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The target of the nearly zero-energy building (nZEB), stated by the European Union, represents one of the most strenuous challenges to reduce energy consumptions and greenhouse gas emissions in the building sector. In Italy, the nZEB concept refers to a set of energy performance requirements, fixed at national level and establishing a maximum allowable mean thermal transmittance value of the building envelope, as a function of the heating degree-days and of the shape factor. The building envelope is becoming more and more thermally insulated; this determines the reduction of the energy need for heating, but on the other hand it can cause the indoor overheating and the resulting increase of the energy need for cooling. In the design of highly energy efficient buildings, the different energy needs should be kept in balance as to increase the overall energy performance. The article aims to investigate the conditions and extent for which the envelope insulation is beneficial for containing overall energy needs. A sensitivity analysis that involves different insulation levels of the building envelope is performed on some apartments of a typical residential building, located in three different Italian climatic zones. The energy performance calculations are carried out by means of a detailed dynamic simulation tool (EnergyPlus). The results point out that, whereas the effect of increasing the thermal insulation causes a stable reduction of the energy need for heating, the energy need for cooling is very sensitive to the apartment storey; specifically, it increases in ground-floor apartments, and decreases for topfloor building units. Its reduction becomes progressively more consistent at the decrease of the heating degree-days. Considering the annual imbalances between the energy needs for cooling and heating due to the hyper-insulated envelope, reference values of thermal transmittance can be derived as to maximise the overall energy performance of the building.

On the compliance of thermal performance requirements for highly insulated building units