Ventilation avoids health problems
Poor air inside can cause health problems such as coughing, headaches, colds, respiratoty problems, etc.

Ventilation avoids humidity problems inside houses
Those arise from :
- The resident (breathing, transpiration, cooking, showering, ...)
- The construction (building materials contain contaminated materials such as Radon and formaldehyde).

Humidity causes fungus and dust mites.

Ventilation helps save energy costs
Most houses are thoroughly insulated these days, which keeps humidity and harmful substances inside.
Humid air is more difficult to heat than dry air.
A well-ventilated house (thus containing dry air) can be heated a lot more efficiently.

Perfect ventilation means ventilation 24 hours a day.
Opening the window for a while is not sufficient. Research has proven that, after the window has been closed for half an hour, the fresh air has disappeared.