Principle of Knapen system : A difference of 16 °C to 24 °C is required to displace the vitiation air from a building by means of a vertical air shaft.
The Knapen Tube : The method of fixing the Knapen tube in the wall. it is a simple porous tube which is fixed with its mouth inclined outwards. As the tubes are porous the moisture of penetrates inside them. Due to difference of temperature the dry air enters in the tube from its top portion as indicated by arrow. This dry air evaporates the moisture and saturated air comes out from the lower portion of the tube. This circulation of air is due to perfectly natural laws, and remains continuous till there is any moisture left in the wall.