When left untreated, damp can lead to deterioration of the building structure. Materials decay and mould can develop, which is a risk to health.
Materials either absorb moisture from the atmosphere, or emit water into the atmosphere. This is the process by which they find equilibrium with the relative humidity of air. When humidity changes, the moisture content of materials will change, and the moisture content of different materials in a given relative humidity will not be the same.
Moisture content of materials is often expressed as a percentage scale called ‘wood moisture equivalent’ or WME. This is used as the benchmark because between different types of wood, there is not as great a difference in moisture content as compared to other building materials.
The % moisture content of materials is an expression of the weight of water as a proportion of the dry weight of the material. Therefore a heavy material such as brick or concrete has a lower % moisture content than a light material with the same amount of moisture contained within.
Under normal, dry conditions, different materials within the same structure will therefore have different moisture content. For example, equilibrium moisture levels in a brick wall with wood battens and plasterboard on one side, and plaster on the other, may show moisture content of around 10% in the wood, 8% in the plasterboard, 3% in the bricks and less in the plaster. These are different levels, but the materials are in equilibrium with the humidity of the local environment. Under damp conditions, all these materials may see an increase in their moisture levels, but they will remain very different levels and they will again find a new equilibrium with the relative humidity of the environment.
The definition of dampness in buildings could be considered to be when a material is wetter than ‘air-dry’, where ‘air-dry’ means in equilibrium with a ‘normal’ relative humidity in the atmosphere (30-70% RH).
Dampness is often most visible as a result of mould developing, which decays wood and spoils decorations. These biological organisms have a dryness limit below which they can not multiply and live. This limit is the line between dry and damp in buildings, and although not precise, can be considered to be between 75-85% relative humidity. Above this level, mould, mites and fungi can develop quickly.
Damp is therefore an atmosphere more moist than 85% RH and a material is damp if in equilibrium with this humidity.
Dampness in materials is not always caused by moisture in the atmosphere. For example, while air is dry, walls can still be wet. This may be from penetrating or rising damp. In this case, if the wall is damp (wetter than air-dry) the thin layer of air immediately next to its surface will be in equilibrium with the wall, regardless of the general humidity level of the room. Mould can therefore grow here, however if the wall dries, and room air remains dry, the mould will die.
How do we measure dampness in buildings?
Measuring humidity emanating from a wall is difficult in practice, so often electrical moisture meters can be used to measure the free water in a material. This gives an indication of the relative dampness of different materials as they are measuring only the ‘free’ water. A high reading would indicate damp in whichever material is being assessed. These meters give a reading of ‘dry’, ‘at risk’ and ‘damp’ expressed often as a percentage reading which corresponds with the humidity equilibrium of most non-metallic materials.