«Cool Color Roofing Materials Hashem Akbari, Paul Berdahl, Ronnen Levinson, Steve Wiel Heat Island Group Lawrence Berkeley National Laboratory ...»
Cool Color Roofing Materials
Hashem Akbari, Paul Berdahl, Ronnen Levinson, Steve Wiel
Heat Island Group
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
Bill Miller and Andre Desjarlais
Oak Ridge National Laboratory
Oak Ridge, TN 37831
Draft Final Report Prepared for:
California Energy Commission PIER Program
Program Manager: Nancy Jenkins
Project Manager: Chris Scruton
This study was supported by funding from the California Energy Commission (CEC) through the U.S. Department of Energy under contract DE-AC02-05CH11231.
1 Disclaimer This document was prepared as an account of work sponsored by the United States Government.
While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor The Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or The Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or The Regents of the University of California.
2 Cool Color Roofing Materials A Collaboration of Industry, Lawrence Berkeley National Laboratory (LBNL), and Oak Ridge National Laboratory (ORNL) Hashem Akbari, Paul Berdahl, Ronnen Levinson, and Steve Wiel (LBNL) William Miller and Andre Desjarlais (ORNL) February 24, 2006 Abstract Raising roof reflectivity from an existing 10-20% to about 60% can reduce cooling-energy use in buildings in excess of 20%. Cool roofs also result in a lower ambient temperature that further decreases the need for air conditioning and retards smog formation. In 2002, suitable cool white materials were available for most roof products, with the notable exception of asphalt shingles;
cooler colored materials are needed for all types of roofing. To help to fill this gap, the California Energy Commission (Energy Commission) engaged Lawrence Berkeley National Laboratory (LBNL) and Oak Ridge National Laboratory (ORNL) to work on a three-year project with the roofing industry to develop and produce reflective, colored roofing products. The intended outcome of this project was to make cool-colored roofing materials a market reality within three to five years. For residential shingles, we have developed prototype light-colored shingles with solar reflectances of up to 35%. One manufacturer currently markets colored shingles with the ENERGY STAR qualifying solar reflectance of 0.25. Colored metal and clay tile roofing materials with solar reflectances of 0.30 to 0.60 are currently available in the California market.
LBNL and ORNL performed research & development in conjunction with pigment manufacturers, and worked with roofing materials manufacturers to reduce the sunlit temperatures of nonwhite asphalt shingles, clay tiles, concrete tiles, metal products, and wood shakes. A significant portion of the effort was devoted to identification and characterization of pigments to include and exclude in cool coating systems, and to the development of engineering methods for effective and economic incorporation of cool pigments in roofing materials. The project also measured and documented the laboratory and in-situ performances of roofing products. We also established and monitored three pairs of demonstration homes to measure and showcase the energy-saving benefits of cool roofs. The following activities were carried out.
In collaboration with the Energy Commission, we convened a Project Advisory Committee (PAC), composed of 15 to 20 diverse professionals, to provide strategic guidance to the project.
In order to determine how to optimize the solar reflectance of a pigmented coating matching a particular color, and how the performance of cool-colored roofing products compares to that of a standard material, we (1) measured and characterized the optical properties of many standard and innovative pigmentation materials; (2) developed a computer model to maximize the solar reflectance of roofing materials for a choice of visible colors; and (3) created a database of characteristics of cool pigments.
3 In order to help manufacturers design innovative methods to produce cool-colored roofing materials, we (1) compiled information on roofing materials manufacturing methods; (2) worked with roofing manufacturers to design innovative production methods for cool-colored materials;
and (3) tested the performance of materials in weather-testing facilities.
One of the project objectives was to demonstrate, measure and document the building energy savings, improved durability and sustainability attained by use of cool-colored roof materials to key stakeholders (consumers, roofing manufacturers, roofing contractors, and retail home improvement centers). In order to do this, we (1) monitored buildings at California demonstration sites to measure and document the energy savings of cool-colored roof materials;
(2) conducted materials testing at weathering farms in California; (3) conducted thermal testing at the ORNL Steep-slope Assembly Testing Facility; and (4) performed a detailed study to investigate the effect of solar reflectance on product useful life.
We developed partnerships with various members of the roofing industry. We worked through the trade associations to communicate and advertise to their membership new cool color roof technology and products. This collaboration induced the manufacturers to develop a market plan for California and to provide technical input and support for this activity. Through the industry partners, many California housing developers and contractors have been convinced to install the new cool-colored roofing products.
4 Acknowledgement This work was supported by the California Energy Commission (CEC) through its Public Interest Energy Research Program (PIER), by the Laboratory Directed Research and Development (LDRD) program at Lawrence Berkeley National Laboratory (LBNL), and by the Assistant Secretary for Renewable Energy under Contract No. DE-AC02-05CH11231. The authors wish to thank CEC Commissioner Arthur Rosenfeld and PIER managers Nancy Jenkins and Chris Scruton for their support and advice. Special thanks go also to Mark Levine, director of the Environmental Energy Technologies Division at LBNL for their encouragement and support in the initiation of this project.
This work has been a collaborative research between ORNL, LBNL, and the industry. We acknowledge the contribution of Tony Chiovare (Custom-Bilt Metals), Lou Hahn (Elk Corporation), Ingo Joedicke (ISP Minerals), Frank Klink (3M Industrial Minerals), Scott Kriner (Akzo Nobel Coatings), Kenneth Loye (Ferro), David Mirth (Owens Corning), Jeffrey Nixon (Shepherd Color Company), Joe Reilly (American Rooftile Coating), Robert Scichili (then BASF Industrial Coatings, currently a consultant to coated metal industry), Ming L. Shiao (CertainTeed), Krishna Srinivas (GAF), Yoshihiro Suzuki (MCA Clay Tile), Jerry Vandewater (Monier Lifetile), Michelle Vondran (Steelscape), Lou R. Zumpano (Hanson Roof Tile).
We also acknowledge the guidance, support, and the directives of the Project Advisory Committee (PAC) members: Gregg Ander (Southern California Edison Company), Aaron J.
Becker (Dupont Titanium Technologies), Carl Blumstein (California Institute for Energy and Environment),Tom Bollnow (National Roofing Contractors Association), Jack Colbourn (US EPA SF Office), Kathy Diehl (US EPA SF Office), Steven Harris (Quality Auditing Institute), Noah Horowitz (Cool Roof Rating Council and National Resource Defense Council), Scott Kriner (Cool Metal Roofing Coalition), Scott Kriner (Cool Metal Roofing Coalition), Shari Litow (DuPont Titanium Technologies ), Archie Mulligan (Habitat for Humanity), Mike Evans (Evans Construction), Jerry Wager (Ochoa & Shehan), Rick Olson (Roof Tile Institute), Mike Rothenberg (Bay Area Air Quality Management District), Steven Ryan (US EPA), Thomas A.
Shallow (Asphalt Roofing Manufacturers Association), Peter Turnbull (Pacific Gas and Electric Company, Jessica C Yen (DuPont Titanium Technologies).
5 Executive Summary The world is facing disruptive global climate change from greenhouse gas emissions and increasingly expensive and scarce energy supplies. Energy efficiency reduces those emissions and mitigates the rising cost of energy. Energy-Efficient Cool Color Roofing is a new technology for the roofs of homes that provides a significant leap in energy efficiency. The application of this technology for developing cool-colored roofing materials has been the focus of this collaborative study between the scientists at the Lawrence Berkeley National Laboratory (LBNL), scientists at Oak Ridge National Laboratory (ORNL), and industry.
Cool Color Roofing uses solar-reflective pigments to reduce a home’s solar heat gain and airconditioning energy consumption in a warm climate by about 10 to 20%. These roofs also lower a home’s peak-hour cooling power demand by about 10 to 20%, helping prevent blackouts and brownouts on hot summer afternoons. We used pigment spectroscopy to identify solar-reflective pigments of different colors and developed software for the design of cool color coatings. We collaborated with a consortium of U.S. pigment, coating and roofing manufacturers to develop novel methods to manufacture asphalt shingle, clay tile, concrete tile, and metal roofing products in a wide palette of cool colors. We estimate that applying cool-colored roofs to houses could achieve a net energy savings in the U.S. worth over $400 million per year (Konopacki et al.
1997). The estimated savings in California (in 2005 $) is about $100 million per year (see Figure EX-1).
Figure EX-1. Potential annual cool roof net energy savings in U.S. cities.
The Need for Cool Color Roofing Cool nonwhite roofing technology is intended not to replace light-colored roofing, but to improve the solar reflectance of dark-colored products, which dominate the residential roofing 6 market. (White residential roofing products stay cool in the sun, but sell poorly in California where homeowners prefer the aesthetics of dark-colored roofs.) Cool Color Roofing technology makes solar-reflective roofing available in any color (dark or light) by selectively reflecting the invisible component of sunlight in the “near-infrared” (NIR) spectrum (see Figure EX-2).
(a) (b) Figure EX-2. (a) Spectral solar power distribution, and (b) solar spectral reflectance of cool and standard brown surfaces.
On a summer day, the peak daily surface temperature of a cool dark roof of solar reflectance 0.35 is about 14 °C lower than that of a conventional dark roof of solar reflectance 0.10. Compared to the conventional dark roof, the cool dark roof conducts about 20 to 40 percent less heat into a home’s conditioned space, reducing the home’s demand for cooling power by about 7 to 15 percent in the late afternoon. These are the same hours during which demand for air conditioning strains the electrical grid and requires utilities to produce additional power using less efficient, more expensive, and more polluting “peak” generators. The cool dark roof yields a net annual energy savings (decrease in cooling energy minus increase in heating energy) of about 6 to 11 percent.. Our findings for sub-tile venting shows the heating penalty is lessened, which will make annual energy savings for cool roofs even more appealing.
Figure EX-3 depicts examples of cool metal panel, concrete tile, asphalt shingle, and clay tile roofing products.
Figure EX-3. Cool metal panel, concrete tile, asphalt shingle, and clay tile roofing products.
7 Development of Cool Color Roofs This multi-year research project is funded by the California Energy Commission’s Public Interest Energy Research program, with an initial grant from the U.S. Department of Energy. The California Energy Commission’s (the Commission) cool-color program at LBNL has three elements: (1) measuring the rates at which many common pigmented coatings absorb (convert to heat) and backscatter (reflect) light at wavelengths in the UV, visible, and NIR spectra; (2) using these rates in a LBNL software tool for the design of color-matched coatings with high solar reflectance; and (3) working with the roofing industry to develop novel manufacturing methods.
Our industrial partners manufactured the prototypes and products. ORNL performed the demonstration work, measuring both the residential energy savings achieved by use of cool colored roofing, and the extent to which exposure changes the appearance and performance of cool colored roofing. ORNL also tested several cool roofing products at their facilities at Oak Ridge. Hereafter, we refer to this national labs partnership as the “Cool Team.” Pigment characterization begins with measuring the reflectance (r) and transmittance (t) of a thin coating (e.g., a paint film) colored by single pigment, such as iron oxide red. These "spectral," or wavelength-dependent, properties of the pigmented coating are measured at 441 evenly spaced wavelengths spanning the solar spectrum (300 to 2500 nanometers (nm), where 1 nm = 10-9 meter.) Inspection of the film's spectral absorptance (calculated as 1-r-t) reveals whether a pigmented coating is "cool" (has low NIR absorptance) or "hot" (has high NIR absorptance).
The spectral reflectance and transmittance measurements are used to compute spectral rates of light absorption and backscattering (reflection) per unit depth of film. These calculations employ a variant of the Kubelka-Munk two-flux continuum model of light propagation through a pigmented coating that Berkeley Lab has adapted to account for the reflectance of incompletely diffused light at the air-coating interface. Such “interface reflectances” can significantly alter the reflectance and transmittance of the optically thin pigmented coatings applied to roofing materials.