For health reasons basic ventilation is needed in all buildings used by people. In summer extra ventilation is needed to reduce solar heat gains (summer ventilation)

Ventilation serves a number of purposes: Supply of fresh air, exhaust of pollution and heat regulation. Important is the control of the air quality. People in a room produce carbon dioxide and vapour. The level of carbon dioxide is an indicator for the freshness of the air. The production of carbon dioxide is accompanied by the production of human malodour. A high level of carbon dioxide is harmful to the health.

Humidity also is a reason for ventilation. In winter humidity produced by people or cooking etc. can result in condensation on windows or cold parts of the construction (cold bridges).

Furthermore ventilation also removes dangerous gasses, micro-organism and dust.