The aim to pull the energy demand of private apartments down, forces the reduction of ventilation losses and, therefore, there is a need for high leak tightness in buildings. New buildings have no problem to fulfill the recommended standard for leak tightness, but this leads to buildings’ hygienic and physical problems (air change rate). Because of increasing energy prices, many consumers try to save energy costs by reducing natural ventilation. However, the minimum ventilation rate cannot be often achieved. A solution to this problem must consider aspects of ventilation, energy and economic efficiency. The window ventilation in new and renovated buildings has limits in application. The increasing mold growth, particularly in renovated residential buildings isn’t only encouraged by insufficient ventilation discipline; but also often the main cause of it. In particular, in renovated flat construction, where the users feel little responsibility for the maintenance of building structure, the conditions of fungus growth are favorable. The trend to decrease energy requirement in house building will continue. These modern building types are not imaginable without machine ventilation systems with heat recovery. But not only the energetic aspect and the building protection affect owner’s decision for a machine ventilation system. The higher comfort through no-draught ventilation and the possibility of efficient filtering the outside air lead in particular allergy troubled persons to the decision for machine ventilation system with filter and heat recovery. Requirements on such ventilation systems under boundary conditions of insulated and air-tight residential buildings are pointed out.