Moisture meters and thermal cameras: what they can and can't see

A damp inspection uses two main instruments: a moisture meter and a thermal camera. Both are useful. Neither sees through walls, and neither gives a number that means "this wall is 12 percent water". Knowing what they actually measure helps you read any damp report, including ours.

Pin moisture meters

A pin meter has two metal pins that are pressed into the surface. It measures electrical resistance between them. Wet material conducts better, so resistance falls and the reading rises. The pins leave two small holes in plaster, usually hidden by a dab of filler.

These meters are calibrated for wood or a reference material. On cement plaster, brick and concrete, the number is comparative, not a true moisture content. Salts left by efflorescence also conduct, so a salty wall can read high even when dry. The useful question is not what the number is, but how much higher it is than a dry wall in the same flat.

Pinless meters

A pinless meter is placed flat on the surface and senses moisture a short way into the material, without making holes. It is quick for sweeping a whole wall and finding where readings rise. Like the pin meter, it gives comparative readings on masonry, and it is affected by what is behind the surface: tiles, metal and dense stone change what it reads.

Thermal cameras

A thermal camera shows surface temperature as colour. A damp surface is often cooler than the dry wall around it, because water evaporating from it takes heat away. That makes a wet patch behind paint show up as a cool shape, often before any stain appears.

But many other things make a cool shape: a cold water pipe, an air gap behind plaster, a draught from a window frame, a reflection, or a wall that was shaded until recently. A thermal image on its own is a lead, not a finding. The camera also needs a temperature difference to work with. When indoor and outdoor temperatures are close, or the sun has just been on a wall, patterns are weaker or misleading. That is why a report should record the conditions on the day.

Why both, and why a dry reference

The method that works is to combine them. The inspector first takes readings on a wall known to be dry in the same flat. Every suspect area is then compared against that reference. A cool area on the thermal camera is called damp only if the meter confirms higher readings there than on the dry wall. The how we inspect page explains the full method.

What neither can do

• See inside a wall or slab. Both work at or near the surface.
• Read reliably through tiles, stone or wood panelling.
• Reach walls behind fitted wardrobes, kitchen units or heavy furniture.
• Find a seasonal leak that has dried out completely before the visit.
• Tell you the source on their own. The source is an opinion based on the shape and position of the wet area, what is above and beside it, the pipe routes and the recent weather.

A good report says what could not be reached, as well as what was found. If a report lists no limits at all, ask why. The sample report shows what a limits section looks like.

Condensation can wet a cold wall surface without any leak behind it. See condensation for how to tell the two apart.

The Megahedge report puts the photo, the thermal image and the reading against the dry reference side by side for every finding.