The energy consumption of buildings depends significantly on the criteria used for the indoor environment, which also affect health, productivity and comfort of the occupants. The indoor environment is mentioned several times in the EPBD. There is therefore a need to specify criteria for the indoor environment for design, energy calculations, performance evaluation and display of operation conditions. The paper describes how design criteria of the indoor environment are set for dimensioning of systems and for energy calculations in the draft standard prEN 15251. The paper highlights some of the new principles in the standard such as several categories of indoor environment, the difference between target values for dimensioning and energy calculations, principles of defining the ventilation rates, and evaluation of the indoor environment. Finally, the paper discusses some of the issues raised during the public review of the draft standard.