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.
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