This study examined the impacts of detailed envelope airleakage distribution and internal partition walls airleakage on the ventilation performance of a detached low-energy house. We used a multizone modelling approach based on CO₂, humidity and formaldehyde to calculate indoor air quality (IAQ) performance indicators. Impacts of uneven envelope airleakage distribution on selected IAQ performance indicators are generally significant whatever ventilation system is present. Impacts of internal partition wall airleakage are considerably greater with exhaust-only ventilation but can be high on some IAQ performance indicators with balanced ventilation. We also conclude that it is relevant to use detailed data on envelope airleakage distribution and internal partition wall airleakage to precisely assess ventilation performance on IAQ. This work highlights the need of regulations or labels requiring performance-based approaches for ventilation at the design stage of buildings, calculating the relevant IAQ performance indicators. In such multizone modelling approaches, detailed airleakage distributions, on envelope and on internal partition walls, should be taken into account.