Monitoring moisture condition

Standard surveyor’s moisture meters with accessories:

Figure 1: Standard pin probes	Figure 2: Deep insulated probes	Figure 3: Thermo-hygrometer
Figure 4: Thermo-hygrometer and surface temperature sensor	Figure 5: Thermo-hygrometer on a lead
Figure 6: Dual-pad type capacitance meter with thermo-hygrometer. Photo © courtesy Tramex Ltd	Figure 7: Capacitance meter used in conjunction with humidity box. Photo © courtesy Tramex Ltd	Figure 8: Meter used to check RH/temperature of a drilled hole. Photo © courtesy Tramex Ltd

Using an inserted electronic humidity/temperature sensor:

In figure 9, below, a plastic sleeve has been tapped tightly into a 16mm hole drilled into the brickwork substrate. On the right an electronic thermo-hygrometer is pushed tightly inside the plastic sleeve, a good seal obtained from its tapered rubbery shape. But would you consider it wise to use sensor ‘H’? (See later advice on checking your sensors.) Wetness in the masonry will create a humid hole in which you have inserted your sensor. The wetter the surrounding masonry, the more humid the air within your drilled hole. The relative humidity (RH) is also influenced by temperature. Measurement of the RH and temperature offer the surveyor a useful indication of how damp the surrounding masonry is, and return visits to take the humidity measurement can help you determine whether the masonry is becoming damper or drier.

Figure 9: Sensor in wall

Figure 10: Sensor sleeve, for insertion into a drilled hole

Figure 11: Thermo-hygrometer sensor (THS); note it has an identification letter H

Measuring conductance of masonry offers quite an indirect way of measuring moisture, but measuring air in a sealed hole gives you a direct measurement of moisture condition inside the drilled hole air – which is directly influenced by dampness in the surrounding substrate. The measurement is within the substrate rather than at its surface, and is not directly influenced by metals or salts.

But you need to know how to interpret the readings – the tests and case studies that follow demonstrate this.

At level 3 we may have begun to be invasive. Drilling holes in walls and floors is certainly invasive, and is a major feature of level 3 monitoring. Leading equipment suppliers have sold ‘insulated deep probes’ for the EMM for many years – but how many surveyors have used them?

Key principles for siting monitoring stations A monitoring station is a fixed position where you check moisture condition over time. It is labelled and usually noted on a table, to be later loaded into a spreadsheet and then translated automatically into a graph. Create monitoring stations where you would learn most, bearing in mind points 2 and 3. Drill holes where it is safest to do so. Drill holes where it creates the least damage or disturbance. Create a monitoring position at the epicentre of dampness, in wall and (if safe to do so) in solid floor. Consider a monitoring position away from the damp zone – as a control. Consider a monitoring position at the margins of the damp area. If you consider there are insufficient monitoring positions for the size of damp area under study, invest in additional sensors or allow more time for site visits.

Hole drilling – the practice and perils

Hole drilling will nearly always be required if probes are to be inserted into sleeved holes. Hole drilling requires practical skills and a knowledge of how buildings are wired and plumbed, so cables and pipes can be avoided. This can be further checked using professional metal detectors. If you do not wish to drill, consider commissioning an independent dampness investigator with the required skills. If you take the plunge, you will get your hands a little dirty. Do you check with a metal detector each time you drill for shelves at home? The same risks apply.

Figure 12: A pack of 5 sensors. You could drill 5 holes, or 10 holes, or 15 or 20 holes. With just 1 pack of sensors each group of 5 holes would take 30–60 minutes to take a reading from. This means our selection of monitoring ‘stations’ needs to be very carefully thought out

Kitchens are a huge problem, as cables are often routed from socket to socket at all manner of angles. And damaging plasters that are guaranteed in association with basement waterproofing or above-ground damp coursing would be another professional embarrassment. You can sometimes access the substrate at a socket or light switch back box position.

But the dangers are greatest in floor screeds - quite apart from where and what depth the dpm may be located, they could even contain grids of heating pipes, and being thicker than wall plasters, your metal detector might not pick up deeper pipes or cables. For this reason I tend not to drill floor holes too often – even armed with a £600 metal detector that is said to detect cables at 100mm depth.

Using inserted thermo-hygrometers

Switch your electric drill to ‘hammer’ mode. As you drill, collect the outpouring dust for later carbide meter analysis. Ideally, vacuum out dust, and tap in a plastic sleeve, followed by a thermo-hygrometer. You will need to wait a minimum of 30 minutes for any worthwhile humidity reading, and further checks after 45 minutes and 60 minutes should help you know that readings have stabilised. You would take the relative humidity and the temperature. Take the readings using a stopwatch.

Label all your sensors, e.g. A, B, C, and use the same sensor each time you check a particular hole. If a sensor malfunctions, it will create less of a problem if you know which monitoring hole it was used in.

Nine easy steps to fitting an inserted monitoring sensor: Get permission for hole drilling. Select wall or floor position for installing sensor Assess the risk of drilling. If a wall is plastered, take care to drill away from cable or pipe runs. Use a professional and reliable metal detector to confirm hidden dangers, or alternatively hack off plaster in area to be drilled – bolstering off plaster is a little less risky than directly drilling. Log sensor position on a plan or in a table in your site notes. Drill the required diameter hole (check manufacturer of sleeve). Collect drillings in a sealed container for possible analysis. Insert patent sleeve. Seal between sleeve flange and substrate (if necessary). Fit your thermo-hygrometer, start your stopwatch.

Using humidity boxes

If you are checking a floor screed in a one-off survey, the humidity box must remain sealed to the floor for at least 4 hours for anything like a useful indication of the amount of dampness in a screed to be obtained.

This amount of time would not usually be available in a standard house survey – so the technique will tend to be used in ‘further investigations’ of damp problems and when monitoring is required. In most cases an ERH of 75% means your floor will be acceptably dry. Always take damp meter readings as well.

Once the humidity box is sealed to the floor, the longer you leave it, the more reflective the humidity reading will be of moisture within the screed it sits upon. After 1 hour you record a reading of 72%, but readings are gradually rising. After about 2 hours the humidity is stabilising, and just increasing very gradually, until after 4 hours we note an ERH of 76.1%.

Figure 13: A standard humidity box is sealed onto a solid floor. For convenience you can seal the box to the floor using duct tape. An electronic thermo-hygrometer is fitted into the side, and measures the relative humidity and temperature of a pocket of air under the box. After time the humidity of the trapped air equates with the humidity condition within the floor

Figure 14: Typical progression of humidity box RH and temperature readings. In this case, after 2 hours a reliable ERH was achieved for the floor under test