The diffusion of the energy certification in Italy is gradually making the users aware of the buildings' energy performance, at least in some regions. For the existing buildings, the challenge is not only the energy performance calculation, but the analysis of the refurbishment strategies to obtain low energy buildings towards the A class. Basing on European Standards EN ISO 13790 and EN 15316, the Technical Standards UNI/TS 11300 (part 1 and 2) represent the procedures defined at national level. They allow different approaches and levels of simplification in the calculation of various representative energy parameters that could bring to variations in the results. The first aim of the work is the analysis of the most suitable solutions for existing buildings and the real possibilities to move towards the A-class requirements. Another considered aspect is the application of different calculation methods to reach this target, showing how different choices in terms of input data and methodologies (allowed by the Technical Standards) can bring to different values of the energy performance indices. This could represent a problem for the reproducibility of the calculations and their reliability in the definition of the class level reachable through refurbishment. The research has been supported by the analysis of some representative cases of common national residential building stock, comparing the energy performance obtained with different calculation methods allowed by National Technical Standards and laws. The results show some differences which can lead to ambiguities about the class definition and the most suitable methodology to adopt. Some considerations to obtain reliable values of energy performance addressing the refurbishment towards the A class are presented. (c) 2012 Elsevier Ltd. All rights reserved.