Green, Vegetated Roofs

“Another alternative to white roofing is garden, or vegetative, roof systems. Garden roofs go back as far as the Hanging Gardens of Babylon, but they are enjoying renewed interest primarily as a means of reducing the effects of UHIs while creating aesthetically appealing, environmentally friendly construction,” reports Ballensky.

Offering benefits beyond UHI mitigation and energy efficiency, green planted rooftops are also known for stormwater retention, rainfall pollutant filtering and evaporative cooling. In fact, the surface temperature of a green roof can actually be cooler than the air temperature on hot summer days.

“Rooftop vegetation can also remove air pollutants and greenhouse gas emissions through dry deposition and carbon sequestration and storage,” adds Denny.

Aesthetically, green roofs are well known as beautiful places of respite, particularly when situated within bustling urban environments. As a habitat for native plants, and potentially animals, a good percentage of garden roofs are accessible and may even offer a small walking trail, trellises or water features.

Putting some numbers behind these ideas, a Chicago study compared the rooftop temperatures on a hot summer day between a green roof and neighboring dark, conventional roof. With temperatures in the 90s, the green roof’s surface temperature ranged from 91 F to 119 F, while the darker roof’s temperature reached 169 F. Meanwhile, the nearsurface air temperature above the green roof was approximately 7 F cooler than the conventional roof.

For a similar Florida study, a green roof’s temperature was measured at 86 F while the rooftop on the neighboring building, which happened to be lightcolored, registered at 134 F.

In an attempt to quantify the extent to which a widespread adoption of green roofs could potentially reduce temperature over a large area, the National Research Council Canada conducted a modeling study for Toronto and predicted that adding green roofs to 50 percent of the downtown area would cool the entire city by 0.2 F to 1.4 F. By irrigating these roofs as well, temperatures could be bumped down another 3.5 F, and extend a 1 F to 2 F reduction over a larger area.

In the same previously referenced NYC simulation study, researchers also found that a 100 percent conversion of the city’s roofs to green roofs would result in city-wide temperature reduction of 0.4 F, with 3 p.m. temperatures falling 0.8 F.

Despite these statistics, Mandel points out that while green roofs have a higher albedo than conventional dark roofs, they aren’t as effective as cool roofs when it comes to the UHI mitigation. Essentially, vegetated roofs’ cooling property is evapotranspiration, which means that their absorbed solar energy is redirected toward vaporizing water, as opposed to heating the building.

“These effects keep cities cooler directly and shave peak load on the hottest days, which has an indirect UHI mitigation effect,” he explains. “However, because green roofs lead to more evaporation, they will cause more rainfall, and the heat absorbed during evaporation is released upon precipitation. This means that green roofs cannot cool the world nearly as effectively as white roofs.”

In fact, a competition between Chicago City Hall, with its green roof, and Chicago’s Cook County Building - which shares the same structure with City Hall, but with a white, reflective roof - resulted in the Cook County Building saving $65,000 on its electric bill in 2011, whereas City Hall only saved $22,000.

Despite these differences, many local governments are promoting, and in some cases, mandating green roofs as a way to combat UHIs. As listed on the EPA’s website⁶ the following cities and states are offering incentives for green roofs as a means of meeting local building codes requirements: California, Chicago, Washington, D.C., Kansas City, Boston, Cincinnati, Portland, Pennsylvania, Austin, Dallas, Houston, Arlington and Seattle.

In addition, LEED recognizes planted green roofs as a way to earn credits for Landscape and Exterior Design to Reduce Heat Islands. And depending on how the green roof is designed and integrated into the building, projects can earn up to 14 credits for stormwater management, recycled content, reduced site disturbance, water-efficient landscaping, local/regional materials and optimized energy performance.

Green roofs have also been documented to reap significant energy savings. For example, a study conducted in central Florida found a green roof’s average rate of heat transfer to be 40 percent less than the adjacent light-colored rooftop, resulting in an estimated summertime energy consumption reduction of 2.0 kWh per day.

Another Chicago study, this one a simulation of City Hall’s green rooftop conducted by ASHRAE, revealed that every one degree Farenheit decrease in temperature would cause a 1.2 percent reduction in cooling energy use. Consequently, if all of Chicago was retrofitted with green roofs, then over a period of 10 years, this would amount to $100 million in annual savings from reduced cooling loads.

Meanwhile, another CRRC modeling study conducted in Toronto found that a 32,000-square-foot green roof on a onestory commercial building could save 6 percent in cooling energy and 10 percent in heating energy usage, amounting to a savings of 21,000 kWh per year.

At the same time, green roofs do require some investment beyond growing a few plants on the rooftop. Rather, they require a waterproofing membrane, root barrier, drainage layer, lightweight soil or engineered growing medium and vegetation.

Building owners also need to determine whether to install an inaccessible extensive, lighter-weight roof with more self-sufficient plant types or a more built-up intensive roof. While there are no studies that official document which green roof type is more effective at lowering rooftop temperatures and UHI mitigation, researchers at the University of Quebec are beginning to look into this question have already gathered data on fine airborne thermal coverage.

However, what is known is that extensive roofs offer better energy savings. Although their installation is more expensive, they require more structural support and need more maintenance over the long term.

State of the Art

With noted roofing technology advancements - both for cool and green roofs - one thing is for sure: building owners are enjoying more durable, lasting and flexible roofing materials.

“More sophisticated engineering of roof assemblies is helping improve roof performance in high wind and hail areas and evolving technologies are providing for better fire, oil and chemical resistance in roofing materials,” notes Ballensky.

And aesthetically, products have broken out way beyond initial offerings of just a few colors with unique colors and patterns currently available for highly visible roofs.

With good quality products and increasing choices, cool and green roofing systems are well positioned to help battle UHIs and reap numerous additional benefits along the way.

Endnotes

1. http://www.ornl.gov/sci/roofs+walls/facts/CoolCalc Energy.htm
2. http://www.roofcalc.com
3. http://coolroofs.org/codes_and_programs.html
4. http://www1.eere.energy.gov/femp/pdfs/coolroofguide.pdf
5. http://www.coolrooftoolkit.org/
6. http://yosemite.epa.gov/gw/heatisland.nsf/HIRI Mitigation?OpenView&count=500&typ U.S. Environmental Protection Agency’s Reducing Urban Heat Islands: Compendium of Strategies on Cool Roofs pg. 7 http://www.epa.gov/hiri

Known as the “World’s Best Roof®,” a Duro-Last® roof is perfect for any flat, low-sloped, new or retrofit application. Precision-fabricated accessories combined with prefabricated deck sheets or roll goods allow for a watertight roof that doesn’t sacrifice aesthetics. www.duro-last.com