Reflect on the sunny possibilities of these two environmental options
Cool (or reflective) roofs and green (or vegetative) roofing systems are two of the most effective ways to increase a building’s energy efficiency. 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 different product types,” says Jessica Clark, marketing manager at Cool Roof Rating Council, Oakland, Calif. “Today there are cool options in all types of roofing materials, from coated metals and coatings, to capsheets and single-plies, to shingles and tiles.”
A cool roof can be aluminum, hot-dipped galvanized, or Galvalume. “Cool metal roofing is available in a wide range of profiles 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 director, Cool Metal Roofing Coalition, Pittsburgh.
You can even have a black roof that qualifies as cool. “Reflective pigment technology can increase the solar reflectance of a metal roof (in wavelengths outside of the visible range) without changing its appearance or color,” Thimons adds.
A cool roof reflects 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 reflectance and thermal emittance.
These cool properties can deliver significant 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 significantly reduce energy costs related to cooling a building. Energy represents 30 percent of a typical office 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 roofing 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 municipal 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 century B.C. The first 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 waterproofing membrane.
Green roof systems may be modular, with drainage layers, filter 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 irrigation system. Green roofs convert carbon dioxide to oxygen, provide insulation, create a natural wildlife habitat, process airborne toxins, offer aesthetic qualities and like cool roofs, mitigate the heat island effect.
A vegetative covering protects the waterproof membrane from daily temperature fluctuations and the ultraviolet radiation of the sun that breaks down conventional roofing systems. Similar to a green roof, a vegetative roof’s lower surface temperature reduces the heat island effect in urban areas. Their increased insulation values stabilize indoor air temperatures and humidity, and reduce the heating and cooling costs for a building.
There are two types of green roofs: intensive roofs, which are thicker and can support a wider variety of plants but are heavier and require more maintenance, and extensive roofs, which are covered in a light layer of vegetation and are lighter than an intensive green roof.
Green roofs are most effective on flat-roof surfaces (maximum slope of 1:12) using the largest continuous roof area. Slightly higher slopes up to 2:12 are feasible but may require additional engineering controls.
Green roof vegetation selection is critical and must be considered in initial planning. It must be chosen with regard to the amount of care it needs, its mature size and roof wind conditions. Vegetation that is oversized may suffer in shallow green roof soils and become uprooted in high winds if there is no shelter. Plants must be able to withstand the harsh climates and weather extremes found on roofs.
Maintenance
Cool metal roofing is virtually maintenance free and can last 30 to 40 years. “Coatings are considered ‘self-cleaning’ in that they resist dirt pickup,” says Lanza. “Some paint manufacturers recommend an annual sweet (tap) water rinse in highly corrosive environments, but unless your building is in a heavy industrial or seacoast environment, no maintenance is required.”
Thimons agrees, saying cool metal roofs require minimal maintenance and cleaning. “Research at the Oak Ridge National Laboratory demonstrates that metal roofing retains its solar reflectance over time better than other roofing products because it resists the growth of organic matter and sheds dirt more readily than other materials. It maintains its surface properties and resists soiling.”
On the contrary, a green roof is a highly engineered living system needing a routine and consistently executed maintenance protocol including watering, weeding, fertilizing, replanting, and replacing dead or dying vegetation. An ineffective maintenance program will result in a wild and overgrown look that lessens the green roof’s success. Weeds, discolored or scorched plants, small plants and plant loss are signs that a green roof is not being correctly maintained. Fungal diseases and insect problems must be checked on a regular basis.
Initial expense
According to Thimons, there is typically no additional expense for installing cool metal roofing when compared with non-cool metal roofing. And, “it pays for itself over time with its energy efficiency and durability benefits,” he says.
However, the initial expense of green roofs is higher than cool roofs. This is because they require more extensive design, careful structural analysis, and multiple layers and systems. Some types of green roofs have more demanding structural standards, especially in seismic regions of the world.
“For initial green roof construction, you are going to have multiple layers and a lot more cost involved with insulation, as compared to metal roofing,” says Brian Partyka, president of Drexel Metals, Philadelphia. “It has to do with the depth of the roof and how many inches you’re going to create with the vegetative roof; there are a lot of variables here.” John Ferraro, general manager at Asphalt Roofing Manufacturers Association, Washington, D.C.,says the same principle applies to asphaltic roofing products. “Construction and installation costs for an asphaltic cool roof can vary greatly depending upon the desired cap sheet or surfacing material. A green roof on the same building could generally prove more cost intensive than a cool roof due to the labor, materials and drainage issues but, of course, each case is different.”
It is important to remember that because a green roof protects the waterproofing membrane from the elements, particularly UV light, the membrane’s life expectancy is doubled or even tripled, leading to recovered initial cost differentials.
Water runoff
One of green roof’s best attributes and one of its biggest pluses over cool roofs is its ability to absorb, collect and reuse rainwater. Most buildings are designed to shed rain; they’re built with hard, impenetrable roofing surfaces. Rainwater bounces off them and collects as runoff, picking up impurities like infectious bacteria from animal waste as well as harmful pesticides and fertilizers on the way to municipal storm sewers. All of this eventually empties into local waterways.
Green roofs decrease the total amount of runoff and slow the runoff rate from the roof. They can retain up to 75 percent of rainwater, gradually releasing it back into the atmosphere via condensation and evapotranspiration (a natural process that cools the air as water evaporates from plant leaves), while retaining pollutants in their soil.
Correct drainage from a green roof’s vegetation is critical. If excess water is not properly directed off the roof, or if the system is incorrectly designed and becomes clogged, this adds far more weight to the green roof than it is supposed to hold. If a green roof is incorrectly assessed for the weight it can bear, or if that weight limit is not heeded, the roof can be damaged or even collapse from the combined weight of plants, soil-holding water, drainage systems and other green roof construction materials.
Also, the excess water can become stagnant or drown the plant roots. Leaks in a green roof arise from poor design or inadequate root barriers. Root barriers are specific materials designed to inhibit the slow but inexorable dig of roots, which can break apart even sturdy roof materials in search of nutrients. Leaks also occur from holes or improper installation of the waterproof lining layer of a green roof. Installers must ensure that no punctures occur, because tiny holes are difficult to find and repair.
Routine monitoring of a green roof’s underlayment is no different than maintaining any comparable roofing system. When leaks are detected under green roofs, a procedure exists for cutting out and removing a section of green roof and reinstalling it after repair.
Acceptance issues
Unlike the growing cool roof industry, the benefits of green roof technologies are poorly understood and the market remains immature in the United States. However, in Europe, green roofs have become very well established. This is the direct result of government legislative and financial support at both the state and municipal level. This support has led to a multimillion dollar market for green roof products and services in Germany, France, Austria and Switzerland. In Germany, it is estimated that 14 percent of all buildings are covered with vegetation, a number that is increasing as the German green roof industry continues to grow 10 to 15 percent per year.
Will green roofs ever catch on in the United States like they have in Europe? According to Michigan State University’s Department of Horticulture’s Green Roof Research Program, several barriers to widespread green roof acceptance exist in North America. These include a lack of awareness regarding green roofs, potentially higher installation costs, limited quantifiable data pertaining to the benefits they provide, no technical information on how to build them, and a lack of government incentives or tax breaks. But these problems are not insurmountable and are being addressed.
Meanwhile, in Chicago, codes offering incentives are available to builders who put green roofs on their buildings. The Chicago City Hall green roof is one of the first and most well-known examples of green roofs in the United States. It was planted as an experiment to determine the effects a green roof would have on the microclimate of a roof. According to the Chicago Department of Environment, on hot days, temperatures atop it are typically 25 to 80 degrees Fahrenheit cooler than an adjacent county office building with a black top roof.
The best of both
When installed correctly, cool roofs and green roofs have excellent insulating properties. But, “cool roofs and green roofs are different technologies used for different applications,” says Clark. “Both have their own environmental and energy efficiency benefits. Cool roofs and green roofs are complementary technologies, and can be combined by using cool pavers on a green roof patio space.”
Ferraro agrees, saying both cool roofs and green roofs can increase a building’s energy efficiency and recommends adopting a “whole building envelope approach” when trying to reduce building energy consumption and achieve maximum efficiency results.
