The paper provides the results of monitoring on the specifics of indoor air quality in Latvian dwellingbuildings. It will present the results of the evaluation of the ventilation systems operation in differentworking regimes in Latvian climatic conditions.Until the end of 90-ties all dwelling buildings in Latvia were equipped with mandatory naturalventilation systems with stack effect. Installation of regular mechanical 'inflow - outflow' ventilationsystems in dwelling buildings was limited by such factors as high noise level during operation time,limited buildings area and high construction expenses. At the beginning of 2007 the part of Latvianmulti-storey apartment buildings which were constructed with natural ventilation systems took 75% ofall buildings. The inflow of fresh air acquired through the gaps between window frame and windowcarcass. The exhaust of air acquired through the vents, which are situated in kitchens and bathrooms.Old multi-storey dwelling buildings in Latvia were mainly built using wooden window frames with lowheat insulation quality. It was assumed that such constructive decision would provide sufficient airexchange rate without any mechanical devices.Nowadays the PVC windows have got a great expansion at Latvian market due to their acceptableprices and high heat and noise insulation qualities. At the same time air tight windows require theintroduction of mechanical ventilation systems that certainly reduces the positive energy saving effect of improvements of thermal performance of building structures.In the scope of this paper the influence of ventilation on building energy performance was evaluated.