How effective is natural ventilation? A study of local mean age of air by modelling and measurement.

A condition often assumed when designing a naturally ventilated building is where air enters at low level and leaves at high level due to the stack effect. It then follows that, at upper levels, the air may be relatively 'stale' since it has previously passed through the lower storeys. An analogous situation may arise when wind is blowing, in which the air entering through the windward face becomes stale as it passes through the building to the downwind sections. It is not well understood how ventilation may, in reality, be affected by this. To address these issues, this report describes a modelling approach using BREEZE and complementary measuremnents using the Passive Tracer Gas technique to study local ventilation rates in multiroomed office buildings. Calculations show that simple ventilation flow rates, as conventionally calculated at the design stage, cannot be relied upon to indicate the true 'freshness'. Measurements show that effective ventilation can be less than current minimum occupancy requirements.