The recast of the energy performance of buildings directive (EPBD) describes a comparative methodological framework to promote energy efficiency and establish minimum energy performance requirements in buildings at the lowest costs. The aim of the cost-optimal methodology is to foster the achievement of nearly zero energy buildings (nZEBs), the new target for all new buildings by 2020, characterized by a high performance with a low energy requirement almost covered by renewable sources. The

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... r presents the results of the application of the cost-optimal methodology in two existing buildings located in the Mediterranean area. These buildings are a kindergarten and a nursery school that differ in construction period, materials and systems. Several combinations of measures have been applied to derive cost-effective efficient solutions for retrofitting. The cost-optimal level has been identified for each building and the best performing solutions have been selected considering both a financial and a macroeconomic analysis. The results illustrate the suitability of the methodology to assess cost-optimality and energy efficiency in school building refurbishment. The research shows the variants providing the most cost-effective balance between costs and energy saving. The cost-optimal solution reduces primary energy consumption by 85% and gas emissions by 82%-83% in each reference building. Besides efforts to design new buildings having low energy demand and available RES [4] [5] [6] , it is essential to tackle the high energy consumption in existing buildings. The contribution of buildings to the total final energy consumption in the EU was 40% in 2012, making the building stock responsible for 38% of the EU's total CO 2 emissions [7] . Despite more stringent building codes and policies, which helped this value to decrease slightly in residential buildings since 2007, the final energy consumption in non-residential buildings remained quite stable in the last decade. Improving and investing in energy renovation of the EU building stock is a key aspect not only for the derived savings and CO 2 emissions reduction, but also for the potential growth and employment that this sector can provide [8] . According to the EPBD, MS have to consider cost-optimality to establish minimum energy performance requirements in buildings at the lowest costs. A methodology is provided by Delegated Regulation No. 244/2012 and its Guidelines to derive cost-effectiveness from a technical and economic perspective [9, 10] . The methodology foresees a comparison in terms of costs [11] and energy performance [12] of construction alternatives to be taken into account both in new and existing buildings undergoing major or non-major renovation. The cost-optimal level is the solution that presents the lowest total costs and it can be located in the graph that reports global costs (€/m 2 ) and energy consumption (kWh/m 2 y) ( Figure 1 ).