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As of 2021 all new buildings within the European Union will be nearly Zero Energy Buildings (NZEB). Different plans are being promoted by member states to reach this goal, to increase the number of NZEBs and to facilitate the retrofit of existing constructions. Within this frame, several analyses of cost benefits have been developed to evaluate the economic effectiveness of retrofit interventions. The economic analysis can be extended to the energy and environmental fields introducing various indicators, such as energy and environmental payback times, to describe the effectiveness of the retrofit intervention from a more extensive point of view.

In this paper, we considered a school building located in Turin (Northern Italy) with the scope of evaluating the energy and carbon payback times of different retrofit scenarios that could be implemented for the existing building. An optimal cost retrofit and two NZEB retrofits were considered, varying envelope insulation, heat generation and lighting systems, shading and control devices. First, we simulated the building's energy consumption and calibrated the results with data from actual energy bills. The calibrated model was used for evaluation of the building's energy demand after the proposed retrofit interventions. We then applied the Life Cycle-Analysis (LCA) for the calculation of environmental payback times. The results showed that the NZEB retrofit produces energy and environmental payback times that are shorter than the life cycle of the building. In the foreseeable future, solutions such as these could prove effective in addressing issues that arise from global warming and energy supply.