The mixing of a tracer gas with zonal air was compared between two zones in an unoccupied test building in both the horizontal and vertical direction. A constant injection of sulphur hexafluoride (SF6) tracer gas was released into each zone separately and its concentration was measured at different positions within the zone. Variations in concentration were observed for different horizontal positions in the southern zone indicating incomplete mixing. The impact of incomplete mixing on the accuracy of subsequent infiltration measurements was determined and compared to the case where good mixing was achieved. Absence of direct solar radiation was identified as the predominant cause of incomplete tracer gas mixing observed in our experiments.