Different types of damp need different treatments so getting the type right is vital

Most people do not realise, that when they notice a 'damp problem', it could be any one of six different sorts of damp, or some combination of the six. As the different types of damp all need different cures, there is no magic single cure for all damp.

The following table outlines some of the key features of the different types of damp.

Type of Damp Features
Condensation - Damp from the air
  • Often shows up as black patches of mildew;
  • Wallpaper may come loose from the wall or develop a ‘tidemark’;
  • Soft furnishings and clothes may smell musty;
  • Visible droplets of water may be seen on affected surfaces;
  • Can be harmful to human health;
  • Can occur inside walls and insulation or under floors;
  • Very common in modern houses and houses that have had standardised energy efficiency upgrades.

Caused by excessively high humidity. Often triggered by the humid air being cooled on contact with a cooler surface, or losing heat in cooler surroundings. As the air cools, it can no longer hold as much water vapour and any excess leaves the air as water droplets. Unrelated to other types of damp and generally requires a ventilation specialist to resolve it rather than a damp specialist.

Rising Damp - Damp from the ground directly below the house
  • Usually only affects ground floor rooms and floors of basements;
  • Skirting boards and timber floors may rot;
  • Plaster may 'bubble' along a horizontal line or form a powdery band;
  • Solid floors may develop a white bloom or get fine white crystals forming;
  • Harmless to human health;
  • Very common in houses that have been ‘damp proofed’ otherwise rare.

Caused by moisture from saturated ground flooding the continuous fine pores in floors or walls and becoming trapped. The moisture will continue to rise until it can drain away or evaporate, or the supply of fresh moisture runs out.

Penetrating Damp - Damp from rain, or damp from ground against walls
  • Patches are often outlined with a brown stain;
  • Plaster can 'bubble' and get a powdery surface;
  • Can affect ceilings and walls;
  • Was often deliberately designed into old cellars to create a cold store and stop rising damp affecting the rooms above;
  • Harmless to human health;
  • Very common at ground floor level, where it is generally misdiagnosed as rising damp. Also common in houses with cement rendered walls or cavity wall insulation.

Caused either by the lack of an escape route for moisture entering the structure, or an increase in moisture that overloads the escape route.

Chemical Damp - Damp from salt contamination
  • Usually starts as intermittent dark patches that become yellowy-brown over time;
  • Intensity of patches can vary with weather conditions;
  • Patches gradually grow in size and intensity over a period of months or years;
  • Can appear on any plastered surface;
  • Harmless to human health;
  • Very common on chimney breasts.

Caused by contamination of the plaster with hygroscopic salts. The salts absorb moisture direct from the air. Contamination can have occurred when the house was built, or have happened at any time since for a variety of reasons.

Damp from plumbing leaks
  • Typically shows up as a small isolated damp patch, without any brown staining, that gradually grows;
  • Plaster can 'bubble' at the edges of the patches.
  • Water can become visible if the leak is near (or above) the surface;
  • Very common in houses with heating pipes buried in solid floors.

Caused either by corrosion of the plumbing, or joints that were not fully watertight when new. Corrosion can be internal as result of more than type of metal being used in the plumbing, or external – often as a result of concrete or cement coming into contact with copper pipes.

Damp from walls and floors drying out
  • Typically shows up as patches, without any brown staining, that may initially grow before gradually shrinking over a period of months;
  • Harmless to human health;
  • Common after a period of building work, flooding, or a long term plumbing leak, particularly if hot air driers have been used to speed up the drying;
  • Damp patches can appear on surfaces that were not affected by the building work or flooding but are near by.

Caused by moisture deep within the structure working its way out. This can take many months or even years to dry fully. Hot air driers only dry out moisture close to the surface, so when switched off, moisture deeper in still needs to make its way out.

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