Building Shell: 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. The objective is to reduce shell temperatures so that less heat is conducted to the inside of the building. One of the most effective measures is the use of light-colored walls and roofs, called cool roofs.
Light-Colored Walls and Roofs
The use of 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, also known as albedo, can reflect up to 85 percent of incident solar radiation, compared with conventional surfaces that may reflect only 20 percent.
Two properties measure the ability of a surface to maintain a low temperature-reflectance and emissivity. Reflectance is measured on a scale of 0 to 1, with 0 being a perfect absorber and 1 being a perfect reflector. Emissivity, which is also measured on a scale of 0 to 1, indicates how much heat is emitted back 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 absorbed heat is radiated back to the sky. White plaster very nearly achieves this combination, as shown in Figure 1.
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 graph. The best performing materials for cooling load reduction, which have both high albedo and high emissivity, are in the upper left-hand section.
Courtesy: Florida Solar Energy Center
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 and nearly all other materials offer a high emissivity. For example, black paint has an emissivity of 0.95, and white paint comes in at 0.90, whereas bare metal has an emissivity of 0.35. 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 they are less effective than white coatings at reflecting incoming radiation and reducing roof temperature, as shown in Figure 2.
Temperatures were measured in August in central Texas, with an ambient temperature of 90 degrees Fahrenheit and clear sky conditions. Materials were applied to two-inch polyurethane foam.
Notes: a. Temperatures for these materials are based on estimates rather than measurements. b. Terra-cotta tiles vary widely, depending on the mix of materials used in manufacture. Researchers at LBNL have measured cooler temperatures for other terra-cotta tile samples under comparable conditions.
Courtesy: Lawrence Berkeley National Laboratory
Using a cool roof can also help increase roof life, because the roof temperature is more constant. The thermal stress of 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.
Is My Building a Good Candidate for a Cool Roof?
In the best applications, cool roofs have no incremental cost, delivering nearly instant payback. However, in the wrong buildings, cool roofs may actually have negative implications. The key is to know 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 energy considerations.
• There is little or no existing insulation. Note that an energy-efficient building should have both a cool roof and adequate insulation. When installing a cool roof or constructing a new building, you should consult local building codes and your contractor.
• 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.
Finding Energy-Efficient Roofing Materials
Two main labeling programs exist in the U.S. to help end users select cool roofing materials. The first is the Energy Star roofing program introduced by the U.S. 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. For more information, including a list of qualifying products, go to www.energystar.gov/index.cfm?c=roof_prods.pr_roof_products.
The second is the Cool Roofs Rating Council (CRRC), which is an independent organization dedicated to providing credible energy performance rating information about roof surfaces. CRRC's label, which is based on reflectance and emittance, is verified through third-party testing. The label is used much like the yellow efficiency labels found on appliances. For more information, including a directory of CRRC-rated products, go to www.coolroofs.org/products/search.php.
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