The typical solution for ventilation of the existing apartment houses – with natural exhaust channels from the toilet facilities and kitchen premises, and air inflow through gaps between the window frame and window carcass – does not ensure return of heat from the exhaust air and cannot ensure a maintenance of room microclimate that would be independent of weather conditions and would correspond to the human comfort requirements. Previous experience shows that renovation of ventilation system is not a typical component of renovation of multi-apartment buildings in Latvia. As a consequence, rooms are not ventilated enough and indoor air quality is reduced. This, in turn, leads to progression of the so called sick building syndrome. Indoor air quality measures in 13 unrenovated multi-apartment buildings shows that CO2 concentration in 19 of the examined 30 apartments was ≥1000 ppm, indicating insufficient ventilation. The study was carried out by Riga City Council in the frame of the project "Energy Efficient and Integrated Urban Development Action (Urb.Energy)" (Baltic Sea Region Programme 2007-2013), activity “Development of an Internet portal for on-line monitoring of indoor climate and consumption parameters of renovated and not renovated buildings in Latvia”. Results of indoor air quality measurements confirmed ventilation problems and indicated a reduced level of comfort in the apartments. Indoor air quality in the apartments is mostly dependent on habits of the inhabitants and technical possibilities.