Supporting the Cool Roofs Standard
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) approved Standard 90.1 at its annual meeting in June of this year. Included in the Standard are norms for reflective roofs, proposed by EETD's Hashem Akbari, staff scientist in the Energy Analysis Department. Akbari was responding to reflective roofs' characteristic of reducing the roof surface temperature, which, in turn (1) reduces the air-conditioning requirements of the building and (2) lowers the ambient air temperature of the building in a large urban area. A lower urban air temperature reduces the formation of smog.
Akbari proposed the inclusion of reflective roofs in the standard and he supported the inclusion with simulation results for roofs for residential and nonresidential buildings. Residential models are intended to apply to hotel guest rooms, patient rooms in hospitals, and high-rise residential apartments. Simulations were performed for 19 of the total 26 different climates used by ASHRAE.
Research shows that the temperature of highly absorptive dark roofs (low-solar reflectance) can be 50°C (90°F) higher than the ambient temperature. "Cool" roofs, i.e., those painted white or of light-colored materials, are only about 10°C (18°F) warmer, and, consequently, are effective in reducing a building's cooling energy use. Coupled with the reflectance of the roof, the emissivity of the materials is also an important factor. The cool roof materials must emit much of the heat energy they receive to help achieve the cooling. When taken into account with the amount of insulation under the roof and the building's thermal integrity, the cool roof can result in air-conditioning energy use savings of between 10 and 50%.
California Standard Now in Place
Revised Title 24 standards (AB 970 Energy Efficiency Standard for Residential and Nonresidential Buildings) now give energy-performance credits for residences and commercial buildings that include cool roofs. The standards required various solar reflectances and thermal emittances for different types of roofing materials. For example, clay tiles must have a minimum total reflectance of 0.40 (when tested with ASTM standards) and a minimum thermal emittance of 0.75. Other materials must have initial higher reflectances and emittances. Liquid roofing products (coatings that reflect light) are measured in tensile strength, elongation and permeance, and weathering strength.
Cool Savings Program
In California the Cool Savings Program (formerly Cool Roof Program) is funding and installing more than 1 million square feet of cool roofs. With an averaged estimate savings of about 0.35 W/sq ft, the Cool Savings Program has saved 523 kW on roofs already "cooled," and will save another 500 kW on the number of projects in the future. One major retailer has replaced its roof in a large store with a cool roof and is planning to test other materials on three of its California stores. When completed, a total of 10 million square feet of commercial space will be kept cooler with this program, saving a total of 3.5 MW of energy and also saving the retailer a considerable amount of money in energy costs.
Heat Island Mitigation Studies
In other California efforts, many institutes and government agencies are looking at efforts to stem the heat island phenomenon. Heat islands are urban areas whose temperatures exceed the surrounding suburban and rural temperatures because the increased amount of buildings and asphalt retain heat. In both Los Angeles and Sacramento, EETD researchers have participated in studies to assess the meteorological and air-quality benefits of mitigating urban heat islands. In Sacramento the utility district, the California Air Resources Board, the regional air quality district, and the energy commission have teamed up to provide demonstration projects advertising the benefits of local heat island mitigation.
In Chicago, Illinois, the Department of Environment has embarked on an Urban Heat Island Initiative in an effort to save energy and also to lessen pollution. The City of Chicago has begun demonstration projects in neighborhoods in the downtown areas to mitigate urban heat islands. These include rooftop gardens (one has been placed on the roof of City Hall), enhanced median plantings, and greening in parking lots, all of which reflect heat and even clean the air. Research will be carried out that will measure the air quality benefits of these strategies. Chicago has included standards in its energy code that mandate light-colored roofs to mitigate the urban heat island effect. Supported with funding from NASA, EETD's heat island group also performed studies in Atlanta, Georgia, to determine heat island impacts on local air temperature and energy use. Because Atlanta has significant air quality problems, the Georgia Environmental Protection Agency will focus more attention on how heat islands and air quality are related.
In Houston, Texas, extensive air quality modeling is ongoing in order to submit heat island mitigation strategies to the Texas Natural Resource Conservation Commission for its mid-course review in 2004. Other U.S. cities that have put studies into place include Salt Lake City, Utah, and Baton Rouge, Louisiana. Funded by the U.S. EPA, these meteorological and air quality modeling studies assess the benefits of increased surface albedo and urban reforestation.
For more information, contact:
- Hashem Akbari
- (510) 486-4287; fax (510) 486-4673
For more information, see the Heat Islands Group Home Page