Doing the replacement of old wooden windows in a new plastic windows, in the old buildings, we get the great reducing of the building heat loss. Simpler maintenance and attendance of window is the next advantage. New windows are characterized by better tightness. The aim of the article is determination due to the performed experimental measurements, how much more are reduce the uncontrolled ventilation that is caused of the infiltration windows. In the article there is presented the experimental measurement of indoor air quality in the room in two phases. In the first phase there is the room installed by 55 year old wood window. In the second phase there is the same room installed by new plastic window. Due to the experimental measurement of indoor air quality it is calculated intensity of ventilation - infiltration. These results of ventilation intensity are reciprocally compared.
 Bullová, I. (2011). Problems when replacing windows in residential buildings. In: Správca bytových domov. ISSN 1336-7919. Vol. 6, No 5, pp. 58-62. (in Slovak)
 Wyon, D. P. (2004). The effects of indoor air quality on performance and productivity. Indoor Air, ISSN 1600-0668. Vol. 14, Issue 7. pp. 92-101
 Budaiova, Z., Vilcekova, S. (2015). Assessing the effect of indoor environmental quality on productivity at office work. SSP - journal of Civil engineering Vol. 10, Issue 1. 2015
 Koskela, H., Maula, H., Haapakangas, A., Moberg, V., Hongisto, V. (2014). Effect of low ventilation rate on office work performance and perception of air quality - a laboratory study. In: Proceedings of Indoor Air 2014, Hong-Kong, Vol. 2, pp. 673-675;
 Mahyuddin, N., Awbi, H. B. (2012) A Review of CO2 Measurement Procedures in Ventilation Research. International Journal of Ventilation: March 2012, Vol. 10, No. 4, pp. 353-370.
 Budaiova, Z., Vilcekova, S., Meciarova, L. (2015). Occupants´ performance in office buildings and indoor environmental quality. In: Healthy Buildings Europe 2015. Eindhoven, Eindhoven University of Technology, pp. 1-8.
 Pilipova, I., Vilcekova, S. (2013) Perceived comfort of indoor environment and users’ performance in office building with smart elements - case study. In: Selected Scientific Papers - Journal of Civil Engineering. Vol. 8, no. 2 (2013), pp. 61-72.
 Persily, A. 1997. Evaluating building IAQ and ventilation with indoor carbon dioxide, ASHRAE Transactions, 103: 193-204.
 Persily, A. 2005. What we think we know about ventilation? in 10th International Conference on Indoor Air Quality and Climate “Indoor Air 2005”, 4-9 September 2005, Beijing, China, 2: 24-39.