Let's consider an old wall, such as the one below. The systematic study of rising damp involves the study of 4 dynamically interacting areas (micro environments):
- The soil
- The wall fabric
- The finish
- The environment (air)
Water moves from 1 to 4, originating from the ground, moving through the w.all fabric and finish, then finally evaporating into the air.
Let’s look at these 4 areas in more detail.
1. The Soil
The soil is one of the primary moisture sources as all walls are resting on the soil. The type of soil (clayey or sandy) affects he drainage of water hence the amount of water readily available to the walls to be absorbed.
2. The Wall Fabric
Various properties of the wall fabric affect how it interacts with water. The main variables are:
- Presence, absence or condition of a DPC: a damp proof course (DPC) is a horizontal barrier designed to keep water away, by preventing the migration of water from the damp soil into the porous building fabric. In lack of a workable DPC, over time, water starts moving into the porous building fabric.
- Material properties: most walls are made of more than one building material. Even a simple wall construction contains at least bricks and mortar. The individual properties of these materials, such as hardness, pore size, water permeability etc. affect how the wall fabric interacts with water.
- Pore structure (voids): every building material is porous to some extent and contains a certain percentage of voids (air). About 25% of a dry brick is air. Sand stones can have 40% or more porosity. The porosity of the wall material (size of pores, its open- or closed nature etc.) determines to a large extent how humidity can enter or evaporate from the wall fabric.
- "Added" or deposited materials (salts): water from the soil contains organic materials which are carried upwards by the water flow into the building fabric. These are known collectively as salts or ground salts and they play a VERY important role in the movement of water. Due to their importance, this topic is covered in detail in separate chapters.
3. The Finish
Many walls are finished with a plaster or render. The finishing serves not only a decorative purpose but a functional one too. A good quality render is not only nice, but it also protects the building fabric from the adverse weather. Thus the physical and chemical properties of the plasters, renders, paints or wallpapers comprising the finish play an important role in keeping the wall fabric dry. If done incorrectly, they can significantly contribute to the decay of the underlying building fabric.
4. The Environment
The temperature and humidity of the surrounding air, the ventilation or prevailing winds can either assist or prevent the evaporation of moisture from the wall fabric.
Quite interestingly, according to latest research, electrical or magnetic fields also seem to affect the movement of water in porous building materials. This is a brand new line of research with phenomena we are just starting to discover and understand.