So, a damp survey has been carried out on your home and the diagnosis has been reported as rising damp or condensation.. two of the most commonly misdiagnosed forms of damp, for two reasons.

Firstly, from a contractor’s point of view, the off the shelf solutions to rising damp and condensation, such as tanking systems and PIV ventilation are extremely profitable. They are a great way to mask the symptoms of a damp related issue rather than addressing the building defect or lifestyle issue, or a mixture of both that are ultimately the cause.

Secondly, these diagnoses are often poorly evidenced by the use of a ‘damp mater’, which may be the surveyor’s only tool with which to evidence the dampness within the property.

Please do note, that damp meters do play an extremely important role in a damp investigation, without them we would be blindly looking at blank walls with no idea of what may be happening behind the surface covering. The key take from this article is that they suggest dampness, they do not evidence it!

A damp meter is not recognised as tool or process that can evidence dampness according to British Standards.

British Standard – BS 5250: Code of practice for controlling condensation in buildings. ‘Accurate measurements of the moisture content of brick or mortar cannot be obtained by the use of electrical moisture meters because the presence of salts increases the electrical conductance of the water, giving falsely high readings.’

British standard- BS 6576: Code of practice for diagnosing rising damp in walls and buildings and installation of damp-proof courses. ‘Surface measurements cannot alone give proof of rising damp, so further evidence may, where permitted, be obtained by measurements taken within the depth of the wall. Chemical (carbide) or gravimetric methods can be used.’

It is important to note that other materials used in historic mortars can also trigger a moisture meter, such as black ash and unwashed ‘sea sand’ – we have some beautiful examples of this in Cornwall, where the sand was taken directly from the local beach.

British Standards clearly state that damp meters are not to be used to evidence any form of dampness and instead other, more accurate methods should be applied. And four of the most accurate methods are:

Psychrometric analysis measures the physical and thermodynamic properties of moist air and is my far the best tool we have when calculating condensation or mould risk within a building. By using a thermohygrometer, we are able to take temperature and relative humidity measurements both internally and externally to measure and compare excess vapour pressure (how much actual moisture is in the internal air compared to external air), absolute humidity an d dew point (the temperature at which air becomes too cold to hold water and condenses).

This rich profile of information, when compared to wall temperatures, greatly evidences the risk of condensation and can also give clues as to the cause being either a building defect or lifestyle.

Carbide moisture meter testing begins with obtaining a sample of the wall, which is typically taken by drilling a hole within the target area of masonry. It is then put into a calcium carbide meter, which is a sealed vessel that is used to mix measured samples of masonry with calcium carbide. Calcium carbide will react with moisture present within the material, produce acetylene gas and the measurement of this gas release can be equated to the level of moisture within the sample.

Gravimetric testing for moisture content involves the precise measurement of weight changes in a material before and after drying. By subjecting a sample to controlled heating, the loss of moisture can be accurately determined based on the reduction in mass. This method provides a reliable means of assessing moisture levels in substances ranging from food and pharmaceuticals to construction materials. Gravimetric moisture analysis is valued for its accuracy and sensitivity, making it an essential tool in quality control, research, and industry, where precise moisture content measurements are crucial for product stability, processing, and compliance with standards.

Salt analysis in the context of rising damp involves examining the composition of salts present in building materials affected by moisture infiltration. Rising damp brings moisture from the ground into walls, and as the water evaporates, chloride and nitrate salts are left behind. Understanding the salt composition within a masonry wall is crucial for evidencing the movement of moisture from the ground and into the walls – rising damp!

The miss-diagnosis of moisture entrapment

For the following example I will make three assumptions, and they are that modern, non-vapour permeable materials have been applied to your building and that the surveyor has not used any other means of moisture testing other than their damp meter, nor did they diagnose any other form of building defect. Their diagnosis was rising damp or condensation. What could have they missed?

When moisture moves through masonry, either as a liquid via capillary action or as vapour via diffusion, it can reach saturation point. The capillaries, which are essentially tiny cracks or air pockets within the material, become flooded with moisture and it is now unable to escape.

Imagine placing a towel (masonry wall) within a plastic bag (cement mortar or plastic paint) that has lots of tiny cuts in it and then soaking (internal vapour pressure or Cornish side ways rain) that bag with a hose. The water will be driven into the bag and soak the towel, but how effective do you think the evaporation process will be with the towel now encased in plastic? The evaporative pressure will be nowhere near as strong as the forces that led the material to become so damp.

Contained within your wall, the moisture can now not easily escape due to the cement, tanking systems and plastic paints, but due to reaching saturation, is now slowly leaching out and presenting as dampness with your home – which is then falsely diagnosed at condensation or rising damp.

Therefore, we have a number of explanations for the damp meter signalling increased levels of dampness, other than condensation or active rising damp, and these could be:

1) Moisture could have been drawn up through the masonry (rising damp) over a prolonged period of time and deposited salts, potentially caused by a broken (and now fixed) drain or rainwater down pipe? However, if we were to carry out carbide or gravimetric tests we may in fact evidence that the wall is dry and these defects are historic. A carbide or gravimetric test plus salt analyses would evidence or disprove any given theory.

2) It may be penetrating damp. Moisture may be penetrating the wall (penetrative damp) due to a building defect such as failed pointing, or render etc that has not been identified by the surveyor as the defects are microfracture or absorbance related.

3) The damp meter may be picking up increased moisture levels within the wall due to moisture entrapment caused by the application of inappropriate building materials. Gypsum plaster is both hygroscopic, meaning it absorbs and retains moisture from the air and surrounding surfaces and hydrophilic, it absorbs and struggles to release moisture.

4) And lastly, false positive readings due to a conductive material within the historic render such as pot ash or some form of salts that have migrated from improper materials or a chimney breast.

Summary – damp meters can not be used to evidence the presence of dampness within a building. A damp surveyor’s role is to both document the symptoms of any dampness as well as evidence its cause and this cannot be achieved with anything less than the application of concise, analytical scientific methods, such as those described within this article.

Please note: all advice given by Truscott Building Preservation is specific to granite, slate and lime constructed buildings located in Cornwall only. It is important to note that buildings constructed in areas with less severe weather patterns and softer masonry types WILL require the use of very different techniques and materials. Our advice is not generic to the whole of the UK, and we suggest that you find a local expert who can best advise on the correct techniques and materials required to preserve your heritage building.