Home > Articles By Issue > Building Envelope & Exteriors > Article Jan. 2003

Green Roofs Vs. Cool Roofs

By Jill Korot

These days, it seems that every industry is veering its focus toward the environment. Every industry is flooded with products that are supposed to be better for the environment than their predecessors. The roofing industry is no exception.

Roof system manufacturers are clamoring to create products that are not only less damaging to the environment, but actually beneficial. In addition to the systems themselves, vendors are also looking to improve the manufacturing and application processes. By improving these processes, they are already marketing a more environmentally friendly product.

Foam roof insulation is the principal target of manufacturing process improvements. Most specifically, the polyisocyanurate manufacturers are phasing out the use of hydrofluorocarbons in the blowing agents used to make the foam. Companies like Denver, CO-based Johns Manville Roofing Systems Group (JM) have already complied with the mandate to be enforced as of January 1, 2003.

"JM has spent over 25 million dollars to convert their polyisocyanurate roofing insulation manufacturing facilities to use an HC (pentane) blowing agent to produce foam insulation. This was a significant investment to manufacture the next generation of zero ozone depletion products," stated Sarah Tholen, manager of marketing communications at Johns Manville.

In regards to application processes, many manufacturers of asphalt systems are examining their application processes to improve the health and safety of the laborers involved in the process. Additionally, they are examining the offgassing that occurs as the systems deteriorate through such natural processes as erosion and evaporation.

According to Ken Hunt, vice president of sales at the Tampa, FL-based Holland Roofing Group LLC, "One of the biggest roofing trends that can be attributed to protecting our environment is the exclusion of roof adhesives from the manufacturer. Suppliers are starting to exclude these adhesives and use alternative methods for application in an effort to reduce fumes."

With these and other modifications, the systems are already more environmentally friendly than their predecessors-before they are even attached to the top of a facility. Any benefits the product can provide the building owners/managers or the surrounding areas of the community are just the icing on the proverbial cake.

Cool Roofs Are Green

Cool roofs-roof systems that have reflective properties that minimize the absorption of heat-are recognized as part of a sustainability program, which makes them good for the environment. Things that are good for the environment have been coined "green." These systems might be green in how they were manufactured or green in how they were applied atop the structure. However, even without those processes being counted, just being light in color makes these systems green.

According to the California Energy Commission, "Most traditional dark roof materials are hot, absorbing 70% or more of the solar energy striking them. Cool roofs absorb less than 35% of this solar energy and stay 50¡F to 60¡F cooler than traditional dark roofs during peak summer conditions. Cool roofing materials are available as coatings (a surface treatment that has the consistency of thick paint), membranes (a pre-fabricated sheet), coated metal roof products, and cool roof tiles. The resulting reduction in the transfer of heat into air-conditioned space below a cool roof results in cooling cost savings of 10%-20% on average."

The commission further claims, "With assistance from California's Cool Savings with Cool Roofs Program, colleges and universities are reducing cooling and other operational costs through the installation of light colored, energy conserving cool roofs." One of the first colleges to take advantage of cool roofs and the Cool Savings rebate program was the College of the Desert in Palm Desert. Located between Palm Springs and Indio in southern California's Coachella Valley, Palm Desert is an ideal location to rely upon cool roofs to reduce energy consumption during blazing hot summers. Given the fact that dark rooftops in Palm Desert can absorb enough heat from the sun to reach temperatures as high as 160¡F during summer, cooling the community college's roofs is a major part of its effort to save money.

Through the college's participation in the California Energy Commission's (CEC) rebate program, nearly 50,000 square feet of new cool roofing was installed on campus buildings-including a gym and three academic buildings. A single-ply membrane was installed on all four buildings.

According to Gene Ingle, the college's director of construction and facilities, "If you maintain the cool roof membrane, you can get another 15 years of substantial energy savings from it."

Ingle continues, "We installed a number of white membranes (like cool roofs) back then. Today, in addition to installing new cool roofs, we are enhancing these older reflective roofs with heavy insulation to increase energy and costs savings further," he says.

Green Roofs Are Cool

In an article posted to CNN.com, the Environmental News Network reports that a grass rooftop can't get any hotter than about 77¡F. This means a green roof can help relieve city smog while cutting pollution and energy consumption.

The article further states, "A 3¡F to 7¡F temperature drop translates to a 10% reduction in air conditioning requirements. For a one story structure with a green rooftop, cooling costs can be cut by 20% to 30%."

The example provided to illustrate these savings featured a recent study conducted by Weston Design Consultants for the city of Chicago. The study estimates that the installation of green roofs atop all of the city's buildings would produce $100,000,000 in saved energy each year.

Green roofs also filter air pollutants and act as sponges, retaining as much as 50% to 70% of the storm water they capture. This reduces storm water runoff, which contributes to contaminated water supplies and flooding.

Urban Heat Islands

According to the U.S. Environmental Protection Agency (EPA), a metropolitan area can reach temperatures as much as 6¡F-10¡F warmer than surrounding regions on a hot summer day. This phenomenon is called the "urban heat island," and it occurs because the buildings and pavement in cities absorb the sun's heat instead of reflecting it, causing a rise in temperature. In addition, cities have fewer trees, shrubs, and other plants to provide shade, intercept solar radiation, and cool the air through the process of evapotranspiration.

These warmer temperatures cause a number of negative effects. People living and working in these areas use more air conditioning, which escalates electricity use. The increased use leads to higher levels of carbon dioxide emissions from power plants. Moreover, the heat island's higher temperatures encourage the formation of ground level ozone, the principle component of smog.

The EPA also says planting trees for shade and replacing roofs and pavement with reflective surfaces can help cool heat islands. Use of lighter colored roofing directly reduces heat conduction into buildings, which reduces air conditioning use. Reflective surfaces and vegetation lower the ambient temperatures in neighborhoods, which further reduces air conditioning use and urban smog.

Both green roofs and cool roofs have excellent insulating properties and can help facilities to reduce energy expenses. They both also serve to improve the ambient air quality in the surrounding areas. So while both fit the popular definition for what makes things green, the terms are not interchangeable.