By doing a simple search on the internet on rising damp, chances are that you will run into some controversy on the subject. Although the phenomenon of capillarity has been observed and documented a long time ago even by Leonardo da Vinci, and it is taught in schools as part of the curriculum, yet some professionals of the UK building industry question whether rising damp, as a phenomenon, exists.
Yet many old buildings exhibit certain symptoms typical to what's commonly known as rising damp, and typically include...
...with more real-world examples shown below:
Rising Damp - The Simplistic Explanation
In a simplistic way, rising damp is explained with capillary action: water is drawn up through the wall pores by capillary forces. Capillary action is often demonstrated with thin glass capillary tubes in which water rises to different heights – the thinner the tube the higher the rise.
This observation is then extended to the walls - which after all, can be regarded as a bunch of tortuous capillary tubes - leading to the conclusion that water in the walls must rise for the same simple reason: capillary action.
Despite of some of the apparent similarities between glass tubes and the wall fabric, there are fundamental differences which makes wall capillaries behave very differently, making the “capillary rise” reasoning incorrect, for several reasons:
- Porosity: building materials are porous, glass is not. Building materials have pore sizes ranging from a few nano meters (one billionth of a meter = 0.000000001 m) to hundreds of microns. This, in comparison to the tiny size of the water vapour molecules (typically 0.3 nm), makes building materials look like swiss cheese; they are full with “holes” through which water vapour can move in and out. Evaporation thus has a very important role in building physics, which does not occur through glass. Glass tubes are solid, non-breathable building materials which instantly pull up water to their maximum height and the effects of evaporation are negligible.
- Water content: while the capillary tubes experiment assumes liquid water at the base of capillaries, houses rarely stand or are built on liquid water; they are typically built on solid foundations, or – in case of very old buildings – on soil.
The Bigger Picture
Research has shown that the rise of moisture in old buildings occurs due to the COMBINED EFFECT of several physical and molecular phenomena rather than just capillary action alone.
This has been extensively researched by various branches of physics and chemistry and can be summarized under the heading of “water transport mechanisms in porous media”.
Despite of all findings, the research is far from being complete. A lot of phenomena into the molecular properties of water are still not fully understood and they are being actively researched. After all, we are dealing with complex nano-scale molecular interactions between water (H2O), the various building materials, and other complex chemicals including ground salts.
The Technical Stuff
The technicalities behind the phenomenon of rising damp are complex as we are dealing with molecular phenomena. These are covered in detail in a separate article here.
This inevitably leads to the question: does rising damp exist? The answer depends on how one defines rising damp.
If – in a simplistic way – rising damp is defined as “the rise of water in masonry due to capillary action alone” then it's no, it doesn’t exist. There is no such thing in nature as “pure” rising damp due to capillary action ALONE. Water rises in porous media due to a COMBINATION of several molecular transport mechanisms; these co-exist, all occur in parallel and can’t be separated. Our technical article describes this.
If, on the other hand, rising damp is defined as “the upward movement of water due to a combination of factors”, then the answer is yes. Doing just a quick overview of the research literature (studies, white papers etc.) the scientific evidence is overwhelming: water CAN rise in building materials and old buildings. The destructive effects of ground salts alone is an area of great interest to researchers as salts account for significant damage to the fabric of old buildings.
In extreme cases, such as Venice, where buildings have been built on seawater, rising damp cases of several meters high have been documented.