Knapen draining siphon technique, named after its inventor, also known as "atmospheric siphon" system, is based on the principle that wet walls may dry out by increasing their inside air circulation.

Knapen siphons: what they are

These artefacts are originally made of porous terracotta and consist of hollow cylindrical or prismatic elements, between 10 and 50 cm long, with a blind internal longitudinal 3 cm diameter hole. Over time they have been produced in various shapes (triangular section, hexagonal, etc.) and with different materials, such as plastic or stainless steel.

Draining siphons: how they work

According to Knapen, drier and lighter outside air enters the siphon, replacing heavier internal humid air, and expelling the latter from the masonry. This air exchange inside the siphon, should increase evaporation inside the wall.

Siphons: how to install them in the walls

After drilling 3-4 holes per linear meter, by hand or mechanical drill, in a single or overlapping rows in a staggered position, about 15 cm from the floor and from each row, you shall insert the siphons inside the damp masonry with slope towards outside. Siphons must be fixed inside the holes with a very porous cement-free mortar, to avoid occluding porosity of the siphon external walls, thus preventing the humidity flow towards central channel of the siphon itself. The outer side is protected by a grid preventing animals, insects and parasites intrusion.

Pros of Knapen siphons

  • fast
  • economic
  • they require no professional installers and can be applied by the "do it yourself"

Cons of Knapen siphons

  1. it has not always had beneficial effects on the walls
  2. tests and trials have shown the atmospheric siphons have no practical use


The cost is around € 170/200 per linear meter, labor excluded (unless you opt for the DIY solution)