Green from the Top Down
Increasing the Use of Vegetated Roof Systems
Greater utilization of “green roofs” throughout the Department of Defense on new construction projects and renovations would offer sustainable economic and environmental benefits.
By Capt. Katherine Sievers, M.SAME, USAF
The commitment of the Department of Defense (DOD) to reduce energy usage and lower dependence on fossil fuels can be difficult to meet with current security requirements. The extensive installation of vegetated roof systems or “green roofs” on current and future buildings across DOD would aid the department in reaching a pair of its key goals: decreasing facility energy intensity by 37.5 percent by 2020; and ensuring that redevelopment projects of 5,000-ft² or more maintain pre‐development hydrology to the maximum extent feasible.
The DOD Strategic Sustainability Performance Plan states the vision for the department is to “maintain the ability to operate into the future without decline—either in the mission or in the natural and man-made systems that support it.” The plan calls for sustainability to be woven into the everyday fabric of DOD’s mission. Green roofs may offer a cost-effective balance between sustainability and security to aid in meeting these goals.
Green roofs have become increasingly popular in the United States and provide a myriad of benefits, including decreasing the amount of energy used for heating and cooling; reducing the intensity and quantity of stormwater run-off; filtering stormwater run-off; mitigating urban heat island effects; decreasing long-term maintenance; improving outdoor air quality; and extending the lifespan of a roof. Green roofs can be considered in two main categories: extensive or intensive roofs. Intensive roofs have a thicker growing medium (>15-cm), which allows for plants with longer root profiles and a larger variety of vegetation. Intensive roofs give the appearance of a park and can be used as green space if access is included in the design. This type of roof provides environmental and social benefits, but generally requires more maintenance and is much heavier. Extensive roofs have a much shallower growing medium (2-cm to 15-cm), and can sustain only small grasses or succulents. Extensive roofs are lighter and may be self-sustaining when designed and planted properly. Due to their lighter weight, extensive roofs often can be retrofitted on existing buildings.
ADVANTAGES AND CHALLENGES
A prevalent alternative to using green roofs to reduce energy consumption in facilities is ground level tree shade. However, this option carries more risk. Shading can reduce annual cooling energy by 10 percent to 50 percent, but the vegetation can provide hiding places for malicious actors. Current security policy directs landscape planting to appropriately screen and protect the facility without providing concealed areas. In some cases, this requirement can eliminate the use of the dense vegetation that provides the most energy savings for facilities.
Although shade may offer benefits at a lower initial cost, green roofs are more sensible for sensitive facilities. Large vegetation on the rooftops of sensitive buildings also may pose a security risk, yet studies have shown that low lying shrubs and succulents, not trees or bushes, provide the best energy reduction. Proper plant selection can optimize energy savings with low-maintenance plants that are resistant to disease, drought and temperature extremes. Using these plants on an intensive roof can provide maximum benefit while providing minimal cover for malicious actors.
There are challenges to implementing green roofs, especially on retrofits:
- The design of the facility may not structurally support the additional weight.
- Installation costs can be much higher compared to traditional roofs.
- Complex root structures and increased water retention may cause leaks if the waterproofing membrane is breached.
- Unwelcome insects may be drawn to the vegetation.
- Plant selection may be limited given varying climate conditions.
Even with these challenges, however, green roofs remain a viable alternative as proper engineering and design will eliminate the majority of concerns—leaving the long-term benefits to outweigh the short-term impediments.
REDUCING ENERGY CONSUMPTION
The building sector is responsible for about 40 percent of all energy consumption in the United States; approximately one-third of that building energy usage is for heating and cooling. Due to the sheer size of this heating and cooling load, incrementally reducing energy consumption can lead to a large net reduction in energy use. Green roofs on multilevel facilities have been found to decrease energy used for heating and cooling by 25 percent to as much as 80 percent, equating to a total annual savings of energy consumption in conditioned buildings of up to 15 percent. As a secondary benefit of less energy use, some facilities have seen an increase in the service life of heating and cooling systems.
Green roofs can extend the life of a roof by three-times that of a conventional roof. They decrease temperature extremes on the surface, shield the membrane from UV rays, and provide a barrier for physical damage. Green roofs also can decrease the average daily maximum temperature of a roof by 40°F relative to conventional roofs. Although the installation of a green roof may cost three- to six-times a conventional roof, the lifecycle extension has been proven to increase its net present value.
A study from researchers with the University of Michigan, “Green Roof Valuation: A Probabilistic Economic Analysis of Environmental Benefits,” assessed changes in net present value based on varying levels of energy consumption, stormwater run-off and conventional roof replacement over 40 years. The study found the net present value increased by up to 40 percent, though it noted the assessment was only for facilities retrofitted with green roofs, not new builds. Savings in reduced on-site stormwater management and decreased infrastructure from reduced HVAC system loads could contribute to increases in net present value for new construction.
MANAGING STORMWATER RUN-OFF
Stormwater run-off has received increased focus due to urban development and growth overwhelming the infrastructure of many cities, especially where sewer and stormwater systems are combined.
The capacity of existing systems is already exceeded in many storms, leading to the release of raw sewage into local bodies of water. Coupled with higher intensity storm events projected for many regions of the country due to climate change, a focus on stormwater and maintaining or improving existing hydrologic conditions is appropriate. Green roofs have been shown to decrease runoff by up to 87 percent, with an average reduction of 60 percent to 70 percent depending on the roof geometry, decreasing overall stormwater flow. Certain municipalities also provide stormwater credits to facilities with green roofs, which can lead to additional cost savings.
New facilities may benefit from reduced on-site stormwater infrastructure and expensive run-off treatment requirements while existing facilities may benefit from stormwater credits. All communities and ecosystems, especially areas sensitive to flash flooding and climate change, would benefit from decreased stormwater run-off.
Green roofs on multilevel facilities have been found to decrease energy used for heating and cooling by 25 percent to as much as 80 percent, equating to a total annual savings of energy consumption in conditioned buildings of up to 15 percent.
LEADING BY EXAMPLE
In 2013, a green roof was installed on the new U.S. Coast Guard Headquarters in Washington, D.C. Measuring 557,000-ft²—making it at the time the second largest green roof in the United States and the third largest in the world—it was designed to reduce stormwater run-off by 47 percent, a significant contribution for an area where 0.75-in of precipitation causes a release of raw sewage. Building occupants can use the roof’s extensive courtyards and gardens for walking, meetings and breaks.
Across the U.S. Air Force, green roofs are installed at Andrews, Ramstein, Peterson, Offutt, Nellis and Tyndall AFBs and the technology is identified in future projects. The U.S. Navy and U.S. Army have utilized green roofs on some new and existing projects. The General Services Administration has incorporated green roofs on 61 of its buildings and projects (including the Coast Guard Headquarters). The agency’s total green roof portfolio is 2.2 million-ft². The use of green roofs should be considered when constructing new facilities throughout DOD and also evaluated as a retrofit option. Starting in January 2016, cadets at the U.S. Air Force Academy will research the most effective green roof construction and plant selection for the Colorado area. Similar research and case studies of existing green roofs in every region will ensure that vegetated root systems are a self-sustaining, cost-effective and energy-efficient solution.
The views in this article are those of the author and do not reflect official policy or position of the U.S. Air Force, Department of Defense, or U.S. government.