Cool Colors: Cooler Roofs
Because of the recent work of Akbari's research group, a coalition of participants in the roofing industry has adopted voluntary standards for measuring the solar reflectance of roofing materials and has set up the Cool Roof Rating Council to develop labels that inform buyers about the relative degree to which various roofing products reflect solar radiation and emit heat through thermal radiation.
The building materials industry has also introduced a number of products that help increase roof reflectance, mainly elastomeric coatings, single-ply membranes, tiles, and metal roofing. The ENERGY STAR program certifies cool roof products with its voluntary label and offers a Web-based guide to ENERGY STAR roof products available on the market.
The Need for Cool Colored Roofs
The lack of cool but colored products has been a major technical barrier to introducing cool roofs on residences. Homeowners typically don't want white materials on high-slope roofs that are visible from the street. The U.S. market for residential roofing materials is currently dominated by colorful shingles, tiles, metal products, and wood shake.
Berkeley Lab's EETD is working with Oak Ridge National Laboratory (ORNL), two pigment manufacturers, and 10 roofing manufacturers to develop a variety of new "cool-colored" roofing products. The manufacturing partners produce the types of roofing materials that cover more than 90 percent of the residential roofs in the United States. The industrial partners are 3M, American Rooftile Coatings, BASF, Custom-Bilt Metals, Elk Manufacturing, Ferro, GAF, Hanson Roof Tile, ISP Minerals, MCA Tile, Monier Lifetile, and the Shepherd Color Company.
Asphalt shingles account for half of the residential roofing market in the western United States, according to industry sources. "Most commercially available roof shingles are optically dark," says Akbari. "Their solar reflectances range from five to 25 percent, depending on color. Even the majority of nominally 'white' roof shingles are grayish and have a solar reflectance of about 25 percent, which is much lower than the 70 percent solar reflectance of white tiles or white metal panels."
Manufacturing a cool-colored shingle starts with finding cooler pigments. Akbari and Berkeley Lab scientists Paul Berdahl and Ronnen Levinson have been measuring the solar spectral reflectance (reflectance versus wavelength over the solar spectrum) of commercially available pigments. For a given color, the ideal pigment reflects as much as possible of the invisible radiation in the near-infrared range.
The research team has developed a pigment database describing a variety of colors, including browns, blues, purples, greens, and reds, that are cool, that is, highly reflective to near-infrared radiation. The cool pigments typically have solar reflectances about 0.30 higher than color-matched conventional pigments.
To the eye, a cool and a standard brown are almost the same color, but the cool brown reflects about 20 percent more incident solar radiation than the conventional color (27 percent versus 8 percent).
In addition to testing materials in the lab, Levinson, Berdahl, and Akbari have adapted a mathematical model (the Kubelka-Munk model) to describe how pigmented coatings scatter and absorb light. They will apply the model to develop more reflective cool-colored roofing materials.
Using the pigment database and the model, the team is now developing cool-color coating design software for the roofing industry. The software estimates the reflectance of a coating using the absorption and scattering properties of the pigment as well as the coating's composition and geometry. The results are recipes for manufacturing pigmented coatings that maximize solar reflectance for a given color.
Cooler Tiles, Metal Panels, and Shingles
The next step is to figure out how to apply pigments to relatively simple roofing products such as tiles, metal panels, and shingles. The team has identified a number of cool pigments appropriate for coating metal panels and concrete and clay tiles.
One manufacturer of metal roofing has already switched most of its product line to cooler coatings because products made with the cool pigments cost about the same as those with conventional pigments, and the solar reflectance adds value for customers.
The research team's current efforts focus on asphalt shingles, a challenging technical problem. Shingles are produced in a multistep process: roofing granules (small crushed rocks) are manufactured, color is applied to them, and the granules are then used to cover asphalt-saturated fiberglass sheets.
The EETD team and its industrial partners have developed a two-layer system for manufacturing cooler roofing granules. In this process, granules are pre-coated with an inexpensive pigment that is very reflective at near-infrared wavelengths. Then, the cool-colored pigment is applied.
The first pigment helps increase the reflectance of granules and reflects even more light than the cool-colored pigment would by itself. The two pigments together significantly reduce the amount of near-infrared light absorbed by the granules' dark surface.
EETD's industrial partners have now manufactured more than 50 prototype cool shingles, 30 tiles and tile coatings, and 20 metal panel prototypes, including a cool black shingle that is 18 percent reflective, well above the four-percent reflectance of conventional black shingles.
Field testing is underway, in collaboration with ORNL. Several new cool coatings are already available through the manufacturing partners. Some of the products resulting from this research will qualify as cool roofs in the California building energy code (the 2005 Title 24 California Building Energy Efficiency Standard).
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Allan Chen is editor of EETD News for the Environmental Energy Technologies Division of the Lawrence Berkeley National Laboratory. For more information about this project, contact Hashem Akbari, director of the Cool Colors Project, Berkeley Lab's Heat Island Group. This research is funded by the California Energy Commission and the U.S. Department of Energy.
This article originally appeared in the Fall 2004 issue of EETD News and is reprinted with permission.