Thermal comfort improvement at the lowest energy consumption is a key issue when dealing with sustainability in buildings. An appropriate passive design is mandatory under those circumstances. Prior to construction, simulation tools help to make designs more sustainable. However, it is recognized a gap between real performance and the predicted one. This article presents the comfort methodology applied in an office building located in the north of Spain, characterized by a continental Mediterranean climate. The edifice is constructed under the principles of the sustainability optimizing its energy performance. The thermal balance between indoors and outdoors and user behaviour shows the proximity of each office to the comfort bands. Warmer or colder set point temperatures correspond to higher energy demands. The application of the Fanger methodology produces high percentages of neutral sensation and low values of people dissatisfied, especially during the wintertime. In this period, warmer sensations increase the percentages of people dissatisfied.