Module 1 · Moisture, Air and Materials

1.6 What a Moisture Meter Actually Measures

9 min read

The electrical moisture meter is the instrument most associated with damp surveying and the one most often misused. It does not measure moisture. It measures an electrical property, calibrated against a moisture content in one specific material, and displays a number on a scale that means something in wood and nothing directly in masonry. Everything in this module comes together here: knowing what the instrument responds to is what lets you use it well rather than abandon it.

Learning objectives

By the end of this lesson you will be able to:

State what resistance and capacitance meters physically measure, and what calibration they carry.
Explain why salts and conductive materials produce high readings independently of moisture.
Use a meter appropriately as a comparative survey tool rather than as a measurement.
Describe a meter reading in a report in terms that will survive challenge.

Key terms

Resistance meter: Measures electrical resistance between two pins. Resistance falls steeply with moisture in wood, and equally steeply with dissolved salt content in anything.
Capacitance meter: Measures the dielectric properties of material beneath a pad, without pins. Non-destructive, reads a shallow depth, and responds to anything with a high dielectric constant — including foil, plaster additives and metal.
Wood moisture equivalent (WME): The moisture content of wood that would produce the observed reading. The correct name for what a resistance meter shows on any non-wood material.
Comparative survey: Using an instrument to find where readings differ across a surface, rather than to establish an absolute value at any one point.

A resistance meter passes a small current between two pins and infers moisture from the conductance. The calibration is for wood, because in wood the relationship between moisture content and conductance is strong, repeatable and well characterised.

In masonry that relationship does not hold. Conductance in masonry is dominated by dissolved ionic salts — and as 1.4 established, an old damp wall is full of them. A salt-contaminated plaster that is holding modest hygroscopic moisture will read at the top of the scale, while a genuinely saturated but salt-free material may read much lower.

This is not a reason to put the meter away. It is a reason to describe what it gives you accurately: it finds where the wall differs from itself. That is a genuinely valuable thing to know, and it is what directs the sampling that will actually answer the question.

Using a meter well

Survey comparatively. Take many readings across a surface and look for the pattern and the boundaries, not the value at any single point.
Record the pattern, not the peak. Where the readings change is diagnostic; the highest number on the wall is not.
Use the meter to decide where to drill. Its job is to direct the sampling that will answer the question, and that is a job it does well.
Read a capacitance meter as a shallow, non-destructive scan and treat anything metallic behind the surface — foil-backed board, lath, conduit, mesh — as a false positive until proven otherwise.
Calibrate against a known-dry control area of the same construction on the same visit.

Writing it up so it holds

A report that says "moisture readings of 25% were recorded to the base of the wall" is making a claim the instrument cannot support, and it will not survive a competent challenge. Two forms that will:

"Elevated wood moisture equivalent readings, up to a maximum of the instrument's scale, were recorded across the lower 900mm of the wall, falling to background above that line."
"Comparative readings indicated a defined boundary at approximately 900mm. Gravimetric samples were taken at three depths at [location] to establish actual moisture content and hygroscopic content; results are at [section]."

The second is the professional standard because it does two things: it reports what the meter can support, and it shows that the question the meter cannot answer was then answered by something that can. Module 8 develops the reporting language; the discipline starts here.

Where this leaves the instrument

There is a fashion for dismissing moisture meters entirely, and it overcorrects. The instrument is fast, non-destructive, cheap and genuinely informative about where a wall differs from itself — which is exactly what you need in the first twenty minutes of a survey.

What it cannot do is establish a moisture content in masonry, distinguish salt from water, or tell you which mechanism produced a reading. Those require the gravimetric and salt methods from 1.4 and 1.5.

The competence being tested here, and in the assessment that follows, is not whether you own the right instrument. It is whether you can say precisely what each of your instruments does and does not establish — because that is what the report has to survive on, and it is what the rest of this course is built on.

Common mistakes to avoid

Reporting a resistance meter reading on masonry as a percentage moisture content. The scale is wood moisture equivalent and the material is not wood.
Concluding rising damp from a meter reading with a boundary, without gravimetric and salt work. The boundary is real; its cause is not established by the instrument that found it.
Treating a capacitance meter reading as a depth measurement. It reads shallow, and it responds to foil, mesh and conduit as readily as to water.
Surveying for the highest number instead of for the pattern. The boundary is the evidence; the peak is a number on a scale.
Failing to take a control reading in the same construction on the same visit, leaving the readings with nothing to be compared against.

Lesson summary

An electrical moisture meter measures conductance or dielectric response, calibrated for wood, and in masonry it responds to dissolved salts as strongly as to water — so its scale is wood moisture equivalent, not a moisture content. Used comparatively it is excellent at finding where a wall differs from itself and at directing sampling; used as a measurement it produces claims that will not survive challenge. Report what it can support, then answer the remaining question gravimetrically.

Module 2 takes this physics into real construction: how solid walls, cavity walls, timber frame and system-built housing each handle moisture, and where each characteristically fails.