Implemented by the European EPB directives, the Energy Performance Certification has been designed by authorities to “provide clear information about the energy performance of a building”. Studies led in Wallonia show however that official results are often too distant from reality and difficult to understand for the lay person. This paper focuses on the comparison between two certified old urban brick single-family houses in the Walloon residential stock: a highly efficiently renovated “exemplary building”, described in detail, versus an inefficient and poorly described second house. After presentation of the case studies, this paper will investigate the uncertainties that surround the description of the “energy systems” and the standardized calculation method. This research explores the possibilities of elevating the confidence in the certification results by adding information on the description of the envelope and systems, by re-integrating the human factor in the equations (through a questionnaire collecting by interview the dwellers’ energy-related behaviours and practices), and by using dynamic simulations. The comparison of the results to the official standardised results indicates that additional data on the household’s composition, practices and energy-related behaviours are as much needed as a more accurate description of the energy system, which is penalized by default values and a severe standardisation of the calculation method.