MONICAIR - MONItoring & Control of Air quality in Individual Rooms - is a pre-competitive field research project of a broad consortium of Dutch ventilation unit manufacturers and research institutes, supported by the Dutch government. The first aim of the project is to investigate and compare the indoor air quality (IAQ) performance and energy characteristics during the heating season of ten different mechanical ventilation solutions in dwellings that meet strict air-tightness standards and comply with current building regulations. The second goal is to further improve the ventilation systems on their IAQ-performance while minimizing their energy consumption. Over a full year 62 residential dwellings were monitored with, in each habitable room, sensors for occupancy, CO2, relative humidity and air temperature. Power consumption of the mechanical ventilation units was also continuously monitored. The ventilation solutions included traditional mechanical exhaust ventilation or MEV systems (systems with mechanical exhaust in the wet rooms), state-of-the-art MEV systems (systems with mechanical exhaust in all rooms), as well as local and central balanced systems with heat recovery (MVHR-systems), with various types of controls. Although all systems under investigation comply with building codes, the data show that huge differences occur in both the IAQ and energy performance. CO2 excess dose (above 1200 ppm) may vary per dwelling from 10 to 850 kppmh per person per heating season, resulting in a situation in which respectively 1 to approximately 85% of the time spent at home, the ventilation is not sufficient in the room that one occupies. Real life primary energy consumption varies from 23 MJ/m2 for centralized ventilation systems with heat recovery to 144 MJ/m2 for traditional MEV-systems.