool pavements and additional shade trees in the parking lots.


Why a cool roof coating works so well

Dark roofs on many homes absorb most of the sunlight hitting them and turn it into heat, which transfers into the building. A white roof prevents as much of that sunlight as possible from being absorbed. According to the EPA, reflectance and emittance help determine how much a roof lowers temperatures on its surface, in the building, and in the surrounding air. The EPA also estimates that in non-air-conditioned homes, cool roofs could reduce max indoor temps by 1.2-3.3 degrees C (2.2-5.9 degrees F), and that solar reflectance from a cool roof could cut peak AC loads for a residence by 11-27%. None of this requires exotic materials; the project in Tucson used an elastomeric coating, a flexible, weather-resistant paint applied by rolling or spraying it onto the roof surface. No roof replacement is required.

What cool roofs mean outside of Arizona

Tucson, Arizona, is in one of the hottest regions in the country, so it's a valid question whether these savings would apply in a region with a less severe climate. The US Department of Energy notes that cool roofs are most appropriate for hot, sunny climates, particularly for buildings with low roof insulation, a description that fits much of the South. The Department also states that cool roofs can be up to 60 degrees Fahrenheit cooler than traditional black roofs during summer months. For homeowners on the fence about investing in cool roof coatings, the Department of Energy's Federal Energy Management Program has developed a mathematical model to determine the break-even point for cool roofs: A cool roof may pay for itself if it costs less than $0.64 per square foot more than a standard roofing product. That threshold rises to $1.11 per square foot for the best-performing cool roofs, which can pay for themselves over their lifespan. The trade-off is worth noting, however. The Department also warns that a cool roof can cause a mild winter heating penalty, as its reflective quality also reflects winter sunlight and reduces solar heating. For most of the Sun Belt, where heating costs are dwarfed by cooling costs, that penalty is far outweighed by the savings in air conditioning costs.

How to know if a cool roof makes sense for you

For renters, a cool roof coating could help persuade a landlord to make an inexpensive fix. For homeowners or small businesses, the decision often comes down to budgeting for the next roof repair, since a coating can be applied over an existing roof. Before investing, consider whether the roof is flat or pitched, whether utility companies offer rebates on reflective roofing, and whether the climate is hot enough for the savings to pay off. The Department of Energy says the biggest gains come in hotter, sun-drenched climates like the American Southwest. The value of the Tucson roof is not that every roof will save $4,000 a year, but that a federal agency measured and verified the savings for this building. For homeowners or renters trying to reduce a high summer cooling bill, it's worth considering before the next heat wave arrives.