Quantifying the performance of natural ventilation

Increasing attention is being paid to natural solutions to building design, e.g. mixed-mode (or hybrid) ventilation, or increased use of daylight. However, the issue of quantifying performance is less clear: is an hour of no ventilation or 5 minutes of acute glare every so often acceptable? Likewise, relying on average values can also create misleading pictures of performance: the annual average irradiance could be acceptable but there is likely to be insufficient daylight for large parts of the winter; similarly an acceptable annual average air change rate may not reveal a ventilation deficit for significant periods of the year. For these cases performance should be time and magnitude based, the  question is how? This paper focuses on the issue of natural ventilation, which is more complicated than daylighting due to the capacitive properties of a volume of air. Given that pollutants build up in air over time and can be quickly flushed from a space the concentration at a particular instant or air supply rate at a particular instant does not necessarily give an indication of acceptability. This paper explores some statistics that could be used to assess natural ventilation. A critique is given and advice provided as to the suitability of the different measures to answer design questions.