## 4 Conclusions  
Starting from data collected in HAPPEN project [2], we propose an innovative application of a well-known methodology for computing the efficiency of retrofitting interventions on the building stock of the Mediterranean area, considering their economic and environmental spillovers. Indeed, the method used, the Directional Distance Function, allows to obtain an efficiency score representing the solution that minimizes the CO\({}_{2}\) emissions and, at the same time, maximizes the saving of final energy consumptions. The paper presents two model formulations, to show the potentiality of the approach. Indeed, through Model#1, authors propose an evaluation of the holistic efficiency because the Life Cycle Costing estimation is introduced as input; instead, a classical environmental efficiency was also estimated using as input the total cost of the interventions. The  
Figure 9: Plot of holistic efficiency scores and primary energy consumptions (kWh/m\({}^{2}\)) for Multi-Family Houses and W3S2 climate zone. (**(a)** France in blue dots; **(b)** Spain in red dots). main purpose of the present experiment is to propose a simple tool that, considering few variables, is able to give a score easy to interpret in terms of efficiency. A methodology frequently applied for evaluating the impact on social and health systems due to a production process (e.g., the retrofitting interventions on buildings) is the Life-Cycle Assessment (i.e., LCA) that is recognized as an appropriate methodology. However, a high availability of data is necessary for computing the LCA; in this work, we propose a combination of LCC and Directional Distance Function as a simpler methodology for evaluating which solutions for each typology of building and climate zone are more efficient. Then, the paper presents an alternative methodology simpler than those suggested by the majority of literature to evaluate the optimal choice. The method proposed can be useful for all stakeholders of a retrofitting investments in different phases. From the one hand, the hybrid-DDF methodology can be used before starting the investment, because considering different scenarios, as it was made in the project, the methodology identifies the more efficient solution for each typology of building and for each climate zone. Nevertheless, also during the interventions, investors can compare easily different solutions and choose the more efficient from the costs point of view. In addition, in this definition of the model, the information on the initial investment is considered also thanks to the LCC estimation, then the investor will be able to estimate which solution is the most attractive alternative for him. Last but not least, the correct evaluation of retrofitting interventions is very crucial when the building to retrofit was built before the 1945 (so called \"historic buildings\"). As suggested by [39], the retrofitting of historic buildings can be very expensive but, at the same time, the spillovers in term of market value can be a key-variable in the decision for the intervention. However, in large part, historic buildings are cultural heritage and, for example in Italy, the interventions are not attractive from the economic point of view [39]. This evidence is supported also by [40] that identifies two main reasons: restrictions imposed by the authorities for the preservation of cultural heritage, and the exclusion of historic buildings from conventional public policies. Starting from these considerations, [40] proposes a methodology able to evaluate the choice between the energy performance and cultural value (an interesting review of problems related to the comparison between retrofitting traditional and historic buildings can be found in [41]. A reference for legislative and regulatory aspects of historic building retrofitting is in [42]). Even if the Directional Distance Function is quite easy to calculate and results are very evident, the methodology presents some weaknesses. The main one concerns the choice of variables for the computation of scores.