Stage 2 of your damp cure - determine the size of the problem

Having worked out what your house is built of, and how, the next stage is to investigate the degree and extent of the damp.

While investigating the construction, I almost invariably find potential damp, rot and woodworm issues the owner wasn't aware of. These are therefore included when checking the size of the problem.

The starting point (and the easy bit), is to record what the damp problems are, and where. Next, is to find their full extent and severity, and whether the problems are ongoing. The severity is determined by the effect (and potential effect) they are having on the building. This is only possible if the construction of the house has been accurately assessed.

Limitations of damp meters

Working out the extent and severity is not always straightforward. It is rarely a simple case of taking a few 'damp measurements'. For instance, contrary to what most people believe, there is no such thing as a general purpose moisture meter (often referred to as a 'damp meter'). These meters are electrical conductance meters. The only thing they are reliably designed to do, is measure the moisture content of the common types of timber used in construction. This is only possible because of the cellular structure of wood. The cell walls lock salts within the cells, but allow moisture to enter and leave. This means the salt content is constant, but the moisture content changes to match the surroundings. The electrical conductivity of the salts varies with their concentration in the cell fluid, which is determined by the amount of moisture in the cells. A moisture meter can therefore convert the electrical conductance of the timber, into a moisture content figure.

Most other building materials contain naturally occurring salts and so conduct electricity. Apart from timber, none of them have a fixed salt content whose concentration is related to moisture. Instead, as moisture moves through the building materials, it washes salts into new places where they can build up. This affects the electrical conductivity, which the 'damp meter' then detects. The high and low salt concentrations are left behind permanently once the moisture has dried up. This means a high 'damp meter' reading could refer to a leak that was fixed 100 years ago!

Tracing damp with thermal imaging cameras

A thermal imaging camera can reveal where damp is currently present in a wall. It does this by showing different temperatures in different colours. Where moisture is evaporating out of the wall, it is cooler so shows up as a different colour. Many other things also affect the temperature, so I have to look for cool patterns that are not caused by things like the shape of the building, construction of the walls, objects in the room, or draughts. When I find a possible damp patch, this can then be checked with a moisture meter for high salt concentrations. If there are not high salt concentrations, the wall probably isn't damp. It also works the other way - high salt concentrations that do not show up any cooler, are probably no longer actively damp. If any timber is fixed into the part of the wall in question, this can be checked for a definitive answer.

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