Adopting natural ventilation as a retrofit strategy for cooling, due to the low impact nature of the installation, is attractive due to the cooling potential of untreated outdoor air for large periods of the extended cooling season, particularly in northern climates. In line with this it is important to characterise the performance of natural ventilation components in low energy buildings 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 louver ventilation system installed in a low energy retrofit application and 13 tests from a pre-retrofit window opening, taken as a control space for comparative purposes. Results for 3 opening configurations for the slot louver ventilation system are compared. Parameters characterising momentum and buoyancy driving forces during each test were also recorded. A number of different permutations for combined wind and buoyancy effects were recorded allowing an investigation of the existence of any underlying patterns as well as the relative effect of the different opening configurations. Owing to the nature of single sided ventilation and the primary airflow exchange mechanisms normally present, the transient evolution of the normalised tracer gas concentration during tests is also discussed and compared. Analysis shows that different patterns emerge for the dominant driving forces depending on opening configuration in the slot louver system. The slot louver ventilation system has led to steadier ventilation rates. Opening height and geometry is 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 dependant. 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.