Opening the Windows

Although this seems simple, it is actually more complex than one might think. During calm weather conditions outdoors, a situation that often occurs at night, an open window is not sufficient to ensure the necessary air exchange.

Venting by opening windows wide, often recommended to save energy, has proven in our experience not to be effective enough to achieve the required air circulation. A permanently open window is necessary, as a single window in a room is usually not enough to ensure adequate circulation. Especially CO2 is not removed from the room in sufficient quantities in this way.

Particularly when sleeping, it is important in our experience not to exceed a CO2 level of 650 - 700 ppm. Additionally, a light breeze should always be flowing past the head to distribute and transport away exhaled CO2, so it does not accumulate in the head area.



  • No acquisition costs


  • Poor energy balance, especially with windows permanently open.
  • Low air exchange, especially in calm weather.
  • No reduction of fine dust in the room air.
  • Difficult to control, as it depends on external weather conditions.

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