Damp meters, or Moisture meters come in all different shapes and sizes. Some are the traditional 'resisitive' ones, where you shove sharp metal prongs into the material to be measured (my ping prong meters!), others are the 'electrostatic' types, which create an electric field in material being measured and calculate moisture content from that. There are also ones that use radio frequencies. They all have their good and bad points but, of much more interest to property owners, are the people who use, or, in the main, misuse them.
A judge in Wales concluded that looking for moisture in a property without using a moisture meter, was 'a bad thing'! When a judge gives a judgement, the rest of the law industry take it as gospel and, unless it's appealed at a later date, it becomes a precedent. So, since then, if you don't use a moisture meter when you're looking for dampness - you're a naughty person. Unfortunately the damp industry have taken that judgment literally, and a moisture meter is the only thing they use - great for selling chemicals.
This applies to most chartered surveyors, who carry out building society surveys, and also to timber and damp contractors, who carry out thousands of fraudulent and incompetent surveys!
Electronic [resistive] moisture meters are designed to give a good indication of the moisture content of timber. I'll say that again - timber! They're calibrated to do this between 7%, up to about 30% moisture content. They're quite accurate, if corrected for timber species and are relied upon by the timber industry in their day-to-day activities.
They are able to perform this task because the conductivity of timber is reasonably constant throughout the different species and is proportional to the moisture content. There are exceptions to the rules but these can be catered for by use of divergency tables. For those interested, a piece of Douglas Fir with moisture content of 7% will have a resistance of approximately 22,400 Megohms - yes Megohms! When it gets to 25% moisture content that comes down to around one half a meg.
But even one half a megohm is substantial resistance and even the slightest amount of moisture in a masonry wall will have a resistance a lot less than that. Consequently you get full-scale deflection, in the shape of red lights and beeps even with only minor instances of moisture! Great for selling injection damp proofing.
So here lies the problem. No-one has worked out the conductivity/resistivity features of a masonry wall. You cannot - there are variations in each one, caused by factors such as the molecular structure of the materials in the wall (lime plaster, cement, gypsum plaster, brick, sandstone, granite, clay...) and even the salt content of the water used to build it!
You cannot deduce the moisture content of a masonry wall by the use of a moisture meter! You can't even guess at it! Don't even try!
A moisture meter can indicate that moisture may be present in your wall but it can't tell you if that moisture is in sufficient quantities to require further consideration; and the lights and beeps are the Red Herring used by the damp industry to con you into thinking you have significant problems.
There is one thing in its favour though: if the wall is actually dry, then it can tell you that quite truthfully; because if it gives a zero reading it means there aren't any water molecules available to allow current to flow.
Surely, if something has moisture in it, it's damp and needs treatment? Not necessarily, most things have a degree of moisture in them. Look at an example: timbers, such as skirting boards etc in a centrally heated house can have moisture content up to 4 or 5 per cent less than those in a house without central heating. But that doesn't mean to say the timbers in an unheated house are in jeopardy, they just happen to reside in a less dry atmosphere, and have therefore absorbed more moisture from the air. Unless their moisture content rises above 20%, they're not in any danger at all.
The individual moisture content of a host of materials: walls, skirtings, joists, etc, recorded by moisture meters, may be higher in some houses than in others; but, if it isn't causing any problems, and further investigations show it isn't likely to do so in the future, why indulge in expensive remedial treatments. Especially those that probably won't work anyway!
Similarly, there are lots of masonry walls that suffer from condensation problems, which can light all the lights and ring all the bells on the meter. But indications on a meter are often all they are; nothing to indicate there's a dangerous degree of moisture present: no deterioration of wall surfaces; and no moisture content above danger point in timbers in contact with it.
Remember: 'Dampness' is present to a degree in lots of materials, and it isn't a problem until it causes problems: a high reading on a moisture meter doesn't alter that truism. Next time you see an 'expert' using one of these things, to the exclusion of common sense and visual observations, beware. Your house just might be in danger of drilling and filling for no better reason than a few over-excited ohms!
The only truly accurate way to measure moisture in a wall is to drill out a sample of the wall, and put it in a carbide chamber - moisture in the sample reacts with calcium carbide in the chamber and produces acetylene gas - pressurising it. The weight percent of water in the sample, expressed as a percentage of the wet sample weight, is shown on the meter. Very simple, quick and easy to use - and definitive - courts love carbide samples - they hate moisture meters!
