Exploring dampness

Instruments vary in how directly they measure moisture. The most commonly used and convenient way to 'measure' moisture content is with an electrical resistance meter (see Surveying equipment and tests). This device is usually calibrated to measure the moisture content of timber, and is quite accurate within the 8–28% range. The surveyor's moisture meter offers an indication of relative moisture content in material other than timber, providing that readings have not been swayed. Readings in material other than timber can be described as relative readings (R/R).

Modern moisture meters often feature plug-in thermo-hygrometers that measure both relative humidity and air temperature using the same probe. Used together with a plug-in surface thermometer, you can know in an instant whether a surface is subject to live condensation and also how many degrees from dew-point temperature the surface is. The hygrometer will quickly tell you whether room air is unacceptably humid.

The same thermo-hygrometer probe can be lowered into a subfloor void or inserted into a sleeved drilled hole so that you can measure its equilibrium relative humidity (ERH) and temperature. From these readings you can gauge whether the material into which the probe is inserted is significantly damp. Such equipment is very useful for monitoring moisture condition.

As we know, different materials placed in a closed void will eventually reach equilibrium relative humidity (ERH); that is, there is no net exchange of moisture with any nearby substance.

A word of caution: moisture meters measure conductivity, so they are influenced by the presence of electrolytes (salts) and metals, and also by the presence of carbonaceous materials (materials consisting of or containing carbon) such as breeze or clinker blocks or aggregates, and plasters or mortars containing ash or coal dust. Carbon is a good electrical conductor. Pulverised fuel ash (PFA) contains carbon and has been used for years as a main constituent of building blocks, load bearing and non-load bearing, and so may cause moisture meters to record very high readings due to the carbon present when the material is not actually significantly damp.