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In Switzerland, 70 % of building refurbishments are realised in stages. When only a window replacement is done, the new airtight windows can lead to a reduced infiltration air exchange and subsequently there may be moisture issues, e.g. mould. The integration of passive window ventilation openings (PWVO) with additional exhaust fans in the kitchen and bathroom(s) can ensure a user-independent basic air change rate. PWVO can be defined as small air inlets integrated in or near the window frame.