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. From the experimental measurement indoor air quality is calculated intensity of ventilation – infiltration. These resultant intensity of ventilation are reciprocally compared. The aim of the article is to find, on the basis of the experimental measurements, the difference in volume flow of air in the room by infiltration, which caused by the replacement of the old wooden window with a new plastic window with insulating double glazing. According to the analysis, it can be stated that by replacing the old wooden window in the room with a new plastic window, the volume of air flow caused by the infiltration decreased by approximately 73 %. The total window heat loss was reduced by
approximately 46%.

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