Ventilation is the replacement of stale air in a building with fresh air. Ventilation is a vital requirement for the comfort and health of building occupants. The reasons why ventilation is so important can be summarised as;

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•  to supply oxygen for breathing (respiration)
•  to dilute pollutants such as body odours and exhaled carbon dioxide.
•  to remove unwanted heat from the building.
•  to supply air for combustion appliances.
•  to lower the relative humidity and so avoid condensation.
•  to clear smoke in the event of fire.

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​Ventilation of domestic buildings is mostly provided by infiltration. Infiltration is the movement of air in and out of the building via cracks and gaps in the building envelope. Air movement is driven by natural forces such as wind pressure and differences between inside and outside temperatures.

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Ventilation of small commercial buildings is carried out in the same way as that for domestic buildings. However as the building size and complexity increases the demand for mechanical ventilation becomes more necessary. A number of situations can be identified where mechanical ventilation is essential these are;

•  Densely populated rooms, where the space per occupant is less than 3.5m³ per person
•  Deep plan buildings. Natural cross ventilation is difficult to achieve through buildings deeper than 15m from outside wall to outside wall. Single sided natural ventilation is also ineffective beyond 6m from an outside wall.
•  Where windows cannot be opened for example due to external traffic noise or pollution.
•  Rooms where accurate control of temperature or humidity is required.
•  Rooms where pollution is created at a faster rate than can be cleared by natural   ventilation.
•  Where pressurisation of rooms is required e.g. in clean rooms to keep dust laden draughts out.
•  Tall buildings which are sealed because wind and stack driven natural ventilation would be excessive

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