This paper examines the impact on domestic background air infiltration of replacing ‘old windows’ with modern double-glazed and draught sealed windows, both with and without controllable ventilation (e.g. trickle ventilators). Methods of estimating the change in infiltration rate produced by such a window replacement are reviewed. A simple model has been developed which, using laboratory measurements of window air permeability, predicts the reduction in infiltration that can be expected when a given number of windows are replaced in a dwelling. The validity of the model has been tested using data from a house both before and after replacing the windows. The paper investigates the impact that replacing windows in the UK domestic building stock (partly stimulated by Part L of the Building Regulations in England and Wales) is likely to have on the adequate provision of ventilation in the domestic stock. The paper concludes that replacing old windows in a significant proportion of UK dwellings can reduce ventilation levels below recommended levels unless controllable background ventilation is installed at the same time as new windows.