Feature Article
Cool Roofs
Communities around the Mediterranean have used them for many centuries
. . . and now building researchers are quantifying their benefits for America.


Some of the homes in the Cavalli HIlls development have “Cool Roofs” in addition to the ARXX Insulated Concrete Form wall system. SMUD will passively monitor these homes for energy savings. Finished homes will be indistinguishable from traditional homes.

 

Cool Roofs can help stave off global warming
by refecting sunlight back into space

    Global climate change is real. There is worldwide agreement among scientists that increased greenhouse gas pollutants from human activities are causing dangerous warming of the earth’s atmosphere.
    Observed and projected impacts include hotter days, additional smog, a rise in the sea level, and a 15 to 30 percent reduction in water supply to California’s cities and farms over this century.
Climate change impacts California. The repercussions of a change in climate are serious. They challenge the state’s infrastructure investments and touch all sectors of the economy: water supply, agriculture, forestry, energy production, health, transportation, tourism and others.
    Climate change is driven in large part by carbon dioxide and other greenhouse gases derived from fossil fuels. While electricity production and industry are universally important sources, transportation is the source of more than half of the fossil fuel carbon dioxide in California.
    It is critical that the state begin to reduce the rate of growth in emissions from this sector.
    Part of the government’s effort to reduce carbon dioxide emissions is Assembly Bill 1493, authored by Assemblywoman Pavley and signed by Governor Davis on July 22, 2002. This statute directs the California Air Resources Board to develop regulations to achieve maximum feasible cost-effective reductions in greenhouse gas emissions from new gasoline cars and light duty trucks vehicles, beginning in 2009.
    California is also pursuing actions related to many other aspects of climate change and can build on these to fashion a comprehensive strategy.
    California can do more, leveraging its vast intellectual and economic resources, in preparing for foreseeable future impacts and on reducing greenhouse gases in the atmosphere.
Responding to climate change need not be an additional burden but can, in fact, promote economic development, ensure energy and economic security, and improve public health and safety.

Facts About Global Warming

    The 1990s were the warmest decade of the entire millennium, and 1998, 1999, and 2000 were three of the hottest years on record.
    Greenhouse gases affect climate by increasing the “greenhouse effect.” They concentrate in the Earth’s atmosphere and trap heat by blocking some of the long-wave energy the Earth normally radiates back to space; the resulting change in atmospheric energy balance affects both weather and climate.
    There is widespread agreement among climate scientists worldwide that human activity is increasing the greenhouse gases in the earth’s atmosphere and accelerating global warming. While some greenhouse gases occur naturally, others are discharged into the atmosphere by certain human activities such as the burning of fossil fuels (coal, oil and natural gas for heating and electricity); gasoline and diesel for transportation; deforestation activities and some agricultural practices. The potential consequences of global warming – some of which are already occurring – include:
    · more extreme weather
    · dislocation of agricultural and commercial activities
    · expansion of desert regions
    · a rise in sea levels
    · damaged natural habitats and ecosystems
    · The planet faces both increased flooding and increased drought. Extended heat waves, more powerful storms, and other extreme weather events will become more common. Rising sea levels will inundate portions of Florida and Louisiana, while increased storm surges will threaten communities all along the US coastline.
    · Global temperatures are expected to rise by between 2.5 and 10.4 degrees F by 2100. This could seriously impact food production.
    · Global sea level is expected to rise by a further 15 to 95 cm by the year 2100.

 

 By Steve Berlin

    If you have ever left a metal hammer lying out in the summer sun, you know that when you go to pick it up it is hot. Even though the hammer is shiny and therefore reflects the sunlight, the metal retains the heat over time, giving it up slowly.
    According to Chris Scruton of the California Energy Commission, the term for this is emissivity. Shiny metal, Chris notes, has a low emittance and does not give off heat energy easily.
    The basic concept of a cool roof is to make the roof out of a material that reflects energy and also emits heat — not retaining it like the hammer left in the sunshine.
    Presently, suitable cool white materials are available for most roof products, with the notable exception of asphalt shingles; cooler colored materials are needed for all types of roofing. The home market has virtually refused to use cool roof materials solely because they are white in color.
    It is the hope and intent of the current Energy Commission research project to develop materials that work as well, or better, than existing materials and come in colors acceptable to both commercial and residential builders.
    The California Energy Commission is engaging Lawrence Berkeley National Laboratory (LBNL) and Oak Ridge National Laboratory (ORNL) to work on a three-year, $2 million project with the roofing industry to develop and produce reflective, colored roofing products and with the homebuilding industry to test these products.
    Last weeks feature story on Insulated Concrete Form Construction noted that progressive homebuilder Mike Evans is working with the Sacramento Municipal Utility District to collect thermal performance data for a new generation of cool roof materials.
    Homeowners have already agreed to have passive thermal monitoring systems installed in the attics and around the house. Data will be collected from housing having only a cool roof, from housing having a cool roof and the insulated concrete form construction walls, and also from standard wood framed homes. Comparison of the data will yield useful information on the energy savings and thermal properties of both the ICF wall system and the new generation of cool roofs.
    Most painted roofs today have a reflectance of about 10 - 20 percent, but special paint made using cool roof color materials can reflect as much as 60 percent of the sun s energy, cutting air conditioning bills by 20 percent or more.
    Steve Weil of the Lawrence Berkeley National Lab, who is project manager for the Cool Roof testing program being funded by the California Energy Commission, explains that they are testing a new generation of Cool Roof paints, coatings and materials. The expectations are threefold.
    First, the new coatings and materials are expected to dramatically lower the surface temperatures of the roof in summer, which will increase the lifetime of the roof.
    Second, the lower surface temperature will mean less heat transfer into the house, and lower air conditioning costs and lower energy usage.
    Finally, the new materials will reduce the ambient air temperature in the locale of the building because the Cool Roof materials actually reflect energy into space.
    If Cool Roofs are adopted widely, this aspect of reflecting energy back into space will have a significant and positive effect on the growing problem of urban heat islands.
    Chris Scruton of the Energy Commission also noted that the high reflectivity of the Cool Roof will have air quality benefits. Smog, says Scruton, forms more quickly when the air temperature is higher. That is why smog is not as big a problem in the winter. If an entire community had Cool Roofs that reflected solar energy back into space, it would have a mitigating effect on smog formation.
    Until recently the major drawback of a Cool Roof was that they came in only one color: white.
    The new generation of Cool Roof materials and paints come in a variety of colors and perform better.
    The intended outcome of this project is to make cool-colored roofing materials a market reality within three to five years. For other materials, the aim is a reflectance of over 45 percent.
    “Raising roof reflectivity from an existing 10-20 percent to about 60 percent can reduce cooling-energy use in buildings in excess of 20 percent,” states Hashim Akbari of LBNL. Cool roofs also result in a lower ambient temperature that further decreases the need for air conditioning and retards smog formation.
    Those reflective roofing products currently available in the market e.g., single-ply membranes and spray-on roof coatings are typically used for low-sloped roofs (mostly commercial buildings) and not for homes with steep-sloped roofs
    LBNL and ORNL will work with pigment manufacturers and roofing materials manufacturers to develop Cool Roof materials that reduce the sunlit temperatures of colored asphalt shingles, roofing tiles, metal roofing, wood shakes, roofing membranes and roof coatings. A significant portion of the effort will be devoted to materials design to continue to improve pigments for cool-colored materials, and to the development of engineering methods for applying colored pigment on roofing materials. The project will also measure and document the laboratory and on-site performance of roofing products.

Cool Roofs must have
at least 0.40 total solar reflectance

    What makes a roof “Cool”? According to the Title 24 cool roof standards, concrete tile and clay tile roofs must have a minimum initial total solar reflectance of 0.40. All other products must have a minimum initial total solar reflectance of 0.70 when tested according to ASTM E903 or E1918. For purposes of calculating credit for a cool roof under both the performance and prescriptive methods, a roof that meets these criteria reduces heat gain through the roof by 36 percent.
    Of course, the impact of this reduction on energy usage will depend on the aspect ratio of the roof: a large, single-story warehouse will have a larger roof area than a high-rise office building with a smaller footprint.
    Studies by Lawrence Berkeley National Laboratory (LBNL) and the Florida Solar Energy Center have found considerable differences in reflectance among roofing materials, such as single-ply membranes, built-up roof systems, metal roofing, and modified bitumen. Many single-ply membranes are available in white with reflectances of 70 to 80 percent or more. For some single
plies, a dark membrane can be combined with a white coating to achieve the same reflectance, provided the coating is properly applied. Built-up roofing systems (BURs) vary widely in their reflectance, from five to 80 percent, depending on the surfacing.
For BURs surfaced with aggregate, LBNL reflectance data
ranges from 10 percent for dark aggregate to nearly 50 percent for white marble chips. Covering dark aggregate with a white cementitious coating can increase reflectance to 55 or 60 percent. Using a white reflective coating on a built-up roof can yield higher reflectances of 70 to 80 percent.
    Metal roof products are available with white coatings, which raise solar reflectances to about 65 percent. And other metal
roof products are available that meet ENERGY STAR® specifications without a coating.
    Modified bitumens’ reflectance ranges from five to 25 percent, but it is possible to increase reflectivity to about 65 percent by adding a white reflective coating.
    Concrete and clay tile may be purchased in white, increasing solar reflectance to about 70 percent, as compared to 20 to 30
percent for red tile.

Cool Roof Benefits
    Energy savings. According to Dr. Hashem Akbari at LBNL, dark roofs are heated by the summer sun and raise the summertime cooling demand of buildings. For highly absorptive roofs, the difference between the surface and ambient air temperatures may be as high as 90°F, while for highly reflective roofs with similar insulative properties, the difference is only about 10°F. For this reason, “cool” roofs are effect ive in reducing cooling energy use.
Individual energy savings depend on many factors, including geographic location and climate, existing insulation levels in the building, the type of roof installed, and how well it is kept clean and maintained. The Environmental Protection Agency says that in the right situations, cooling energy savings can be as high as 50 percent.
    Downsized A/C equipment. A reflective roof can reduce peak cooling demand by 15 percent, meaning that the capacity of a commercial building’s HVAC system can be downsized.
Extended roof life. Roofs undergo significant expansionand contraction as they heat and cool throughout the day. Heat absorbed by the roof can also accelerate degradation by ultraviolet rays and water. A reflective roof can reduce the amount of thermal shock that occurs on the roof surface and make the roof last longer. This also means that less retired roofing has to go to landfills.
    Reduce Smog from Urban “Heat Island” Effect. On a summer afternoon, temperatures in Los Angeles are typically 5°F higher than the surrounding suburban and rural areas. Hot roofs and pavements, baked by the sun, warm the air blowing over them. The resulting urban “heat island” raises air conditioning bills and speeds up the formation of smog.
    LBNL researchers have found that increasing the albedo (solar reflectivity) of roofs can limit or reverse an urban heat island effectively and inexpensively. Measured data and computer simulations studying the impact of the temperature on Los Angeles smog show that a significant reduction in ozone concentration
is achieved by lowering the ambient temperature and decreasing necessary cooling demands.
    This shows that, when widely used in a community, cool roofings can decrease air temperature and cooling demands, which reduces the amount of smog in the air and benefits the entire community.

ENERGY STAR¨ Roofing Label
    The Environmental Protection Agency’s ENERGY STAR program now offers the ENERGY STAR label for reflective roof products that lower roof surface temperature by up to 100°F.

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