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Design criteria for a residential nZEB in Mediterranean climate are discussed. The integrated design procedure focuses on the problem of a large number of available building variants concerning the building envelope. The aim is to search the ones that minimize winter and summer energy demand without compromising thermal comfort. The adopted methodological approach combines the use of dynamic energy simulation tool (EnergyPlus), based on a one-dimensional conduction finite difference solution method, and a constrained multi-objective optimization algorithm. For four cities (Madrid, Nice, Naples, Athens), several passive strategies are compared: thermal properties of the building envelope, adoption of phase change materials with different melting temperatures, cool roof solutions, several window/wall ratio values, some external and internal shading systems. The results allow to evidence that it is difficult to understand the best trade-off between summer and winter performance, by assuring high standard of thermal comfort when the aim is to reach NZEB objectives in Mediterranean climate. However, some guidelines are indicated, starting from the discussed results.

Thermal bridges are weak areas of the building envelope, determining heat flows higher than those characterizing the common dispersing surfaces (i.e., walls without discontinuities). This phenomenon induces uncontrolled thermal losses and hygiene problems, connected to the possible vapor condensation and mold growth. Presently, the numerical codes for the energy audits, as, for instance, EnergyPlus, carry out zero-dimensional (indoor air) and one-dimensional (conduction heat transfer) analyses, approximately estimating the thermal bridge effects on the seasonal heating demand of buildings. In this paper, results obtained by means of the simplified 1-D models are compared to those obtained by more sophisticated models 2-D or 3-D, in order to point out differences in terms of equivalent conductivity and thermal transmittance. Then, dynamic simulations compare the outcomes in terms of seasonal energy consumption. A typical office building has been considered, analyzing – according to three different approaches – the thermal bridge represented by the roof structure. The outcomes, with reference to several Italian climates, show that a proper modelling is necessary. In particular, an over estimation of the heat losses, determined by an approximate evaluation, induces higher cost of refurbishing, higher cooling energy requests in summer, minor thermal comfort in naturally ventilated buildings.