Displacement ventilation, chilled ceilings and mixed-mode air conditioning can save a considerable amount of energy and CO2, reducing the atmospheric pollution associated with fossil fuels and greenhouse gases. In these buildings there is usually the option for the occupants to use natural ventilation by opening windows. However, natural ventilation is the key element for ventilation in some buildings, and summer cooling and detailed strategies, including summer night ventilation, are employed. Where natural ventilation is the sole means of summer cooling, it is important to have good control and careful design to reduce the heat gains, especially from solar radiation. This is because natural ventilation is variable, due to the wind and insideoutside temperature difference, and it can only provide about 20 W m-2 of cooling. With a very high ventilation rate and an inside-outside temperature difference of 6 K, this can at best rise to 50 W m-2, although this is an optimistic value .