The two kinds of meter, and what they really measure
| Pin / conductance meter | Radio-frequency / capacitance meter | |
|---|---|---|
| Method | Electrical resistance between two pins | Reads a field a few cm into the wall |
| Measures | Surface and just below | Sub-surface, non-destructively |
| Marks the wall | Yes, two small holes | No |
| Fooled by | Salts, foil, metal, carbon in paint | Density changes, pipes, mesh |
| Best for | Comparing points on one material | Mapping a pattern across an area |
Neither instrument measures moisture directly. Both infer it from electrical properties — which is exactly why hygroscopic salts in the plaster can produce an alarming reading on a wall that is not especially wet.
How to read a wall properly
- Take readings at several heights up the same vertical line — roughly 100mm, 500mm, 1m, 1.5m.
- Repeat on two or three verticals across the elevation.
- Take a control reading on an internal wall in the same room that you have no reason to suspect.
- Note the material at each point. Plaster, brick and concrete are not comparable to each other on the same scale.
- Look at the shape of the numbers, not the numbers themselves.
Falling as you go up, with a control that reads normal, is consistent with rising damp. Highest in the middle of the wall points at penetrating damp through a defect. Uniformly raised everywhere including the control suggests a humidity problem, not a wall problem. A sharp localised spike is usually plumbing.
Where meters mislead
- Salts read as water. The most common false positive by far, and the reason efflorescence has to be assessed alongside the readings rather than separately.
- Foil-backed plasterboard, metal lath, conduit and mesh can all peg a meter regardless of moisture.
- Some paints contain carbon and conduct.
- Surface condensation gives a high surface reading on a wall that is dry 20mm in.
This is why a defensible diagnosis pairs meter readings with what the wall actually looks like — the height and edge of the patch, whether there are salts, and how it behaves with the weather. The readings support the diagnosis; they are not the diagnosis. That logic is laid out on the four types of damp.
Readings only tell you how wet the wall is, not why. Once the pattern points at capillary rise from a failed damp-proof course, the treatment is damp proofing in Cape Town.
Related reading
Start with The Four Types of Damp for the overview, or:
- Musty Smell in the House
- What a Damp Survey Should Actually Cover
- Water Ingress Explained
- Efflorescence and Wall Salts
Common questions
What is a normal moisture reading for a wall?
It depends entirely on the material and the meter, which is why absolute numbers are close to meaningless out of context. What matters is the comparison — the affected area against a control point on the same material in the same room.
Can a damp meter tell the difference between damp and salts?
Not on its own. Hygroscopic salts hold moisture and conduct, so they read high. Distinguishing them needs the visual evidence — a level tide mark and a white bloom — or a laboratory analysis of a drilled sample.
How long does a wall take to dry?
Roughly one month per 25mm of thickness once the source is stopped, so a 230mm solid wall takes the better part of a year. It can be replastered and decorated well before that with the correct salt-inhibiting render.