Currently, the built environment accounts for about 40% of the total energy consumed in the European Union and this level is continuing to increase. This high level of energy consumption is partially a result of attempts to create an optimum indoor environment for the occupants of buildings. New buildings and renovation projects on existing buildings need to improve their energy performance while maintaining a good and healthy indoor environment. One way of saving energy is to build passive houses. This paper presents the results of a case study of some of the first certified passive houses in Denmark, called Comfort Houses. The indoor environment is evaluated using both quantitative measurements of these houses and qualitative interviews with the occupants about their experiences of the indoor environment. The combination of these two sets of knowledge leads to a more complete and holistic picture of the indoor environment. It is shown that attention needs to be paid to the objectives in the design process, the documentation of the designs and on the occupant’s lifestyle and behaviour. Finally, it is important to educate the occupants on how to live in an appropriate manner to achieve a comfortable indoor environment.