Adopting natural ventilation as a low impact retrofit strategy for space cooling is attractive due to the cooling potential of untreated outdoor air for large periods of the extended cooling season, particularly in northern climates. Furthermore, it is important to characterise the performance of natural ventilation components in successfully transferring the cooling potential of outdoor air to the occupied zone. This paper presents an analysis of the results from 25 individual ventilation rate tests of a single sided slot louvre ventilation system installed in a low energy retrofit application and 13 tests from a pre-retrofit window opening, taken as a control space. Parameters permitting characterisation of different permutations for combined momentum and buoyancy driving forces during each test were also recorded, allowing an investigation of the existence of any underlying patterns as well as the relative effect of the different opening configurations. Analysis shows that different patterns emerge for the dominant driving forces depending on opening configuration in the slot louvre system. Owing to the primary airflow exchange mechanisms normally present, the transient evolution of the normalised tracer gas concentration during tests is analysed using the concentration fluctuation amplitude. The slot louvre ventilation system has led to steadier ventilation rates. Opening height and geometry are shown to have a significant effect on the net contribution from momentum driving forces and the fluctuation amplitude of the ventilation rate and this effect is wind direction dependent. Ventilation rates are shown to correlate well with fluctuation amplitude. The nature of the ventilation rate during tests for different wind directions is shown to vary depending on wind patterns at the building envelope.