Cost Optimal Levels of Energy Requirements for Nearly-Zeb: Application to an Italian Reference Building for Existing Offices

The recast of the Directive on the Energy Performance of Buildings has set out that Member States must ensure that minimum energy performance requirements are set with a view to achieve cost optimal levels for buildings, building units and buildings elements. A cost optimal level is defined as the energy performance level which leads to the lowest cost during the estimated economic lifecycle. It must be calculated in accordance with a comparative methodology framework that is based on the global cost method. This one considers, for each energy efficiency measure, the initial investment, the sum of the annual costs for every year (including energy costs) and the final value, all with reference to the starting year of the calculation period. In this study, the global cost method is applied to a reference building for existing offices customized to the Italian context to assess the cost optimal levels. In detail, different packages of energy efficiency measures, which consist in the implementation of envelope thermal insulation and the improvement of systems efficiency, were considered. Moreover, the utilization of renewable energy sources was taken into account with the installation of PV system on the building roof. Then, the energy consumptions of the reference building and the impact of the improvement measures were assessed. Finally, the costs of the different packages were estimated, according to the European Standard EN 15459:2007, in order to establish which of them has the lowest global cost and, consequently, represents the cost optimal level