Rising damp is widely misdiagnosed in existing buildings, based on the incorrect interpretation of visual evidence and the readings of moisture meters. Because of a highly successful sales campaign for over 30 years by specialist remedial contractors installing injected 'chemical dampproof courses', this misdiagnosis of rising damp has also become synonymous with a diagnosis of a lack of an 'injected chemical damp-proof course'. Although this has been very good for business, it has often resulted in a waste of the clients' money and resources; original plasters and finishes have been destroyed in the process of installation, and unnecessary damage has been caused to original structures by the drilling of irrigation holes. In addition, money that might have been spent on more cost-effective maintenance or repair works has been wasted.
Whilst injected chemical damp-proof courses may provide some protection for certain types of structure if properly specified, their general application is rarely the most cost-effective way of controlling damp problems in buildings, and may be wrongly specified and ineffective. In particular the more generally available water based products may only form an effective ‘hydrophobic band’ if applied to a dry wall after it has dried out. This can prevent their effective installation in damp walls.
CAUSE AND EFFECT
Rising damp actually describes the movement of moisture upward through permeable building materials by capillary action. It becomes a problem if the moisture penetrates vulnerable materials or finishes, particularly in the occupied parts of a building. This moisture will dissolve soluble salts from the building materials such as calcium sulphate, and may also carry soluble salts from its source. If the moisture evaporates through a permeable surface, these salts will be left behind and form deposits on or within the evaporative surface. Where there is a large evaporative surface, salt crystals are deposited as a harmless flour-like dusting on the surface. If evaporation is restricted to localised areas such as defects in an impermeable paint finish, then salt deposition is concentrated, forming thick crystalline deposits with the appearance of small flowers; hence the term 'efflorescence'. When evaporation occurs within the material, salts can be deposited within the pores. The expanding salt crystals in these locations may result in fractures forming in the material and spalling of the surface. This type of decay may be seen in porous brickwork or masonry.
When there has been a long-term problem with moisture penetration, evaporation at the edge of the damp area leads to a distinctive 'tide mark' as a result of salt deposition. Where this occurs at the base of a wall, the tide mark is often taken as a typical diagnostic feature of 'rising damp'. However, these salt accumulations may remain even when the water penetration that originally caused them has long gone. Similarly, water penetration may have occurred from causes other than 'rising damp'.
The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of such factors as rising ground levels, the failure of ground drainage systems, and the increased use of concrete or finishes around buildings without consideration of drainage slopes.
The accumulation of 'moisture reservoirs' in the foundations may also arise as the result of chronic plumbing leaks or floods from catastrophic plumbing or drainage defects.
Damp conditions at the foot of walls may be greatly increased by condensation. This occurs when warm moisture-laden air cools to dew point (the temperature at which moisture condenses) against a cold surface. Such cold surfaces commonly occur when the insulation value of the external wall is reduced by water penetration, as described above. Intermittent occupancy with intermittent heating provides the conditions for condensation of further water on these cold damp surfaces, particularly in ground floor bedrooms. These phenomena are the main causes of damp in the base of walls rather than 'rising damp' alone.
Concentrations of hygroscopic salts, which are often found in masonry, can also absorb moisture from the air, especially at relative humilities above 75 per cent. In a room that is sometimes unoccupied, with fluctuating relative humidity levels, this can result in the regular appearance of salt blooms on the surface (‘cyclical efflorescence and deliquescence’), resulting in damage to vulnerable materials, and giving the appearance
of rising damp.
Damp masonry at the base of walls may lead to a number of problems:
- The moisture content of the structure may rise to a level at which decay organisms may grow, or the materials themselves may be adversely affected. For example, timber skirting boards or built-in bonding timbers along the base of walls may become infected and decayed by dry rot, wet rot, weevils or woodworm.
- In very damp conditions, the inorganic materials themselves may lose their structural strength. This occurs most spectacularly with walls made of cob (earth) soaked with water.
- Damp conditions on the surface of walls, particularly in conjunction with condensation, allow the growth of moulds both on the surface and within porous or fibrous materials, such as wallpapers or carpets fitted against the base of the wall. Not only is this aesthetically unacceptable and damaging to finishes, but it can be a significant health hazard to occupants.
- Where evaporation takes place, the deposition of soluble salts on the surface or within the pores of materials can cause aesthetic and structural damage.