The Siphon

The damp siphon was supposed to cure dampness in a masonry wall. It’s a method proposed by Professor Achille Knapen, as a result of his research and experiments. In Paris, you can see the siphons on the external joints of significant historic buildings (see photos). The idea was introduced into the UK around 1919 and marketed by British Knapen Ltd.

Theory

Introducing a small hole in the masonry creates a slight change in pressure. Moisture migrates to the surface of the hole and evaporates from the surface pores of a clay tube. The tube is installed at a slope toward the outside of the building and secured in place with a porous dry-pack mortar mix. Additionally, a metal grill is fitted. This allows the damp air to evaporate from the masonry, thereby drying the wall.

Conclusion

A Knapen Siphon would have had an initial effect, but would then stop. This is due to the pour structure getting blocked by salts, which are carried in the water to the evaporation point of the tube. These salts solidify due to evaporation. This method is unsuitable for drying a wall because it does not control the source of water (e.g., rising damp) entering the wall. Nor does it control the major effect of migrating salts.