The valuation of public and private benefits of green roof retrofit in different climate conditions

Green roofs represent a suitable option for industrial buildings refurbishment, providing private and social benefits. Nowadays, in Italy green roof technology is still uncommon, because of high installation cost that drives the choice towards more traditional performing solutions, such as cool roofs (CRs). Thus, a thorough valuation of green roofs benefits in urban contexts is required to encourage their diffusion. This paper aims to valuate private and social costs and benefits generated by extensive green roofs (EGRs) compared with cool roofs in three Italian cities: Trieste, Ancona and Palermo. These contexts are characterized by different Mediterranean climate conditions (North, Centre and South Italy); moreover, residential areas overlooking industrial settlements take place in each of them, so it is possible to hypothesize a potential enhancement of landscape value in these properties, allowing to compare aesthetic benefits due to green roofs upon industrial buildings. The study has been based on literature review and on simulation of energy performances of EGR and CR alternatively considered as refurbishment solution for a reference industrial building taking place in each industrial settlement. For the social side, the externalities deriving from EGRs and CRs, such as aesthetic enhancement, biodiversity preservation and natural habitat provision, carbon reduction, air quality improvement, stormwater control, have been monetized according to available data for the cases of study. The analysis demonstrates that a private investor has a poor convenience to implement EGR rather than CR. On the other, a positive Net Present Value (NPV) derives from social cost-benefit analysis comparing EGR and CR, due to the EGR positive externalities. The valuation of the positive externalities let the calculation of economic incentives amount to promote the diffusion of green roofs in the Mediterranean area. On this basis, an annual reduction of local property tax has been considered as incentive form. Further, a sensitivity analysis with Monte Carlo simulation of both private and social benefits evaluation has been performed for each of three case studies. This technique properly evaluates the final effects on private investments in the presence of random unpredictable variables, depending either on climate conditions or on market observation, that influence their economic affordability. Finally, through the comparison of the three contexts, an overall discussion about of the influence of climate and urban conditions on the economic analysis has been conducted