In this paper the energy impact of natural cross ventilation is examined conducting a set of crossventilation experiments in a well insulated apartment of a 5-storey building. The experimental resultscompared with simulation results derived from the combined use of the multizone air flow modelCOMIS and the thermal model Suncode.A 24-hour lasting natural cross ventilation experiment was conducted, to monitor thermal comfortventilation mainly during the day and night time cooling ventilation. A short lasting cross ventilationexperiment was also conducted to monitor the impact of ventilation in midday hours. Both experimentscompared with a 24-hour lasting single-sided ventilation experiment. Surface and air tempelature,heat flux in building elements, ambient wind and solar conditions were monitored.The ventilation and infiltration phenomena in the above experiments were modelled in detail using30min / 1h time-step, taking into account landscape, wind environment, the building's shape as wellas the leakage characteristics of the building's envelope.