USE OF ELECTRICAL
MOISTURE METERS
When damp investigations are undertaken using electrical moisture meters, one frequently hears the comment that such meters don't measure dampness in walls or other masonry substrates and therefore they are misleading. As a result some authorities would argue that quantitative results should be obtained, but try persuading a vendor to allow you to remove samples from their newly decorated walls in the course of a general survey!
Electrical moisture meters, however, have an enormous advantage for the general survey — they are clean and non-destructive. But there are also limitations — they do not quantitatively measure moisture on/in masonry substrates, such as plaster, brick, stone, mortar, etc. The majority record electrical resistance between two applied electrodes; a few measure capacitance, and more recently some measure the reflection of radio frequency emissions from the meter: moisture, together with one or two other materials, affects these properties. Thus, such meters can reflect the presence (or absence) of moisture and/or other factors which may be present.
The actual readings also depend on the area of contact of the probes/electrodes with the substrate. For example you will get much higher readings on a material where the probes can fully penetrate than on a similar material with similar moisture factors where the probes cannot fully pnetrate. Therefore, do not try to make simple comparative readings between substrates - these meters are qualitative and it may just be reflecting the probe contacts and nothing else!
NB. It must also be fully appreciated that electrical moisture meters, especially conductance meters, are very sensitive to very small amounts of free moisture and/or certain types of soluble salt contamination. For example, free moisture contents of less than 0.1% can result in high meter readings.
Hygroscopic salts
It is essential to appreciate the fact that on masonry substrates the readings may not always reflect moisture alone. For example, contaminant hygroscopic salts, mostly chloride and nitrate, even in very low concentrations in plasterwork/wallpaper, especially under conditions of high humidity, will cause moisture meters to respond quite positively. Therefore examination of, say, an exposed stone wall in a property that has been damp-proofed or a property that was once a barn (likely to be salt contaminated from past use), should be undertaken with extreme care; one cannot distinguish between residual hygroscopic salts and rising damp/dampness in these situations, and it might therefore be prudent to resort to other methods of moisture evaluation.
It is possible for the material itself to be significantly electrically conducting but this is rare and is generally quite evident to the investigator from the unusual distribution of readings; almost all plasters, bricks, cements and finishes are unlikely to cause such problems. Nevertheless, this factor should be borne in mind. But, most importantly, where the meter records zero readings the surface is both ‘dry’ and uncontaminated.
Description of readings
The meter, whilst providing a true quantitative moisture content for wood, does not do so on masonry substrates. The readings obtained from masonry surfaces should be read as ‘per cent full scale deflection’ of the meter needle or ‘per cent wood moisture equivalent’ depending on the type of scale provided. A figure above 15% full scale deflection (fsd) or 20% wood moisture equivalent (wme) is considered by the manufacturers to be of significance. The fact the meter does not record a quantitative measurement is of little consequence. The diagnosis is based on the overall distribution of meter readings and not so much the actual reading itself.
Most building materials (brick, mortar plasters, render) are free from materials which affect electrical resistance. Therefore, it must be appreciated that in over 99.5% of cases changes in electrical resistance is reflecting either water and/or soluble salts! Thus, it is quite valid to use electrical moisture meters to look at damp/salt problems.
An electrical moisture meter should be used to plot a ‘profile’ of readings: this involves recording the pattern and distribution of surface readings, for example, in a vertical series up a wall. It is the pattern and distribution that gives an indication as to the problem. (NOTE: 'profiles' cannot currently be obtained with the Protimeter SM Radio frequency meter in 'search mode'.
Below are given a number of possible interpretations of such moisture meter ‘profiles’ taken with a resistance/capacitance meter. Please note - it is the overall pattern of readings that is important. All the interpretation of patterns recorded have been confirmed by authoritative laboratory and other analytical techniques.
Pattern types
The following describe moisture meter profiles obtained by taking readings from the surface at ever increasing heights. The actual readings obtained on the moisture meter are given below these patterns - and remember it is the overall pattern that should be interpreted.
(Please note: The green areas in the graphs below indicate non significant moisture meter readings.)
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Pattern A High surface readings, often with slight decline, finishing with a sudden cut-off-- typical of active rising damp. You should be aware that rising damp can reach heights in excess of one metre. |
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Pattern B: Very low readings at base, increasing with height and then a sudden cut-off — well controlled rising damp, the increase in readings reflecting the increase in ground salt contamination up to the ‘salt band’ marking the past maximum height of rise. This pattern is often misdiagnosed as falling damp. However, salts alone can also give a pattern similar to A above depending on their final distribution. |
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Pattern C: High readings at base declining then increase — partially controlled rising damp, i.e. water still rising into the lower part of wall only, and increasing salt contamination above causes high readings to be obtained especially at the ‘salt band’. The pattern is sometimes misdiagnosed as a combination of rising and penetrating dampness. |
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Pattern D: High readings just above skirting or at very base of wall — typical of floor/wall junction problem: this may also lead to high readings along the perimeter of a solid floor. |
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Pattern E: Zero or very low readings the lower part of a wall, then a band of very high surface readings — usually occurs following damp-proofing and replastering works. The replastering is preventing water/salts in the underlying wall from passing to the surface (the surface is both dry and uncontaminated), but the damp-proof course is not effective — water/salts have risen above the new plasterwork. |
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See table below |
Pattern F: The damp proofers nightmare! -- No problem. |
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Pattern G: Steady decline of readings up external facing wall usually with a wallpapered or permeable finish - may be condensation or very high humidities present. Note: condensate will absorb into permeable materials for a time before dampness becomes visible. Long term severe condensation, however, usually results in mould growth, and other signs may be visible. In such cases one can lift a small area of wallpaper, scratch the exposed surface to about one millimetre in depth and check the cleaned area, or drill into the substrate and use insulated deep probes (resistance meters only); if zero or very low readings are obtained then the problem is surface orientated, e.g., condensation. Should condensation the thought to be a factor then it is advisable to undertake a proper evaluation (calculating dew point temperatures, relative humidity, etc.) for the presence or risk of condensation occurring, especially in the colder months of the year. It should also be appreciated that the risk of condensation is very low in the warmer months of the year, and at most times on party or dividing walls between rooms where temperature differentials between the rooms are usually insufficient to drop surface temperatures below the dew point. Thus, in general terms, should high surface readings be obtained during the warmer months then it is highly unlikely to be to condensation; rising damp or penetrating damp should be investigated, especially the former on party walls. |
Actual moisture meter readings providing patterns above:
Electrical moisture meters can also be used to plot isolated areas of dampness/hygroscopic salt contamination, for example, to evaluate the extent of penetrating dampness/water leaks, the latter usually being very localised. But do take care in coastal areas; over many years salt spray can be a problem, or salt water flooding can leave sea salt (sodium chloride) in walls — both can cause problems with meter readings.
Use surrounding timbers
One should also use timber to reflect dampness in the substrate. For example, timber skirtings in inhabited parts of buildings would not normally be expected to record moisture content in excess of 16 per cent (assuming no condensation/ maintenance of high relative humidities). If moisture contents are in excess of this figure then it is likely that there is some degree of dampness in the substrate. If such timbers are close to timber suspended floors, e.g., base of door post, skirtings, etc., then concern should be given to conditions beneath the floor.
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When using a moisture meter on skirtings, for example, test the top and bottom where they are in contact with the substrate. Why? wood doesn't transmit water readily across the grain and therefore testing the middle section of the skirting may not give true results since it is the thickest part of the skirting and may not be in direct contact with the underlying wall; this may not properly reflect moisture in the underside. The very top can usually be seen to be in direct contact and even if square edged skirting, the probes can be embedded very close to the substrate. |
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Please note that where high relative humidities are persistent, e.g. poorly vented subfloor areas, moisture contents in excess of 16 per cent can be obtained solely due to atmospheric moisture absorption by the wood. the figure below illustrates the relationship between relative humidity (RH) and the moisture content of wood. In this respect it is most important, of course, to appreciate that whether any dampness is manifestly visible or not, all wood in contact with that dampness will be at risk to rot; this cannot be over emphasised.
Relative humidity and moisture content of wood.
Conclusions:
To conclude this guide, and it is only a guide, the careful and intelligent use of an electrical moisture meter together with an appreciation of its limitations, coupled with the investigator’s skill and experience (and eyes!) should give the building professional a high probability of a correct damp diagnosis. However, occasionally situations arise, usually in disputes, where a definitive answer is required. This will necessitate evaluating the distribution of free and air dry moisture fractions in removed samples, and obtaining other site data pertaining to dampness; this usually requires laboratory and other specialist services.
© G.R.Coleman 1999