Plant selection for green roofs and their impact on carbon sequestration and the building carbon footprint

Abstract

One of the most critical determinants of a green roof's performance is the type of vegetation planted on its surface. This study examines the factors influencing the choice of plant for green roofing, such as sunlight requirements, water requirements, and cold tolerance, in order to identify the preferred green roof plants for use in cold and dry climates such as Mashhad, Iran, and to provide a roadmap to assist decision making in this regard. For this purpose, initially, fifty different plant species were evaluated from four perspectives: (i) applicability in extensive green roofs, (ii) photosynthesis rate, (iii) availability, (iv) low cost. Then, green roofs with selected vegetation were used in a field test, and the amount of carbon uptake of each of them was measured over one year. Finally, by modeling these green roofs in Design Builder software, the reduction of building energy consumption was evaluated to comprehensively investigate the overall impact of green roofing on the carbon footprint of the building. This study found that the best plants for the climate experienced in the field test are Sedum acre, Frankenia thymifolia, and Vinca major, which enjoy good tolerance and performance characteristics and offer the best energy demand and carbon emission. Green roofs with these three plants could reduce a typical building's annual energy consumption by 8.5%, 8.0%, and 7.1%, respectively. After implementing a green roof with Sedum acre, Frankenia thymifolia, and Vinca major atop a 4-story building and measuring these plants' dry weight monthly over one year, the annual CO2 absorption of these plants through photosynthesis was estimated to be 0.14, 2.07, and 0.61 kg/m2. In addition to absorbing carbon through photosynthesis, the green roofs with these plants also reduced the building's CO2 emissions by 28.16, 26.48, and 23.44 kg/m2 respectively, by reducing the energy demand.