Green roofs at Judson University in Elgin, Ill., have produced lower energy costs. (Photo courtesy of credit McElroy Metal)
Cool Versus Green Roofs: Reflect on the sunny possibilities of
Cool (or refl ective) roofs and green (or vegetative) roofi ng systems are two of the most effective ways to increase a building’s energy effi ciency. While similar in some aspects, they are not interchangeable. The two have distinct attributes that should be evaluated and compared prior to installation.
Cool characteristics “Cool roof is a term encompassing several differ- ent product types,” says Jessica Clark, marketing manager at Cool Roof Rating Council, Oakland, Calif. “Today there are cool options in all types of roofi ng materials, from coated metals and coatings, to cap- sheets and single-plies, to shingles and tiles.” A cool roof can be aluminum, hot-dipped
galvanized, or Galvalume. “Cool metal roofi ng is available in a wide range of profi les and colors, and is able to produce many different looks for a commercial or residential structure,” says Mark A. Thimons, P.E., LEED AP BD&C, executive direc- tor, Cool Metal Roofi ng Coalition, Pittsburgh. You can even have a black roof that quali- fi es as cool. “Refl ective pigment technology can
18 METAL CONSTRUCTION NEWS September 2012
increase the solar refl ectance of a metal roof (in wavelengths outside of the visible range) without changing its appearance or color,” Thimons adds. A cool roof refl ects and emits the sun’s heat
back to the sky instead of transferring it to the building below. Coolness is measured by two properties: solar refl ectance and thermal emit- tance. These cool properties can deliver signifi - cant energy savings. In the United States, annual cost reduction is typically between 7 and 20 cents per square feet, according to an energy savings model developed the U.S. Department of Energy. Cool roofs signifi cantly reduce energy costs
related to cooling a building. Energy represents 30 percent of a typical offi ce building’s costs and is a property’s single largest operating expense. Cool roofs lower ambient temperatures helping to reduce the urban heat island effect associated with densely populated cities and suburbs. They lower internal building temperatures improving occupant comfort. Cool metal roofi ng has been designed to meet
the criteria of Energy Star, the United States Green Building Council’s LEED program, California’s Title
24, ASHRAE 189.1, and numerous state and munici- pal programs. “The ability of a cool roof to reduce energy consumption has been widely documented,” says Laura Lanza, marketing chair, National Coil Coating Association, Cleveland. “The EPA estimates that a cool Energy Star labeled roof on a building can save up to 40 percent in annual energy costs.”
Vegetative variables Green roofs started in Babylon in the 7th cen- tury B.C. The fi rst was the Hanging Gardens of Babylon, a majestic structure built by King Nebuchadnezzar II for his wife, Amyitis. Today, a green roof is a partially or completely covered roof with vegetation like shrubs, trees, durable grasses and sedum plants in a growing medium, over a waterproofi ng membrane. Green roof systems may be modular, with
drainage layers, fi lter cloth, growing media and plants already prepared in movable, interlocking grids, or each component of the system may be installed separately. They may also include additional layers such as a root barrier and irriga- tion system. Green roofs convert carbon dioxide