Top down natural ventilation systems, usually referred to as ‘windcatchers’, have been used recently in modern non-domestic buildings in the UK. These systems combine inlet and outlet into a single roof mounted terminal, which is split into sections. Literature exists on theoretical, scale modelling and wind tunnel tests to evaluate the performance of the systems; however there is a scarcity of performance in-use tests. This paper presents the results of air exchange rate tests using the tracer gas decay method carried out in three operational buildings with windcatchers. It was found that air exchange rates are related to wind speed. However, for low wind speeds a correlation was found to internal external temperature difference indicating the effect of buoyancy forces. It was also identified that, during periods when wind and buoyancy pressures are of a similar magnitude, then the determination of the air exchange rate within the space becomes more complex.