Monitoring moisture condition case studies

The author made 2 site visits to Tenterden. The first was on 19 July 2003, a ‘dampness investigation’ as drying seemed to be far more protracted than expected for a localised kitchen water damage. A further inspection by the author on 2 October was to help confirm whether the property was by then dry, and if drying equipment could be switched off and removed from site. The author was not involved in the daily monitoring of the drying process.

Figure 1: Ground floor plan of Kent chalet bungalow. The wall between the garage and the kitchen was right at the epicentre of the water damage. Is this blockwork partition wet or dry, drying or not?

We looked at how the screed was checked using a capacitance meter in this property in Case Study 3. But what about the walls? How were they monitored to check drying progress?

In Case Study 3 we explained how a drying contractor set up equipment to dry out a property suffering badly from a plumbing leak. During drying the contractor found it difficult to know for certain if a blockwork wall between the kitchen and the garage was actually drying.

The contractor was unsure whether moisture meter readings on either side of the wall could be relied upon - after all, this type of lightweight blockwork contains carbon, doesn’t it? Checks on the garage side could be influenced by this integral carbon - and be magnified due to the inherent conductance of the PFA (pulverised fuel ash) block material.

It was necessary to make sure. Moisture meter readings might not reflect the actual moisture content - so on 19 July we first set up 3 humidity sensors in the wall (1 on the garage side and 2 on the kitchen side). This would at least offer a humidity reading uninfluenced by carbon in the blockwork. We will concentrate on sensor ‘A’ on the garage side. In addition to taking readings on the garage side of the wall from an inserted thermo-hygrometer - the author gouged out a sample of blockwork using a large screwdriver. The sample was duly tested on site by carbide meter. Results of the blockwork testing are presented in the table below.

Setting up a monitoring station at A:

The blockwork was tested for moisture content (MC) at about 250mm above garage floor level. Remember that the blockwork was wettest at its base, having sucked up moisture from a wet kitchen floor. The carbide test confirmed a total moisture content (TMC) of 18% (i.e. with respect to wet weight). What would a moisture content of 18% in lightweight blockwork signify? Was the wall here saturated? Author tests on blockwork at South Bank University found such blockwork able to contain around 55% of its dry weight as water. So 18% MC means the wall here contains about a third as much as it potentially could. This is very damp!

Figure 2: The sensor inserted at ‘A’ can be used to assess whether the blockwork has dried at all on a subsequent visit. Note the sensor is exactly where the blockwork is very darkly stained

Refer to the Appendix to find out how much water 1m2 of such blockwork can hold – it can be surprising.

On 19 July we had at ‘A’ a marker - a starting point we could really be certain of, a reliable actual moisture content, and also a monitoring position from where ERH readings could be taken. After gouging out a blockwork sample for carbide testing, the same hole can be fitted with a sleeve in which your humidity/temperature sensor can be inserted - 2 tests for the price of 1.

Analysis of monitoring results:

How had the blockwork changed between July and October?

Between 19 July and 2 October you would have thought there had been no change at all in the moisture content of the blockwork if you had relied solely on capacitance readings. The 1,000 reading stayed put. Surface readings using the standard EMM pins showed a decline, but we could not rely on a surface reading to be sure the wall was drying at depth. But the carbide monitoring result was very positive – a drop in actual moisture content from 18 down to 11.8%. The samples were taken from the same block. The ERH had also dropped from 100% down to 93.6%. But the blockwork was still significantly damp. More drying was needed.

More drying continued until the contractor informed the author that ERH had dropped down to an acceptable 75%. The author did not take any further carbide readings after 2 October, but would have anticipated a carbide reading of say 2–3% when the ERH had fallen to 75%. This would be is pretty dry for a lightweight block that can hold at least 50% of its dry weight as moisture. Such blocks would float if you tossed them into your local canal.

The carbide meter reading really helped us know how damp the blockwork actually was – and we didn’t even need an electric drill.

And the strange dark patches were nothing really to do with moisture. We learned from the client that her husband used to drive a large car with double exhaust pipes. Hence the 2 peaks – of carbon from the exhausts. No doubt the carbon content affected the surface EMM readings.

Monitoring at ‘A’ - Kent blockwork