Air exchange rates in a naturally ventilated apartment: a multi-box modelling approach using occupant-generated CO2 decay measurements

Indoor air quality significantly impacts on human health, particularly in residential spaces where natural ventilation is the primary mode of air exchange. This study investigates natural ventilation rates in a modern apartment using the occupant-generated carbon dioxide (CO2) decay method under different ventilation scenarios, including window-only, door-only, and combined window-and-door openings. To deepen the understanding of ventilation dynamics, we developed and evaluated an Indoor Multi-Box (IMB) model that simulates CO2 variations across interconnected rooms.Our results demonstrate that Natural Ventilation Rates (NVR) are most strongly influenced by indoor-outdoor temperature differences, followed by wind speed and wind direction. Specifically, NVR increases with significant temperature differences, high wind speeds, and air flows in windward direction, particularly in the window-only ventilation scenarios. When ventilation occurs only through internal doors, the NVR is governed by the CO2 gradients between two adjacent indoor spaces, with greater initial differences leading to higher NVR. However, internal air exchanges through doors are generally much less effective than the exchange with outdoors, due to the limited driving forces and lower capacity for air renewal. The IMB model accurately captures the temporal variations in CO2 mixing ratios, showing close agreement between experimental observations and simulations. The model highlights the critical role of windward and leeward wind directions on the ventilation efficiency, with windward conditions facilitating effective air exchange.This research provides a robust framework for assessing ventilation strategies in naturally ventilated indoor spaces, offering key insights into options to optimise indoor air quality through targeted ventilation practice.