Replacement of old wooden windows in a new plastic windows, in the old buildings, we achieve the massive reducing heat loss of the building. New windows are characterized by better tightness. The question is, how much more are reduce the uncontrolled ventilation. In the article is presented the experimental measurement indoor air quality in the room in two phases. In the first phase is in the room installed 55 year old wood window. In the second phase is in the same room installed new plastic window.