Normally, one wouldn’t walk bare-foot on the roof in midsummer. But, it’s a different story when your roof is green. A green roof is covered with living vegetation rather than conventional roofing materials – and it can provide a cool soft place to rest your feet.
In recent years, green roofs have emerged from an obscure niche and entered the mainstream. Cities around the world, from Chicago to Stuttgart, are adopting initiatives to promote, or even require, the installation of green roofs. As a testament to this growth, green roof start-up companies are sprouting like… green roofs.
While proponents extol the advantages of green roofs, not enough research has been conducted to quantify their benefits. It is commonly accepted that green roofs reduce the urban heat island effect, improve storm water management, reduce building energy demands, extend roof life cycle, and provide other benefits. Although it seems straightforward that green roofs can provide many of these benefits, questions remain about green roofs.
For example, what benefits do different types of green roofs provide? Which roofing materials/designs work the best? How does performance vary in different environments? To answer these questions, Drs. Wade McGillis, Patricia Culligan, and other members of the Columbia Green Roof Consortium are applying ecological techniques – techniques usually reserved for field research – to urban green roofs.
Wade uses the eddy correlation method to measure evapotranspiration, CO2 flux, and heat exchange from his green roof experiments. An LI-840 CO2/H2O Analyzer configured with a flux profiling system, and a customized LI-8100 Automated Soil CO2 Flux System are used to measure mass flux of water and CO2 from their experimental green roof at Columbia University in New York City.
According to Wade, the LI-8100 system has been “continuously monitoring the surface fluxes, basically monitoring the green roof 24/7 over the seasons.” That, he says, has generated direct quantitative measurements that other indirect techniques cannot provide.
Of the potential benefits provided by green roofs, one of the most relevant is reduced storm water runoff. Many cities, including New York, use a combined storm water and wastewater sewer system. Rainfall events can overwhelm the sewer system and cause combined sewer overflows (CSOs), resulting in discharge of untreated waste into waterways.
Green roofs mitigate theseevents by providing additional water-holding capacity, which causes a reduction in the total runoff and forces the runoff to occur over a longer time period. Both effects can lead to fewer CSOs and cleaner water. In fact, the Columbia Green Roof Consortium estimates that green roofs could mitigate 40% of CSOs in New York City, given historical precipitation and the available roof surface area.
Green roofs can have temperatures up to 40 degrees cooler than conventional urban surfaces. Green surfaces provide higher surface albedo, and they result in evaporation and transpiration that typically are absent from urban roof surfaces. These two features both contribute to the cooler temperatures that green roofs provide.
While the ecological measurements of heat, H2O, and CO2 flux are just as meaningful in urban areas, working in urban environments does present some challenges. For example, dense, tall buildings often result in unusual airflow patterns that must be taken into account. Some green roofs, however, are natural in situ wind tunnel experiments. The Columbia University Green Roof on West 118th street and Amsterdam Avenue often experiences wind flows that are straightened by the canyon-like roadway. The roof top parapet also converges like a wind tunnel, resulting in a well-developed boundary layer.
While questions remain about the costs/benefits of green roofs, and which green roofing techniques should be used where, one question is certainly answered – green roofs are another tool that can be used to help improve efficiency and sustainability. As long as green roofs bring change to the world, LI-COR will continue to provide tools to measure that change.
Thanks to Dr. Wade McGillis, Department of Earth and Environmental Engineering and the Lamont Doherty Earth Observatory, Columbia University, for contributing to this article. Photographs courtesy W. McGillis and the Columbia University Office of Environmental Stewardship.