The acceptance and appreciation of ventilation systems is mainly determined by the perceived indoor air quality, thermal comfort and noise. Noise in relation to ventilation systems can be divided into three categories: (I) outdoor noise (entering the dwelling through ventilation openings, cracks, mechanical supply and exhaust openings, etc.); (2) noise generated by components of the ventilation system and; (3) the impact of ventilation systems on sound reduction of partitions (between dwellings, rooms, etc.). Depending on the type of ventilation system, one or more of these aspects are of concern. Noise related to the ventilation system and components, can result in turning off the ventilation system or closing vents, etc. This can have a negative influence on ventilation and indoor air quality. In the framework of IEA ANNEX 27, several noise aspects of domestic ventilation systems have been evaluated. Outdoor noise: In noise loaded areas. the selection and the applicability of different types of ventilation systems are determined by the noise level on the facades. A simplified tool is developed to select ventilation systems as a function of the required noise reduction of the facade, room dimensions and design and construction of the facade. System noise: Controlling noise levels caused by ventilation systems is in practice one of the most important factors to contribute to the satisfaction with a ventilation system. Air duct systems in dwellings transport noise generated by fans and aerodynamic noise generated by bends, control valves, grilles, etc. ANNEX 27 gives basic equations to estimate sound power levels of fans and grilles, indication of sizes for silencers and guidelines for design. Impact on noise reduction of partitions: The composite sound reduction is the result of different sound channels from one room to the other. One of this sound channels may be the ventilation system (cross-talk). Cross-talk can be brought about through the air duct system, overflow grilles and ventilation openings in partitions, duct transitions, etc, ANNEX 27 provides guidelines for sound reductions of partitions and insertion losses for ducts to eliminate the influence of cross-talk.