Ventilation is the cornerstone of good indoor air quality. Ventilation requirements have major attention in building regulations, but ventilation in practice is often poor, resulting in increased concentration of pollutants and hence exposure to health risk. Inspection of 500 houses with interviews of occupants provide the data to link residential ventilation with regulations, performance and user behavior. The chain of activities from design through execution to use and maintenance of ventilation systems shows weak links. Noise of fans limits the use of higher set points for the required ventilation volumes. Bedrooms become polluted because air circulation is poor and the ventilation capacity is not sufficient. Ventilation services are poorly maintained and after a few years the capacity is reduced with 30-50%. When compensation for the effects of technical deterioration is not available, complaints about poor indoor air are more likely to occur. Recommendations are given for ventilation functions per individual room, for regulations in support of higher volumes in practice and for maintenance protocols. The study highlights the difference between theory and practice of residential ventilation.