A moisture meter measures how wet a material is. Pin meters read electrical resistance between two probes pushed into the material — wetter material conducts better. Pinless meters use a radio-frequency field to sense moisture in the top few centimetres without marking the surface. You use one to map where a wall is wet, compare against a known-dry area, and track drying over time. It is what tells you whether gypsum can be saved or must be removed.
Key takeaways
- Pin meters read resistance between two probes; pinless meters scan below the surface.
- Readings are comparative — always check a known-dry reference area.
- They map the extent of a damp area, not just whether it is wet.
- Drying is confirmed by readings coming down and matching dry reference, not by touch.
Pin vs pinless meters
Pin (resistance) meters have two sharp probes. Pushed into gypsum or timber, they measure electrical resistance between the tips — more moisture means lower resistance and a higher reading. They give a localised reading at a specific depth and leave two small holes. Pinless (capacitance or RF) meters have a flat sensor plate held against the surface; they read average moisture in roughly the top 20–40 mm over a small area without marking anything, which makes them ideal for quick scanning.
What the readings mean
Meters give either a percentage moisture content (calibrated for a specific material) or a relative scale. The absolute number matters less than the comparison: a reading on the suspect area versus a reading on the same material somewhere you know is dry. A wet patch might read two or three times the dry reference. Different materials hold different “normal” moisture, so you compare like with like.
Using one on a damp wall
- Take a reference reading on the same wall material well away from the problem.
- Scan outward from the visible damp or stain in a grid, marking where readings are elevated.
- You will usually find the wet area is larger than the visible mark — that hidden margin is where growth spreads.
- Re-measure the same points every few days during drying; you want them falling towards the reference.
What it decides
Whether a material can be saved. Lightly and briefly damp painted gypsum that dries back to reference can often be cleaned; board that stays high after extended drying, or reads high on the paper itself, generally needs removing. Readings also tell you when it is safe to close a wall up and redecorate — not before. See mold on gypsum walls.
“It feels dry” is not a measurement. A painted surface can feel dry while the board behind it is still holding water — only a meter reading tells you.