Rising damp is a combined action of several molecular phenomena. Although many of its aspects have been clarified and understood, the research is ongoing and it's far from being complete.

On this page we attempt to summarize the most important phenomena that contributes to the development of rising damp.

Water Transport Mechanisms in Masonry

Rising damp is a combination of vapour and liquid transport mechanisms due to the co-existence of liquid and vapour phases inside the capillaries.

The wetting process of a dry building material is a gradual process. It starts with vapor diffusion and adsorption, and leads through capillary condensation, capillary surface diffusion, then through a saturated flow to a water-saturated building material - as shown below:

Here is a brief explanation of these scientific terms:

Vapour diffusion 
(gas)​​

The passive movement of vapour molecules between areas of different concentrations until equilibrium is reached.
E.g. perfume or smoke spreading in a room

Adsorption
(gas)​​

The attachment of atoms (e.g. water) onto a solid surface, creating an extremely thin film, initially one then several molecules thick. (monolayer and multilayer adsorption).
E.g. misty window

Capillary condensation
(gas > liquid)

The occurrence of liquid water and wetting of solid surfaces, the filling of pore spaces with water, A change of phase occurs from gas to liquid.

Capillary flow
(liquid)​​

The flow of liquid water inside the capillaries.

How Rising Damp Starts

The wetting process of a dry wall, in the earliest phase, is triggered by temperature differences. The base of the walls is colder while upper areas are warmer, due to the cooling effect of the ground and the existence of heating. Because warm air is lighter and cool air is denser, the temperature differences translate to pressure differences - higher pressure at the cold base and lower pressure towards the top of the wall. These pressure differences create a pumping mechanism, pulling the humidity inside the initially dry capillaries, where the deposition of water can begin, initially in the form of a very thin, microscopic film.

Temperature or pressure differences move the water upwards inside the wall capillaries

Liquid Films and Salt Transport in Nanopores

Small, narrow capillaries can get saturated very easily, leading to the presence of wet films or the build-up of liquid moisture. 

Very narrow capillaries (with diameters less than 200 nm = 0.2 microns) in which liquid water flow is not possible due to the surface tension of water, still can develop liquid films in which salt transport can commence - this can occur in the absence of a liquid capillary flow.

Very narrow capillaries where water flow is technically not possible can develop liquid films

The theory of wetting and liquid films in extremely narrow (2 - 50 nm) capillaries has been studied since the mid 1950s.

Capillary Flow - Electrokinetic Phenomena

Once capillary flow has been established, a whole new host of phenomena enter into play and things get significantly more complex. These liquid phenomena are known in the scientific literature as electrokinetic phenomena.  

Electrokinetic phenomena (electro = electric, kinetic = motion) describe a number of small-scale molecular phenomena that occur at any solid - liquid interface (e.g. wall capillary - water) and has to do with the movement of water and / or particles suspended in water.

These include some known phenomena such as osmosis or electro-osmosis, but there are a lot more - the most important ones are summarized in the table below: 

Electrokinetic phenomena contributing to the presence of rising damp

Due to their complexity and their extremely small-scale nature and interaction, the full effect of these forces is not fully understood.

These forces play a significant role at the wall-water interface also known as the electrical double layer. The negative wall surface attracts the positive end of the water molecules and bonds them to the capillary surface by electrostatic attraction - opposite molecules attracting each other. 

The electrical double layer "bonds" the water to the surface of the capillaries through electrostatic forces

Small local charges (e.g. crystallized salts) as well as electrical and magnetic fields can also significantly affect these forces and thus the wall-water interface - especially alternating-current (AC) or pulsed signals, which are difficult to model and describe theoretically even with complex mathematical equations - hence their effect on the electrical double layer hasn't been studied extensively.

The Presence of Ground Salts

Liquid moisture transport brings up various diluted salts from the ground (chlorides, nitrates and sulphates) which over time are deposited into the building fabric. There can be high concentration of accumulated salts deposited in the upper area of the building fabric (e.g. 1 m high) and the high concentration of salts will turn on another lifting force: osmosis - water starts migrating towards high concentration salt areas, reinforcing the mechanism of rising damp.

The presence of salts can damage the wall fabric significantly through crystallization, and the effect of hygroscopic moisture onto the brickwork is also not negligible.

Summary

The brick-mortar-water-salts mix forms a complex electrical-physical-chemical system subject to many small-scale molecular forces.

This makes rising damp is a very complex phenomenon generated over time by a combination of vapour and liquid transport mechanisms, which include:

  • Vapour diffusion (gas)
  • Monolayer adsorption (gas)
  • Multilayer adsorption (gas)
  • Capillary condensation (gas-liquid)
  • Non-saturated capillary flow (liquid)
  • Saturated capillary flow (liquid)
  • Capillary osmosis (liquid-solid)
  • Electro-osmosis (liquid-solid)
  • Streaming potential (liquid-solid)
  • Diffusionphoresis (liquid solid)
  • Evaporation (liquid-gas)
  • etc.

Condensation (also known as capillary condensation or interstitial condensation) is just one interim mechanism of the many that all together contribute to the development of rising damp.

Rising damp is NOT condensation as some superficial or uneducated observers claim, but a lot more, hence it can't be fully solved by simple ventilation.

References

Here are some of the research papers used for the compilation of the above information: