Building rating devices which are currently in use to perform the new „Directive on the Energy Performance of Buildings EPBD” only calculate the energy demand, but not the achieved thermal comfort. This study shows that a comprehensive method is necessary to determine the true energy efficiency. The BEEP method (Building Energy and Environmental Performance Tool) is able to do this while it relates the energy demand with the achieved thermal comfort. A comparison of different cases in modernization an existing office building is carried out with the mentioned different approaches and will show that looking at the energy demand only may lead to wrong decisions. Modernization with emphasis on high thermal insulation achieves a good value in terms of energy demand und would be judged by the “Energypass” of the “Energieausweis-Vorlage-Gesetz which implements the EPBD in Austria as an excellent solution. However, thermal room conditions can’t be predicted. Only when using BEEP, it is visible that 10% of the occupants feel uncomfortable during 35% of occupancy, which is even more as before modernization. BEEP also allows a direct comparison to another variant has renewed windows and utilizes night-time ventilation. This variant when weighted in the achieved thermal room comfort to the expended energy is more energy-efficient and shows only about 11% uncomfortable hours during occupancy.