If you suspect your house has rising dam or if you have some research on this topic, chances are you might have run into some controversy on the subject. Although there is a lot of information on this subject, indicating that it's a valid phenomenon, some others claim the opposite: that it doesn’t exist, or "true rising damp" is very rare - making you wonder whom to believe?
This writing intends to systematically address this topic and bring some much needed clarity into an over-complicated and seemingly confusing subject.
Lacking a clear, simple, workable definition creates confusion and unnecessary complexity. The definition of rising damp is simple: Rising damp is the upward movement of water inside the building fabric.
2. Four Main Areas of Study
The understanding of rising damp involves the study of four dynamically interacting areas (micro environments), which are:
- The soil
- The wall fabric
- The plastering / 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 sections in more detail:
1. THE SOIL: the soil is one of the primary moisture sources as all walls rest 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 absorb.
2. THE WALL FABRIC: various properties of the wall fabric affect how it interacts with water. The main variables are:
- DPC condition, presence or absence: a damp proof course (DPC) is a horizontal barrier designed to prevent 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.
- Solid 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.
- Voids, pore structure: every building material is porous to some extent and contains a certain percentage of voids (air). About 25% of a dry brick is air. Sandstone 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 by plastering or rendering. 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 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 gradual or severe 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 have a significant impact on 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.
3. The Water & Salts Cycle
There are 2 major cycles occurring inside old walls:
- The water cycle and
- The salts cycle
The Water Cycle
The water cycle follows the following path: Soil > Wall fabric > Finishing > Air.
Water comes and goes, it evaporates. If the wall fabric and finish is breathable, most of the water can easily evaporate, resulting in a relatively steady moisture content of the walls.
The moisture content of walls at any time depends on the ratio of water intake vs evaporation. If evaporation is blocked (e.g. by using non-breathable materials), dampness gradually accumulates inside the building fabric, making the walls damper and damper over time.
Conversely, if the evaporation rate is higher than the water intake (e.g. by using breathable materials such as lime), the walls tend to stay relatively dry.
The Salts Cycle
The salts cycle follows the following path: Soil > Wall fabric > Finishing.
Along with the water, diluted organic salts are also transported into the building fabric. Unlike water which evaporates through the pores, salts stay and accumulate over time inside the building fabric.
Even in professional circles the true effect of salts and the damages they can create onto the building fabric is to a large extent unknown or severely underestimated.