A thorough building refurbishment generally includes window replacement and thermal insulation of the opaque building envelope. However, quite often only a window replacement is done and the building envelope is left uninsulated. This could lead to mould problems, because of the reduced air change. A possible solution can be the integration of passive window ventilation openings (PWVO). This paper deals with two questions: Firstly, can PWVO guarantee the necessary ventilation rate to avoid moisture damage? Secondly, how do PWVO compare to a ventilation system with heat recovery in terms of heating demand?