The Energy Performance Assessment of nZEBs: Limitations of the Quasi-Steady State Approach

The European Directive 2010/31/EU on the energy performance of buildings (EPBD recast) establishes the target of nearly Zero Energy Buildings (nZEB) for all new buildings and major renovations of existing buildings by the end of 2020. This paper investigates the accuracy of the quasi-steady state method, according to the Italian technical specification UNI/TS 11300, in predicting high performance buildings’ energy consumptions. Both the terms of the building energy balance and the simplified dynamic parameters are assessed by comparing the simplified model with dynamic numerical analysis. The two calculation models are applied to some real low energy buildings, which are representative of the Italian building stock. The envelope U-values are assumed as complying with Italian official nZEB requirements. Weather data from some Italian locations, two inertial mass configurations and different system operating chedules are considered. The comparison between the dynamic and static calculations for low energy buildings’ energy performance assessment reveals some discrepancies: the quasi- teady state model generally overestimates the energy need for space heating and underestimates the energy need for space cooling; the gaps are bigger among various Italian locations and inertial mass configurations than among different system operating schedules. The reasons of this gap are discussed in the paper. It is highlighted that in some particular cases the national regulations should introduce the dynamic numerical analysis as reference calculation model.