Since about a decade the European policies are impelling researchers to find solutions to reduce the energy demand of buildings, for both new constructions and renovations. The first step towards energy consumption reduction is to carry out a reliable energy performance analysis of the building envelope. At present, the Italian professionals are required to assess the energy demands of buildings according to the quasi-steady state method specified in the Italian Technical Specification UNI/TS 11300-1:2008 that follows the UNI EN ISO 13790:2008. The paper aims to investigate the common professional practice in the assessment of the energy performance of an existing building. Thus, this study analyses a real building block situated in Turin (Italy) and develops a 3D parametric architectural model and an energy model by means of an Italian certified commercial software, commonly used for energy diagnosis and certificates. This paper highlights the importance of the interoperability between the 3D parametric model and the energy model in speeding up the acquisition of precise data and making the work of a professional more accurate and more cost-effective. In particular, the paper focuses on the impact of the projections, due to the geometry of the building, on the energy demands for heating and cooling