On the limits of the quasi-steady-state method to predict the energy performance of low-energy buildings

The recent European energy policies progressively introduced more restrictive energy performance requirements aimed at achieving the nearly zero-energy building target for all new buildings and major renovations. To check compliance with these requirements, the building energy performance can be evaluated through different calculation methods, as widely presented in literature. The present article is aimed at identifying in which boundary conditions (e. g. climate, use category, building size, thermal insulation level) a simplified steady-state calculation method can predict with sufficient accuracy the energy performance of low-energy buildings if compared with a dynamic simulation model. The analysis was performed on two building types, representative of the Italian residential typology, located in three different climatic zones and characterised by two insulation levels. The insulation levels fit the U-values of the notional reference building, established by the Italian legislation for checking compliance with energy performance requirements in two different steps; the first level is in force until 2020, while the second level is that of a reference nearly zero-energy building in force from 2021 onwards. The building energy performance, in terms of net energy needs for space heating and space cooling, was assessed by means of both the monthly calculation method of CEN standards and the detailed simulation model of EnergyPlus. Consistency options were applied to the models to guarantee that their outputs could be comparable. The quasi-steady-state method demonstrated to predict the cooling energy need quite well, but to lose in accuracy when the weight of the thermal transfer in the energy balance increases.