What Happened
A new study from the Natural Resources Defense Council and UCLA says that green roofs and “cool roofs” can have a major impact on stormwater runoff, energy usage, and an urban environmental phenomenon known as “heat islands.”
The Findings
The report, titled, “Looking Up: How Green Roofs and Cool Roofs Can Reduce Energy Use, Address Climate Change, and Protect Water Resources in Southern California,” has a local emphasis, but national implications.
It is premised on several interconnected findings regarding runoff and “heat islands” in major cities, such as:
- Urbanization and development transform landscapes into impervious surfaces, which increases the volume of runoff that results from precipitation;
- More runoff means more pollution carried by stormwater to rivers, lakes, and beaches;
- Climate change is threatening regional water supply, particularly the availability of freshwater resources like the Sierra snowpack; and
- Dark, impervious surfaces in our cities absorb and radiate heat back into the surrounding atmosphere at a far greater rate than the natural landscape does, causing a “heat island” effect that raises ambient air temperatures in developed areas, which of course fuels additional energy use for building cooling.
According to the report, green roofs and cool roofs offer a solution, because:
- The plants and growing medium of a “green roof” provide shade, thermal mass, and evaporative cooling that reduces temperatures on the roof surface and in the building interior below;
- Cool roofs use reflective materials, often but not always light colored, to reflect more of the sun’s energy than traditional dark roofs, and to more efficiently transmit heat from the building’s interior;
- Green roofs have substantial capacity to both absorb and delay rainfall runoff, reducing the volume of rainfall runoff and pollutants that flow to rivers and lakes.
Heat Data
Data is provided in all cases to demonstrate the efficacy of the programs. Of particular interest is the data on “heat islands,” which any city dweller (especially New Yorkers) will appreciate. In cities like New York, the percentage of impervious surface in the landscape—such as rooftops, roads, parking lots, and other paved surfaces—is increased. This greater amount of impervious cover, in turn, absorbs and radiates heat back into the surrounding atmosphere. For example, temperatures in highly developed city cores like New York and Boston were an average of 13 degrees to 16 degrees higher than temperatures in nearby rural areas. Green and cool roofs can have a material impact on this effect, and details are provided in the report.
The study also provides extensive detail about how to operate green roofs successfully, and provides comparisons between running green roofs, cool roofs, or solar-powered roofs. Also provided are analyses of costs, benefits, and installation challenges, as well as policy recommendations for southern California, which may be interesting to those in other regions. For example, the study recommends financial incentives for residential and commercial development, noting that Chicago recently introduced fast-track permitting procedures.
More Information
The University of California at Davis has published a quick overview of green roofs, with details on what they are, how they work, and benefits and considerations. A much more-detailed Canadian study is available from GreenRoofs.org, complete with quantifiable benefits and numerous case studies. For implementation assistance, the non-profit Pomegranate Center has published a Green Roof Manual, and a Michigan State study analyzed selecting plants for green roofs in the U.S. The Department of Energy also published guidelines for selecting cool roofs, and a more entry-level document, titled, “Everything You Need to Know About Cool Roofs,” is available from CoolRoofs.org.
For additional data on heat islands, review this EPA “compendium of strategies,” which focuses on the impact of green roofs.
In addition to the NRDC report, Gov1 recently reported on a separate NRDC study on stormwater infrastructure and new ways of financing stormwater collection methods.