Rising damp is the most common form of dampness encountered in buildings; this is left to condensation. However, it is very likely that a high proportion of older buildings are affected by rising damp to some degree or another, and it does cause problems with positive identification, appropriate remedial action and ancillary works.
Rising damp is normally indicated by high free-moisture content in the base of the wall, reducing with increasing height. Often a characteristic ‘tide-mark’ is visible, typically up to 1m above floor level, but occasionally even higher. Hygroscopic salts (particularly chlorides/nitrates) are almost invariably present in ground water and will therefore be found in walls and plaster suffering from rising damp. These salts normally concentrate in a band in the upper area of dampness and may cause electrical (conductivity) moisture meters to give spurious readings.
Rising damp in buildings may be defined as the vertical flow of water up through a permeable wall structure, the water being derived from ground water. The water rises through the pores (capillaries) in the masonry by a process loosely termed 'capillarity'. In other words, the masonry acts like a wick.
Ground water contains small amounts of soluble salts, the most significant of which are chlorides, nitrates and sulphates. These pass with the water up the wall and are left behind when the water evaporates. Over many years of active rising damp large quantities of these salts accumulate within the masonry and decorative surface, most becoming concentrated in a general `salt bond` towards the maximum height of rise. Frequently, the concentrations of these salts are very low towards the base of the wall.
Both chlorides and nitrates are usually hydroscopic, i.e. They can absorb moisture from the surrounding environment and in general, the greater the amount of salts the greater the absorption of moisture, especially under humid conditions. Thus, even though rising dampness may have been controlled by the insertion of a remedial damp proof course these salts alone can cause the wall and any contaminated decorations to remain damp.
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