Cool Roofs

Although most heat gain in a facility comes through the windows, eliminating heat gain through the exterior roof and walls can be a cost-effective and low-risk way to reduce cooling loads and peak demand. One of the most effective measures for commercial buildings is light-colored walls and roofs—the latter is commonly referred to as a cool roof.

Using light-colored building surfaces is a time-honored means of keeping buildings cool in the Mediterranean region, the Caribbean, and other sun-drenched locales. Light-colored roofing materials with high reflectance (known as high albedo) can reflect up to 85 percent of incident solar radiation—compared with conventional surfaces that may reflect only 20 percent.

A cool roof can also last longer thanks to less temperature variation. The thermal stresses on conventional roofs can expand and crack the roofing material, but because cool roofs stay cooler on hot sunny days, they expand and contract less and should therefore last longer than conventional roofs.

Two properties measure the ability of a surface to maintain a low temperature: reflectance and emissivity. Reflectance is measured on a scale from 0 to 1, with 0 being a perfect absorber and 1 being a perfect reflector. Emissivity, also measured on a scale of 0 to 1, indicates how much heat is emitted from the surface to the environment. An ideal exterior surface coating for a cooling-dominated climate would have reflectance near 1.0 and infrared emissivity near 1.0, so that little infrared is absorbed and any existing heat is radiated out to the sky. White plaster very nearly achieves this combination, as shown in Figure 1.

Figure 1: Spectral characteristics of building materials
Even though many metals have a high solar reflectance, if emissivity is low the material will not reject heat effectively. For example, polished aluminum foil has a very high solar reflectance, but its emissivity is low, so it retains heat. Note its placement in the lower portion of the chart. The best-performing materials for cooling load reduction, which have both high albedo and high emissivity, are in the upper left-hand section.

Commercial building roofs typically have solar reflectances in the 0.20 to 0.35 range, although dark roof reflectance can be as low as 0.05. Metallic surfaces have low emissivity while nearly all other materials offer a high emissivity. For example, black paint has an emissivity of 0.95, white paint comes in at 0.90, whereas bare metal has an emissivity of 0.35. As a result, the inclusion of metal in paints, such as aluminized roof coatings, may reduce emissivity.

Both walls and roofs can be treated with light-colored paints or other finishes to increase reflectance to 0.70 or more. Walls can be treated with light-colored, exterior-grade latex paints (which are unsuitable for roofs), and special white waterproof coatings formulated and marketed specifically for heat load reduction are available for roofs. Single-ply membranes are factory-fabricated roofing sheets that are installed in the field; they are available in light colors. Aluminized roof coatings are also available, but due to their low emissivity they are generally less effective than white coatings at reducing roof temperature, as shown in Figure 2.

Figure 2: Surface temperature and albedo of common roofing materials and paints
Temperatures were measured in August in central Texas, with an ambient temperature of 90° Fahrenheit and clear sky conditions. Materials were applied to two-inch polyurethane foam.
What are the options?
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Currently there are both white and “cool color” options available for low- and steep-slope rooftops. Additionally, most standard roofing materials also come in a “cool” version that features a reflective surface (this typically adds about $1 per square foot to installation costs).

White roofs are generally coolest, but in cases where roofs are sloped and visible from the street, a white roof may not be desirable. In such cases, a reflective coating option may be more practical. These coatings can be found for metal roofs, in clay and concrete tiles, and in the multicolored granules that make up shingles. These colored cool-roofing products, developed by Lawrence Berkeley National Laboratory for its Cool Colors Project, look like their standard counterparts but reflect more long-wave radiation from sunlight and stay cooler as a result. For roofs that have a pitch below 2:12 (ratio of height over distance), thermoplastic membranes, elastomeric coatings, and metal products are available. For steep roofs, options include cool asphalt shingle, clay tile, concrete tile, and metal products.

How to make the best choice
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In the best applications, cool roofs have no incremental cost, delivering an instant payback. According to the US Department of Energy (DOE), installing a white roof on a commercial building can result in annual savings of up to $0.20 per square foot. However, in the wrong buildings, cool roofs may actually have negative effects; the key is knowing when a cool roof makes sense. Cool roofs are most effective when one or more of the following conditions exist:

  • The building has high air-conditioning use, and the cooling season dominates HVAC energy consumption.
  • There is little or no existing insulation (an energy-efficient building should have both a cool roof and adequate insulation).
  • The climate is hot and sunny, at least in the summer.
  • New construction is planned or the existing building is scheduled for reroofing or roof maintenance.
What’s on the horizon?
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Currently, cool roof technology and implementation is well-established, though small gains in technology may continue to arise. There are also alternatives to white-colored cool roofs that may gain in popularity going forward. One such approach involves putting plants on roofs (known as a green roof), and this is gradually becoming more common. Green roofs keep roofs cooler than conventional black or colored roofs, and they offer some of the energy benefits of white roofs. What makes green roofs appealing is that they provide additional nonenergy benefits like stormwater management and unique aesthetics, although they are generally much more expensive to implement.

In Economic Comparison of White, Green, and Black Flat Roofs in the United States from the Lawrence Berkeley National Laboratory, researchers conclude that white roofs are likely to be most cost-effective over a 50-year period when compared with green and black roofs. The study acknowledges that this is from a purely economic standpoint and does not factor in the additional benefits of green roofs. The researchers found that white roofs generally cost $0.30 per square foot less than green roofs and estimate that white roofs are three times better at countering climate change.

Who are the manufacturers?
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With numerous manufacturers currently producing cool roof products, two main labeling programs exist in the US to help end users select cool roofing materials. The first is the Energy Star roofing program introduced by the US Environmental Protection Agency (EPA). EPA awards the Energy Star label to low-slope roofs with an initial solar reflectance of at least 0.65 and to steep-slope roofs with an initial reflectance of at least 0.25. After three years, the roof products must maintain a reflectance of at least 0.50 and 0.15, respectively.

The other labeling program comes from the Cool Roofs Rating Council (CRRC), an independent organization dedicated to providing credible energy performance rating information about roof surfaces. CRRC-rated products are labeled based on reflectance and emittance, verified through third-party testing. The label is used much like the yellow efficiency labels found on appliances.

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