What the meter measures
The hand-held meters used across the industry measure electrical resistance (or capacitance) between two points and translate it into a moisture reading. They are genuinely useful for mapping where moisture is and how it is distributed. What they cannot do is tell you where the moisture came from — and the source is the entire question.
Why readings mislead
Several things push a meter high without any structural damp problem:
- Surface condensation on a cold wall
- Hygroscopic salts in old plaster, which conduct electricity and inflate readings
- Materials the meter is not calibrated for — foil-backed plasterboard, dense renders, some masonry
- Recent redecoration or surface wetting
The rising damp trap
A common pattern in the industry: a high meter reading at the base of a wall is presented as proof of rising damp, and a chemical damp-proof course is quoted on the spot. But condensation, bridging, leaks and salts all produce exactly the same reading in exactly the same place. The meter confirmed moisture. It said nothing about rising damp.
How readings should be used
In a proper survey, meter readings are one input among several: the moisture pattern up and across the wall, the construction and any damp-proof course, external ground levels and defects, internal humidity and ventilation, and salt analysis where the cause needs confirming. Readings interpreted in context inform a diagnosis; readings alone are just numbers.
Need this looked at properly?
If this sounds like your property, a proper investigation will tell you which cause you're actually dealing with.