Surveying equipment and tests

The humidity box is a reasonably priced piece of equipment suitable for house surveyors. It indicates whether there is residual moisture in a floor slab or screed, which might cause floor coverings to fail. The flooring industry considers the substrate 'dry' where ERH readings of 75% or less are recorded.

The box is placed on the solid floor to be tested, and is best weighed down with a heavy object such as a brick. The box has a moulded void underneath into which a humidity sensor is pushed. The box seals the pocket of air to be tested. The thermo-hygrometer sensor normally used will read temperatures too – an important thing to know when assessing the significance of a humidity reading.

The only problem for hard-pressed house surveyors is again the length of time it takes for a stable ERH to be achieved in the pocket of air under the box. Ideally the box needs to be left undisturbed for 24 hours. You could improve the seal of the box to the screed by using sealing material between box and floor. It is also possible to speed up the whole process by pre-planning; that is, placing impervious materials over the floor to be tested before the day of survey (see Oliver, A., Dampness in Buildings, Oxford, 1997). This would probably only be done by a specialist dampness investigator where there is reason to believe a solid floor to be significantly damp itself or an important contributory factor in a dampness problem. If a screed or solid floor is quite damp you would know this from the RH reading even after just an hour's wait.

Figure 1: The humidity box – probably the easiest piece of moisture-measuring equipment to use. An electronic thermo-hygrometer measuring humidity (RH) and temperature is inserted into the side of the box and connected to the moisture meter

Figure 2: The underside of a humidity box, showing where a pocket of air is trapped to equilibrate with moisture deep into the floor, perimeter sealant and an inserted electronic thermo-hygrometer

Depending on the condition of the floor, type of coverings etc., a more reliable humidity reading could be obtained after, around 4 hours. Bear in mind that if only 1 humidity box is used, you would only obtain information for 1 floor position, without the benefit of a control reading elsewhere. In practical terms there are therefore difficulties in obtaining a horizontal profile of moisture content for a solid floor in a house. You must also remember that the ERH you record could be influenced by surface moisture conditions of the floor under investigation, rather than conditions deeper in the slab. Any conclusions must therefore be based upon a collection of evidence, including humidity box readings of ERH, details of construction, age of floor, likely position and condition of dpm, and the likelihood of moisture ingress from various sources. One manufacturer of humidity sensors and boxes has suggested that a reliable stable ERH can be found more quickly within a drilled void than at the surface of the floor.

You can use a capacitance meter to help build up a picture of changing moisture conditions across an area of floor, and return another day to check if the floor is improving.