Exploring dampness

Dampness is a subject littered with misconceptions and confusions over terminology and thinking. For example, in some texts 'rising damp' means 'dampness rising past the damp-proof course (dpc)', while in others it (rightly) means 'dampness rising from the ground'.

Building Research Establishment Digest 245 suggests that dampness rises in masonry walls via 8 different routes - all of them by way of bridging the physical horizontal damp proof course. Please note there is no example of rising dampness by way of a failed physical horizontal damp proof course (see research undertaken by Mike Parrett in conjunction with the BRE).

It is important to understand the physical and chemical processes that are the root cause of dampness.

Water is all around us, in liquid or vapour form (or in winter as a solid). The physical properties of water – for example that it expands rather than contracts when cooled – mean that it can wreak havoc in buildings.

Moisture in buildings is a problem because:

• it expands and contracts with fluctuating temperatures, possibly causing cracks, but certainly causing materials to degrade over time;
• it is rarely pure: most moisture in buildings contains a range of elements and compounds;
• it can be detrimental to physical as well as mental health, as it causes an increase in house dust mite population and enables moulds to grow, as well as the resultant collateral damage to building elements and subsequent expense of remediation;
• it can lead to attacks from boring insects such as woodworm, which are attracted to damp timber; and
• damp timber can be subject to dry or wet rot.

Most construction materials are porous. How porous they are is related more or less to their density. These materials generally function satisfactorily within their own particular 'safe' range of moisture content, depending on the location of the material in the construction and the function it is required to carry out. Damp in a material usually refers to its content of free moisture (i.e. water that is not chemically bound to the material), probably more often when it is in a condition between 'dry' and 'wet'. Effectively, the free water is the content of dampness that should not be present if a material is to be acceptably dry (normal hygroscopic state).

A small amount of moisture in a material should not be a concern. However, when porous materials are subject to persistent high humidity (i.e. above 75% relative humidity [RH]), or when moisture has arrived from an outside source by soaking through, the material may be damp enough to exhibit a problem, or can cause other associated material to suffer a problem, and the material may then be described as 'significantly damp'.

A completely 'dry' porous material can only be found in a laboratory, after all its 'free' moisture has been driven out by oven-drying. The total moisture content of a material that is oven-dry is 0%. 

Therefore, to determine whether the moisture in, for example, a masonry wall exhibits a vertical profile consistent with rising dampness or whether the dampness in the wall is affected by internally generated condensation, a gravimetric profile can be undertaken by extracting core samples from the wall, weighing each sample to determine the wet weight, then oven drying the samples back to dry to determine the dry weight. The dry weighted sample is then subjected to a controlled 75% relative humidity environment until moisture equilibrium is reached. It is then a simple mathematical equation to determine which has the greater moisture content - the original wet weight sample or the hygroscopic sample (dried and subjected to 75%rh). Rarely would the internal atmospheric conditions have any major influence on the total found moisture contents (free water) deep in substrate masonry walls covered by internal cementitious renders and wall plasters. Although the wall plaster and render may be affected by any atmospheric internal moisture that eventually condenses on cooler internal plastered wall surfaces.

The calcium carbide method is also very useful to determine actual total moisture contents in core samples of masonry. (For further information refer to BRE Digest 245: 2007.)