Passive House standard is a rigorous voluntary scheme for high energy efficiency buildings. Passive behaviour implies that solar heat gains and internal heat gains due to people, lighting and appliances are able to nearly balance the heat losses due to transmission and ventilation in heating mode. This standard is mostly applied in Northern European countries, where heating represents, in most of the cases, the predominant part of energy consumption. However, the adoption of these strategies in the Mediterranean area has to be carefully evaluated to prevent overheating in intermediate seasons an increase of cooling loads in summer. In other words, the use of this standard in the Mediterranean area, characterized by a mild climate, can be debatable. Further, optimistic assumptions are often considered in the design phase and semi-stationary calculation methodologies are still commonly used. Additionally, occupants’ comfort preferences and behavior can determine a high variability in terms of energy performance, in particular with respect to the cost of energy services and the interaction with infrastructures. The research presented concentrates on the analysis of the potential performance variability due to occupants’ behaviour and envelope quality, starting from a real case study building constructed following Passive House standard in Italy. The research compares the initial design hypothesis, obtained with a semi-stationary calculation tool, with a larger spectrum of data generated by means of a parametric simulation, following a Design of Experiment (DOE) approach. The approach aims to detect potentially critical assumptions already from the preliminary design phase, making a proper use of available simulation methodologies to ensure the robustness of energy performance evaluation in the design phase and the possibility to monitor performance transparently and easily in the operation phase.