If you engage three different people to investigate the same damp ceiling, you may well get three different answers — not because one of them is incompetent, but because they are measuring three genuinely different physical quantities and calling all of them "moisture".
Understanding what each instrument actually detects is the difference between a survey that finds a leak and a survey that produces a colourful report about a wet patch you already knew about.
Infrared thermography: it measures temperature, not water
This is the one most often misunderstood, including by people selling it.
A thermal camera detects infrared radiation and renders surface temperature. It cannot see water, cannot see through a surface, and cannot see inside a wall. What makes it useful on a moisture survey is indirect: evaporating water cools the surface it is evaporating from, and wet material has a different thermal mass from dry material, so it heats and cools at a different rate.
That produces a visible thermal pattern — and that pattern very often traces the shape of a leak far better than the visible stain does.
Where it is strong. Covering large areas fast. A whole ceiling, façade or roof can be scanned in minutes, revealing the extent and shape of an anomaly, which is what tells you which direction water came from.
Where it misleads. A cool patch is not necessarily wet. Air movement from a vent, a thermal bridge at a stud or beam, a shaded area, a recent repair with different material, insulation that has slumped — all produce the same signature. Equally, genuinely wet material with no evaporation happening (sealed under vinyl, for instance) may show nothing at all.
The other catch: it needs a temperature difference. With no thermal drive — a still, overcast, mild day with the building at equilibrium — there may be nothing to see. Survey timing matters, and anyone who tells you it does not is not paying attention.
Thermography is a targeting tool. It tells you where to point the instruments that actually measure water. Read more on what an infrared camera can and cannot resolve.
Moisture meters: they measure electrical properties near the surface
Two types, commonly confused.
Resistance (pin) meters pass a current between two probes and measure conductivity. Wet material conducts better than dry. Simple, cheap, quantitative, and the probes must penetrate — which means small holes in the finish.
Capacitance (pinless) meters apply a field to the surface and read the material's dielectric response over a shallow depth. Non-destructive and fast, and they are the ones commonly seen on television.
Where they are strong. Confirming, at a specific point, that a surface really is damp. Cheap, immediate, and a good sanity check on a thermal anomaly.
Where they mislead. Resistance meters respond to anything conductive. Salts in masonry, efflorescence, foil-backed plasterboard and metal fixings will all read as very wet material. Capacitance meters read a shallow layer and are affected by density, thickness and surface finish, so the same real moisture level reads differently on render than on plasterboard.
And critically: neither one tells you how deep the water is. A surface reading of "wet" is compatible with a thin surface film and with a saturated slab. Those two findings lead to completely different repairs and completely different costs.
Microwave reflection moisture mapping: it measures water through the build-up
A microwave sensing head emits a low-power field into the element and measures how that field returns. Water has a far stronger dielectric response than concrete, render, screed, brick or insulation, so the change in the returned signal is driven overwhelmingly by water content — largely independent of what the material itself is made of.
Different heads reach different depths, so the same area can be surveyed at several depths and the moisture distribution built up layer by layer, to depths up to around 300 mm.
Where it is strong. Answering the question that decides the scope of a repair: is the water in the tile bed, in the screed, or in the slab? Also excellent on materials that defeat conventional meters — high-salt masonry, dense render, insulated build-ups.
Where it needs care. Reinforcement, embedded services and voids affect readings, so the survey has to be interpreted against what is known of the construction rather than read off a screen. It is a specialist instrument and a specialist interpretation.
There is more detail in our article on microwave reflection moisture mapping.
Side by side
| Infrared thermography | Moisture meter | Microwave mapping | |
|---|---|---|---|
| Measures | Surface temperature | Electrical properties at/near surface | Water content through the material |
| Depth | Surface only | 0–20 mm typical | Up to ~300 mm, layer by layer |
| Destructive | No | Pin type makes small holes | No |
| Speed over large areas | Very fast | Slow, point by point | Moderate |
| Main false positive | Thermal bridges, draughts, shadows | Salts, foil backing, metal | Reinforcement, voids |
| Answers | Where to look | Is this point damp | How much, and how deep |
Why a credible survey uses all three
Each instrument covers the others' weaknesses.
Thermography finds the pattern across a whole ceiling in minutes and shows which direction the water travelled. A meter confirms, at chosen points, that the anomaly really is moisture rather than a draught or a beam. Microwave mapping then establishes how deep the water sits and how far it extends through the build-up — which is what determines whether the repair is a localised patch or a full strip and re-membrane.
A survey using only one of them is not necessarily wrong. It is just answering one third of the question, and the missing two thirds are usually the expensive ones.
What this means when you are choosing a provider
Ask what instruments will be used and what each one is for. A specific answer is a good sign. "We use a thermal camera" on its own, for a leak investigation that will inform a repair scope or an insurance claim, is not enough — thermography alone cannot confirm moisture, and a report built on it can be challenged on exactly that basis.
Ask, too, what happens after the finding. Because we hold a QBCC waterproofing licence, the crew that identifies the defect can also rectify it — which removes the gap where an investigator blames the repairer and the repairer blames the investigator, and the owner pays for both.
Tell us what you are seeing and we will tell you which combination of methods will settle it.