Surveying equipment and tests

Some modern electronic moisture meters include a plug-in thermo-hygrometer (humidity and temperature sensor) that can be used to quickly measure temperature and relative humidity of air in a room, or of air in a readily accessible void. If more time is available, the device's probe can be used to measure temperature and relative humidity of air inside a solid material when the probe is inserted into a hole drilled into the material. Such techniques are growing in popularity among surveyors who monitor moisture conditions.

A key advantage of thermo-hygrometer measurement is that it can be used to confirm 'live' condensation, by taking an air temperature reading, and a relative humidity (RH) reading, and then checking surface temperatures.

Modern instruments calculate how far air or surface temperature is from dew point at the touch of a button. You can track surface temperatures from floor to ceiling height in the same positions where you have already taken relative moisture meter readings. If you then plot the readings on a graph your mapping may indicate an actual or potential condensation threat at the wall surface. If this is the case, remember to log surface temperature readings at other parts of the building to confirm a difference in building condition, and remember that you will already have caused air conditions in the property to equilibrate with the outside by opening the external door and internal doors.

In the above example the use of the multi-function instrument meant that it was possible to confirm condensation as the cause.

Another advantage of these devices is that they can be used to check whether condensation occurs after you have left the property. And when combined with a data logger, you can log building conditions over time.

Relative humidity sensors are now increasingly being used to assess moisture conditions within materials and to monitor moisture conditions over time (figure 1). RH measured in drilled voids, or in enclosures placed on floors, is usually termed the equilibrium relative humidity (ERH).

Figure 1: The hygrometer can be used to monitor the changing humidity conditions within a material

ERH readings offer a great opportunity for monitoring dampness conditions, after flooding for example. Rather like the resistance meter, you can be fairly confident that changing readings reflect changing moisture conditions, as long as you are sure that changing temperatures have not influenced the relative humidity readings unduly. With practice you can become familiar with levels of relative humidity that indicate possible risks to buildings. For example, it is not uncommon to find cases where damp air in floor voids has condensed on cold timbers, whether they are joists, boards or the backs of skirtings. The backs of skirtings may be physically wet where 'rising damp air' has condensed. Such a scenario could give the impression of having been caused by 'rising damp' in walls, because plasters will indeed be wet just above the skirting, gradually becoming less wet as relative readings decrease further up the wall. This problem can be cured by improving the ventilation of the floor void or removing the source of the moisture. If, as often happens, a dpc had been installed to cure it, only the symptoms of the dampness would have been cured by provision of new waterproof plasters and a new isolated skirting. Such dampness problems in floors have been known about for some years. In cases like this the careful use of a humidity sensor would lead to an accurate diagnosis, and a different remedy.

How accurate are ERH readings?

We have occasionally observed high ERH readings (up to 85%) when the material under investigation has been found (by carbide testing) to contain less than 0.5% moisture by weight. This should not cause undue concern. Always bear in mind that there is a degree of interpretation required from readings obtained by any meter or sensor, but changes in readings will strongly indicate a change in the percentage moisture content, providing that variables such as ambient temperature have not affected readings significantly. After all, if you drill a hole and insert a sleeved humidity sensor, you will not obtain an accurate reading of ERH unless you leave the sensor in situ for at least an hour. After, say, 30 minutes (the available time you might have on a typical house survey) you will obtain an approximate indication of ERH in the wall. A more accurate reading could be obtained if you inserted a plastic- capped sleeve into the hole (so that you do not need to leave the sensor in the hole on site) and you then returned on another day to insert the sensor and take a reading.