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Distribution of Moisture and Soluble Salts in Masonry


The total moisture within a material is the sum of the moisture due to moisture absorbed from the air and any wetting from some source of water, eg, rain penetration, rising damp.

Hygrosopic or Air Dry moisture content:

All materials composing the basic fabric of walls, eg, stone, brick, render, plaster, contain various amounts of moisture, the amount depending on the specific material and the moisture equilibrium of the surrounding environment. This standing level of moisture is known as the air dry or hygroscopic moisture content of the material.

The Air dry or Hygroscopic moisture content represents the natural or air dry condition of the material and is the amount of water absorbed from the air - it is waht we would expect for a 'dry' material. So, in practice no material is completely dry; there is always some water present.

Effects of hygroscopic salt contamination on moisture content
Material Uncontaminated @ 80%rh Contaminated @80% rh
Carlite 0.6% 4.2%
Carlite 0.7%
Lime plaster
1.5% 12.2%
Cement render 2.0% 10.1%
Brick 0.4% 6.2%

Some contaminant salts which can arise in masonry by one means or another, eg, through rising dampness, may be hygroscopic, ie, they are capable of absorbing moisture from the surrounding environment. In masonry the most significant of these salts are soluble chlorides and nitrates, both of which may arise from ground water and therefore be present as the result of long term rising dampness. However, both can also arise occasionally from other sources of contamination.

Because these salts can absorb moisture from the surrounding environment their presence in masonry causes the air dry (hygroscopic) moisture content to increase purely as a result of increased absorption of atmospheric moisture. Depending on the nature of the hygrosopic salts, the amount and the humidity of the sourrounding environment the air dry (hygroscopic) moisture contents can be very high even though, for example, active rising dampness via which the salts originated has been controlled.

Free or Capillary moisture:

Free (or Capillary) moisture is that moisture which is filling the pores of the material and is in excess of the air dry (hygroscopic) moisture. It is indicative of active water ingress into the masonry, eg, as the result of active rising dampness, rain penetration, water leak. Sometimes, it may only be residual moisture of construction depending on conditions and the time since building work had been completed, or residual during drying out following remedial or building works.

Although free moisture may be present it might not be sufficient to cause decorative spoiling, staining or initiate fungal decay. It should also be noted that free moisture content might vary significantly between different materials as a result of their differing moisture carrying capacities (permeability and porosity); for example mortars are generally more permeable than bricks and so are capable of holding more free moisture. Thus, free moisture content may not reflect that of immediately adjacent materials and direct comparisons of free moisture content of different material should not be made.

It should finally be noted that very small amounts of free moisture can cause very high electrical moisture meter readings.

A 'dry' wall:

Technically a 'dry' wall could be regarded as one that is not subject to active water ingress; in other words it doesn't contain free moisture. However, in practice, it could be damp solely due to hygroscopic moisture, ie, water that is solely being absorbed from the atmosphere.

Sometimes the results obtained in the laboratory tests show that the hygroscopic moisture is in excess of the total moisture, ie, the free moisture content is negative. This indicates that the environment within the wall at the time the samples were removed was less humid than the conditions used for the analysis. Such results also indicate that the wall was 'dry' (no free moisture), ie, no water ingress. The above evaluations form the basis of moisture analysis as described in Building Research Establishment Digest 245, 'Rising damp in walls; diagnosis and treatment'

In the case of evaluating rising dampness, a 'moisture and salt profile' examines the distribution and relationship between free moisture and contaminant salts (chloride/nitrate - see below). Should free moisture be present up to the same height as the salts then there has been no control of the rising damp However, if free moisture is present below the maximum height of contaminant salts, this indicates that drying back of rising dampness has occurred. Conversely, the presence of free moisture above the height of contaminant salts would indicate that some other source of water ingress is occurring such as penetrating dampness.


Most building materials are free from soluble chlorides and nitrates but they can arise in significant quantities as the result of a long term rising damp complex where they origin in the ground water prior to it rising up the wall. Over many years of rising damp and the continued evaporation of the rising water from the wall these salts are left behind in the masonry.

Generally, these salts are distributed within the affected wall with a concentrated 'band' of salts towards the maximum height of rise of the dampness, lower amounts normally being detected at lower levels down the wall. It is possible for the base of the wall to contain no significant levels of such salts even though a rising damp complex has existed for many years.

Both chlorides and nitrates arising from ground water are hygroscopic and can cause visual signs of dampness and decorative spoiling when present in significant quantities even though the rising dampness may no longer be active. The degree of moisture absorption by such salts is governed by the level of contamination and the humidity of the surrounding environment. It is unlikely that the lower levels of contamination would cause any significant problems.

Chlorides alone can frequently be found in coastal areas where they originate from salt water in the atmosphere; sometimes they can be found where special processes have taken place, eg, in old converted kitchens where it is derived from baking salt having been stored.

Sulphates are common in a lot of masonry at some level or another: gypsum plasters contain a high level of calcium sulphate and 'efflorescence' seen on surfaces where water is passing through and evaporating is usually sodium sulphate. They can be found around chimney flues in association with hygrosopic salts described above. Ordinary portland cement contains around 3% sulphate (measured as SO3). Where sulphates become of particular importance is where cement based products become contaminated with them. This causes the formation of calcium alumino-sulphate (Ettringite) under conditions of dampness and high alkalinity which causes the cement to expand in excess of 200% leading to disintegration.

Ammonium salts can sometimes be detected around old chimney flues where it originated from the combustion of wood and other fossil fuels. Chlorides and nitrates can also be found in this situation, and all these salts are usually associated with discolouration derived from the 'extractives' formed from the combustion of the fuels. Ammonium salts are usually hygroscopic.

Final comment:

The method described in BRE Digest 245 for deteremining the full distribution of moisture in masonry is a powerful tool in dampness diagnostics. By producing 'Moisture profiles' this can give us a very accurate and precise method of evaluating dampness and its distribution.

However, what the Digest fundamentally lacks is the use of this method coupled with soluble salt analyses. When used together to determine moisture and soluble salt profiles, we get an extremely effective and now very powerful definitive and objective tool for identify dampness problems. Perhaps its greatest use is identfying the performance of remedial damp-proof courses, and giving a definitive evaluation of hygroscopic ('salt damp') moisture problems.

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