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 roofs and walls.
Light-colored roofs and walls
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% of incident solar radiation, compared to conventional surfaces that may reflect only 20%.
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 is also measured on a scale of 0 to 1 and 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 .
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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 graph. The best performing materials for cooling load reduction, which have both high albedo and high emissivity, are in the upper left-hand section. Source: 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. 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 exterior-grade latex paints (which are unsuitable for roofs) and special compounds formulated and marketed specifically for heat load reduction are available for roofs. The differences between standard paints and "specialty paints" are relatively small and relate more to durability, fire protection, application ease and waterproofing and crack-sealing performance than to cooling load reduction. 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.
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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 degrees Fahrenheit and clear sky conditions. Materials were applied to two-inch polyurethane foam.
b: Terra cotta tiles vary widely, depending on the mix of materials used in manufacture. LBL has measured cooler temperatures for other terra cotta tile samples under comparable conditions. Source: Lawrence Berkeley Laboratory |
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 while bare metal has an emissivity of 0.35. The inclusion of metal in paints, such as in aluminized roof coatings, may reduce emissivity.
Installation of high-reflectance roof coatings or paint is most cost-effective if done during new construction or when buildings are scheduled for reroofing. For commercial applications, flat, poorly insulated roofs are the best candidates for retrofitting with high-reflectance materials. Although wall reflectance can also affect cooling load, the roof is a much more significant solar receiver and is typically more difficult to cool by shading.
Surfaces that can benefit most from an increase in reflectance meet all of the following criteria:
- Dark or metallic existing surface
- Poor insulation
- High solar insolation
- Large ratios of surface area to enclosed volume
- Cooling season dominates energy considerations
Finding energy-efficient roofing materials
To help select energy-efficient roofing materials, the U.S. Department of Energy and Environmental Protection Agency have introduced the "Solar Reflectance Index" (SRI). It is used in a manner similar to the yellow efficiency labels found on appliances. SRI is a measure of a material's ability to reflect solar heat. EPA's Energy Star label is awarded 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 (for more information, go here). Traditional roofing materials have SRIs of 5% (brown shingles) to 20% (green shingles). White shingles with SRIs of around 35% were used in the 1960s but were discontinued because they appeared dirty after a short period. Today, manufacturers treat white shingles with a fungicide to prevent discoloration. Smooth "self-washing" shingles are also available with SRIs of up to 62%.
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Last Modified: May 9, 2009
