Comparative environmental and economic evaluation of green roofs under Mediterranean climate conditions – Extensive green roofs a potentially preferable solution

Abstract

Green roofs constitute a prominent urban green infrastructure solution and are considered to be able to enhance urban sustainability and resilience. Existing literature, however, lacks a holistic evaluation of their life cycle impacts. In this study, a comparative environmental and economic life cycle analysis between green and flat roofs under Mediterranean climate conditions was conducted, examining all green roof types (extensive, semi-intensive, intensive), two types of drainage layer (granular and synthetic) for each one, all green roof layer materials, life cycle stages (production of materials, use and maintenance, disposal) and their quantifiable costs and benefits, aiming to address this research gap. Dynamic whole-building energy demand simulations were also conducted in its context, which additionally investigated the effect of various factors on green roofs' energy saving potential. Thermally insulated green roofs provided a small improvement in the energy consumption for heating and cooling (up to 8.30% and 3.50% respectively) but significant reductions in total life cycle energy consumption (8–31%), CO2 emissions (24–32%) and waste production (15–60%) rendering them a cleaner environmental option compared to flat roofs. However, they also produced a very important increase in total life cycle water consumption (279–835%). The extensive green roof types were the ones to perform better in all these indexes. They were also the only type to constitute a better economic choice than a flat roof for the private owner in a low discount rate (0.25%) scenario, both with or without public benefits. As this study shows, the green roofs' potential is not limited only to the improvement of the buildings’ energy efficiency, as numerous studies in the field of energy efficiency highlight, but compared to flat roofs, expand to reduce the embodied energy, greenhouse gas emissions and material waste. By means of environmental footprint, they are superior to flat roofs when only energy and emissions are considered, while in regions where water availability is a key environmental factor, this superiority is in question, together with their economic feasibility in countries where this technology is not financially supported.