The Potential for Reducing Impacts of Solar Radiation on a Crop Producing Green Roof, and Modifying Roof Microclimates, through the Utilization of an Adjacent Crop Producing Green Façade. (FUNDED by USDA-NIFA)
The District of Columbia has millions of square feet of roof area. This roof area negatively impacts the urban environment and climate in several ways: the roof surface area absorbs a significant amount of heat, and is the greatest factor in residential energy consumption in certain climates (Kliman, 2001). Light colored roof coatings can ameliorate this problem by reducing the heat absorption; however, these coatings require regular maintenance, darken with dirt accumulation and age, and do nothing to address the increased runoff created when the previously vegetated site was replaced by an impermeable surface. These reflective roofs, coupled with the higher thermal mass of typical building materials like concrete and brick, are significant factors in the urban heat island in urban environments such as the District of Columbia.
Vegetated, or green, roofs can mitigate the effects of the urban heat island and water runoff in several ways. By improving the thermal performance of a building, a green roof can reduce the annual load for cooling the building (Garrison, 2012). A smaller mechanical system translates into reduced CO2 emissions. The plants also reduce the radiation reflected back into the atmosphere from the roof. Finally, water retention and evapotranspiration of rainfall helps reduce runoff. These green roofs can contribute to an urban environment in other positive ways. A well-designed green roof can have park-like setting, and serve as a nice amenity for building users. A green roof can also be used to grow crop-producing plants. In this configuration, the roof serves as an important component in helping to ensure food security in an urban environment.
One of the challenges with fully utilizing the roof for cultivation of plants is the fact that many roofs have a penthouse for the mechanical system. This penthouse has solid vertical walls that reflect the sunlight and heat back onto the roof. The reflected light and heat is deleterious to the growth of many plants – particularly crops. This study proposes to determine the viability of using a three-dimensional modular lattice system to support crop-producing vines that would cover the walls and reduce this reflected sunlight and heat.
The proposed research project is a parallel study that would utilize two existing green roofs – the one on the UDC campus, and one at a k-12 independent school located due west in McLean, Virginia. The UDC roof will test conditions and impacts in a dense urban environment, five floors above grade. The Virginia site will test conditions and impacts in a heavily vegetated suburban environment, two floors above grade. The test areas will be divided into three sections. The lattice on one section will support dense vine crops. The lattice on the second section will support moderately dense vine crops. The third section will serve as the control, with no lattice or vines. The roof area adjacent to the walls will be planted with micro greens. Measurements will be taken at regular intervals away from the wall to determine temperature, relative humidity, and solar radiation.
It is anticipated that the dense vine crops will provide shading and reduce the reflected sunlight by as much as 20%. It is further anticipated that the air temperature adjacent to the vines will be reduced by 2-8°C (Connelly, 2012). These two variables should provide a better environment for the plants located in the adjacent shallow beds, and result in a higher crop density. With thriving crop producing vines, and an increase in the usable roof area for planter beds, the overall potential for increased food production on the roof is significant.
The ability to increase food security, while also mitigating the urban heat island and reducing the harmful runoff, simply by using existing roofs in the District has an enormous potential to positively impact the overall sustainability of the city. Furthermore, this condition of reflected heat is not limited to roofs with penthouses or partial floors on the same level. The results of this study will have relevance to the creation of urban farms, and even the homeowner with the garden plot. Student participation will provide hands-on learning experiences for college level and k-12 students, and there are numerous outreach opportunities through the activities of the Center for Urban Agriculture and the Co-operative extension services of UDC.
Susan Schaefer Kliman, PhD, FAIA
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