Cool Colors Project: Improved Materials for Cooler Roofs
Roofs and the rainbow of colors used in roofing materials are getting cooler and cooler, thanks to research by scientists in the Lawrence Berkeley National Laboratory (Berkeley Lab) Environmental Energy Technologies Division (EETD). The cooler roofs get, the more energy and money they save. A new research program in cool materials is developing the first cool shingle for residential roofs.
Since the mid 1980s, EETD researchers have studied the effects of increasing the solar reflectance of roofs. "Cool" roofs reflect more of the sun's radiation than do conventional roofs, lowering temperatures inside buildings, decreasing air-conditioning energy use, and reducing the "urban heat island," an elevation of air temperatures in urban areas relative to surrounding rural areas.
Traditional cool roofs are white because light surfaces absorb less solar radiation than dark ones. EETD research has demonstrated that raising the solar reflectance of a roof from about 20 percent (dark gray) to about 55 percent (weathered white) can reduce a building's cooling energy use by 20 percent. Although white materials may find acceptance on flat-roofed commercial buildings, U.S. homeowners typically demand non-white roofs for aesthetic reasons. EETD researchers, working with industry, have found that non-white cool roofs can be manufactured using colorants (pigments) that reflect the invisible, "near-infrared" radiation that accounts for more than half of the energy in sunlight. "Our research estimates that the potential net energy savings in the U.S. achievable by applying white roofs to commercial buildings and cool colored roofs to houses is valued at more than $750 million per year," says Hashem Akbari, head of the Heat Island Group at Berkeley Lab.
The group's research has shown that widespread regional application of cool roofs can reduce ambient air temperatures and retard smog formation. Cool roofs can also reduce peak electricity demand in summer, which helps reduce strain on the aging electricity grid when relief is most needed. The lower temperatures of cool roofs may also increase the roofs' serviceable lives, according to some preliminary research by the group.
Because of the recent work of Akbari's research group, 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
Lack of cool colored products has been a major technical barrier to introducing cool roofs on residences. Existing white and non-white cool coatings are fine for the low-slope roofs of commercial and industrial buildings and apartment structures. But most homeowners don't want white on their high-slope roofs, which are seen from the street. The market for home roofing materials is dominated by colorful shingles, tiles, metal products, and wood shake.
As a result of a research project funded by the California Energy Commission, homeowners will soon see a variety of new "cool-colored" roofing products. Berkeley Lab's EETD is working with Oak Ridge National Laboratory, two pigment manufacturers, and 10 roofing manufacturers. The manufacturing partners produce the types of roofing materials (shingles, clay tiles, concrete tiles, and metal roofs) that cover more than 90 percent of the residential roofs in the U.S. 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 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. Since many homeowners prefer non-white roofs, we are working to develop cool colored roofing products."
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. Figure 1 shows the distribution of solar power as a function of wavelength. 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, i.e., highly reflective to near-infrared radiation.
Figure 2 shows some color-matched concrete tiles. Along the bottom row are tiles colored with standard pigments compared to the top raw tiles colored with cool pigments. The cool pigments typically have solar reflectances about 0.30 higher than color-matched conventional pigments (e.g., 0.40 versus 0.10).
Figure 3 compares a cool and a standard brown. To the eye, they are almost the same color, but the graphic to the left shows that the cool brown reflects about 20 percent more of the 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. (Tile roofs are increasingly preferred on more expensive houses in the western and southern states.) One manufacturer of metal roofing has already switched most of its product line to cooler coatings because the product made with the cool pigments costs about the same as that made with conventional pigments, and the solar reflectance features adds value for customers.
The research team's current efforts focus on asphalt shingles, a challenging technical problem. Shingles are produced in a multi-step 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 if it were 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 and Market Acceptance
To test the field performance and durability of these cool coatings, EETD is collaborating with Oak Ridge National Laboratory. The Oak Ridge team has set up a steep-slope assembly test apparatus (Figure 4) on its grounds in Oak Ridge, Tennessee, to evaluate a variety of samples from the manufacturing partners. Andre Desjarlais, William Miller, and their associates have installed representative cool roofing materials at the Oak Ridge roof testing facility and are measuring the changes in physical composition and appearance of the samples as a result of exposure to ultraviolet light, weathering, and temperature changes.
The joint Berkeley Lab-Oak Ridge team has also set up seven test sites throughout California, in six climate zones ranging from mild to severe, to monitor the performance of roofs using test materials from the manufacturing partners. The sites range from the far north of the state, to the California-Mexico border. The project team is also collaborating with the Sacramento Municipal Utilities District to measure energy savings and changes in temperature and humidity inside test houses with cool roofs in Sacramento, California.
Several of the new cool coatings are already available through the manufacturing partners. "Since the start of this research," says Akbari, "the solar reflectance of commercially available clay and metal products has increased from the five to 25 percent range to the 30 to 45 percent range. Working with our industrial partners, we hope to produce shingles with a solar reflectance of 25 percent or higher, qualifying for an ENERGY STAR cool roof label. Some of the products resulting from this research will also qualify as cool roofs in the California building energy code (the 2005 Title 24 California Building Energy Efficiency Standard)."
For more information, contact:
- Hashem Akbari
- (510) 486-4287; Fax (510) 486-4673
Berkeley Lab's Heat Island Group
California Building Energy Code (Title 24)
This research is funded by the California Energy Commission and the U.S. Department of Energy.