Economics of green roofs and green walls: A literature review

Green roofs and green walls are effective resilient and adaptive solutions that provide multiple ecosystem services when implemented in urban environments. In the past years, research efforts have been made to understand the real value of these solutions. However, there is still a lack of comprehensive calculation of the economic merit of green roofs and walls, which have restricted the decision-making process. The objective of this study is to provide a state-of-the-art of green infrastructure economics, namely, to identify research methods, summarize range values for each green system and highlight research gaps. A systematic review with a particular focus on green roofs and walls is conducted as well as a qualitative and quantitative analysis of 79 scientific articles. Overall findings suggest that green infrastructure is preferable to traditional alternatives. Financial performance is typically low, quite often negative. However, economic evaluation improves when adding ecological and social benefits. The current weight of evidence suggests that balancing costs against benefits plays a significant role in the implementation of green infrastructure. However, this evidence is still not clear as studies have been reporting contradictory results. Firstly, the listed benefits are not always supported with empirical data, especially intangible ones, where valuation can be extremely difficult. Also, there seems to be a lack of awareness about the multiple benefits and co-benefits of green infrastructure as studies analyze just a few in-depth. Thus, the full potential of green infrastructure is not fully exploited. Secondly, results depend on the dynamic behavior of green roofs and walls. A wide range of possible values for the benefits and costs come out of different study conditions, depending on the influence of design characteristics and external factors on systems performance. Lastly, research methods are not systematically laid out. All this hampers the discussion and comparison of results and further progress. This review is expected to contribute to the body of knowledge on green infrastructure, by helping to set general guidelines to assess their value, identify research gaps and, thus, better support the decision-making process of greening the built environment. Current analyses focus primarily on the most acknowledged benefits, but also extrapolation of results from previous modeling. Future research should be inclusive and based on local studies, using a more collaborative and interdisciplinary approach.