Last edited 03 Aug 2017

Rising damp

Rising damp.jpg

Excess moisture is a common problem in buildings, and may be apparent from; damp patches, mould growth, mildew, salts, staining, ‘tide marks', blistering paint, bulging plaster and so on.

The most common causes of persistent damp in buildings are:

Rising damp is said to be caused by capillary action drawing moisture up through porous elements of a building’s fabric. It has never been successfully reproduced in laboratory conditions - extensive research by Jeff Howell at South Bank University failed and it was concluded that rising damp was a phenomenon invented by the chemical industry to sell remedial 'damp' treatments.

Rising damp might be apparent from:

  • Condensation or damp patches (typically up to 1m above the floor).
  • Corrosion of metal elements such as beading.
  • Damp odours.
  • Timber decay, such as skirting boards.
  • Damage to surface finishes.
  • Tide marks and staining (typically up to 1m above the floor).
  • The presence of white salts.
  • Health problems.

It should be noted that these problems are common to other sorts of damp, such as; cold bridges, lateral penetrating damp, surface condensation and interstitial condensation. It is claimed capillary action can only cause damp to rise approximately a meter above the source of the damp (depending on the nature of the materials, the presence of salts and the rate of evaporation), and so problems above this height probably have a different cause.

Capillary action has never been demonstrated to act in this way in laboratory research. Dampness seen in walls to a metre high will always disappear when the wall is cleaned of impermeable materials and normal evaporation allowed. It is concluded that rising damp is, in fact simply interstitial condensation being trapped into building fabric by impermeable materials such as gypsum plaster and cement render.

Dampness can be measured with electrical resistance meters, either on the surface, or within the building fabric itself. Generally, if the meter reading indicates that the fabric is dry, then it is dry. However, electrical resistance meters were developed for use in timber, and if the reading indicates the fabric is wet, this does not necessarily mean that it is wet, as the presence of other substances such as soluble salts will give a similar reading, and in older walls, salts may be present even where damp is not.

Carbide meters are likely to give a more accurate measure of moisture content, and this can be further enhanced by testing samples drilled from the building fabric and tested for hygroscopic moisture content. This may require appointing a specialist.

Further information about testing techniques, the cause of dampness and remedies is available from BRE Digest 245, Rising damp in walls - diagnosis and treatment.

Rising damp has been attributed to faults to, or the absence of a damp proof course (DPC). Up to 25% of Britains housing stock does not have a damp proof course, and yet is still perfectly dry. In Holland, damp proof courses are rarely used even in new build. Every case of rising damp can be very easily explained as being caused by human occupation - the average family produces nearly a gallon of atmospheric water every day. If this is not removed from the building it ends up within the fabric, and can easily condense - forming the mythical rising damp. Dampness issues can be enhanced by

  • The moisture content of the building fabric itself.
  • Raised ground water levels.
  • Raised ground levels around a building.
  • Leaks to pipework or guttering.
  • Flooding.
  • The presence of salts in the building fabric.
  • Crystallisation of salts on surfaces resulting in reduced evaporation.
  • Cool internal temperatures or internal humidity reducing the rate of evaporation.

'Rising damp' or interstitial condensation can reach a height of more than 1m, with the height depending on:

  • Temperature gradients in walls - coldest being near ground, and closer to dew point
  • The rate of evaporation from the wall.
  • The porosity of the wall.
  • The salt content of the wall and soil.
  • Heating in the building.

If there is a specific source of damp, this should be corrected, for example, fixing leaking pipes, altering ground levels, installing drainage and so on.

Treatment of rising damp is responsible for the massive damage and deterioration of historic housing stock in the UK. It has involved creating a moisture impermeable layer within the building fabric, either by the insertion of a damp proof course, or by injection of water-repellent chemicals. These scientifically unfounded measures have created more damage to old buildings in the UK since the early 1960's when the chemical industry first started selling 'damp treatment' than any other period of architectural history.

Treatment generally also involves remedial work to any corroded or decayed elements of the building fabric, as well as hacking off and replacing existing plaster to a height of 1 m. This can be disruptive as it involves removing skirting, sockets and so on. This is almost always done using chemically modified cementitious compounds which simply trap more interstitial condensation until these new coatings fall off in a few years - to be renewed with yet another diagnosis of 'rising damp' The permanent solution is to return the building to breathability, removing impervious coatings, and restoring original breathable plasters and paints. Damp problems do not reappear if internal moisture conditions are also monitored and kept below around 7 grams per cubic metre.

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