Measurement techniques

Moisture in buildings and inspection methodology

The first step in the investigation of a problem building is to carry out a thorough inspection of the building for defects.

The next steps are to:

establish moisture contents in affected materials, such as timber, plaster, masonry, insulation materials and textiles;
establish the humidity, temperature and dew point in the environment (both internally and externally); and
investigate in greater detail (as necessary) the moisture profiles in large dimension timbers and across masonry masses.

Timber moisture content: This may be estimated at the surface by the use of a resistance-type moisture meter, fitted with insulated needle probes. The results gained will fluctuate depending on relative humidity and temperature - a rafter may have a surface moisture content of 16% in summer, which might rise to over 20% in winter. This would not necessarily indicate an increased water content due to a fault in the roof, it might be water absorbed through a drop in temperature. The core of the timber will remain relatively dry and a hammer probe with insulated electrodes is recommended for measuring the subsurface moisture content. Probes can also be used to measure moisture contents at depth in large section timbers and those built into masonry.

Masonry moisture content: Estimating surface moisture content in plaster and mortar is of limited value except for comparison. A surface capacitance meter may be used on plastered walls and panelling to detect areas requiring further investigation. Surface moisture readings in plaster and masonry can be taken using moisture meters. These will indicate if a wall is dry but can give false readings of dampness. Specialist tailor-made probes can be used to measure moisture across masonry walls ranging from domestic housing to the several metres thick walls, say, of castles. Absolute readings should be made by means of a carbide-type pressure meter or by the oven drying method. Moisture contours, both on the surface and within the thickness of the wall, will help to define the source and type of moisture giving rise to decay. Where possible, take mortar samples of the areas affected to determine accurately the moisture and salt content of the masonry. This is, however, a destructive form of testing.

Data loggers can be used to measure the environmental parameters, temperature, humidity and dew point, both internally and externally.

Hygrometer test: This involves drilling a hole into concrete and inserting a plug into the hole, leaving it to stand for 72 hours to allow the air in the hole to achieve equilibrium. A probe is then placed into the plug in the hole to measure the relative humidity. After this time the seal is broken and the relative humidity noted after a period of equilibrium has been reached. The concrete is deemed 'dry enough' when the relative humidity is no more than 75%. The hygrometer test is the most widely used method in Europe for determining whether the concrete is dry enough.
Readings of 75% Relative Humidity or less indicate that the material is dry (BS 8203 and BRE Digest 163).

Detailed investigation

The findings from the initial investigations are followed up by more detailed study. The aim is to determine the distribution and extent of all significant decay organisms in the building, the distribution of all micro-environments with a predisposition to timber decay, and the building defects that cause them. The distribution of moisture and its movement through the structure is particularly important. The extent of significant timber decay should also be determined. Active decay organisms may not yet have caused significant timber decay. Conversely, there may be significant decay even when the decay organisms that caused it have been dead for many years.

Key factors to note are:

species and viability of decay organisms;
moisture content of materials;
ambient relative humidity; and
ventilation.

Timber species and previous chemical treatments may also be significant.

It is important to coordinate the results of the investigation with the building structure, bearing in mind the characteristics of particular periods and methods of construction. Carefully record these and quantify where possible. This allows analysis of the results by other experts, reduces the 'grey' area in which differences of opinion can arise, and forms a basis on which future investigations can be planned. Recording of data is especially important in the current legal climate, and photography can be particularly valuable. A detailed investigation of this sort might take about 5 man-hours for a typical 3-bedroom house.

The results of some or all of the above tests will establish the cause and enable a solution to be developed.