Is the Wall Damp, or Just Cold?

A meter reading on a plaster wall is not a moisture percentage.

Pin-type moisture meters measure electrical conductivity, and they are calibrated for timber. On plaster and masonry, conductivity is affected by salts — and a wall that was damp years ago can read high while being perfectly dry, because the salts left behind conduct.

Which matters, because the two things people are trying to tell apart have completely different fixes. Condensation on a cold wall is ventilation and heating. Damp arriving through the structure is a building defect. Buying a dehumidifier for the second one is money spent on a symptom.

A note on this guide: this rests on how these instruments work and how the two problems present, not on meters tested here. Persistent or spreading damp needs a professional survey — nothing here substitutes for one. See how we test.

The problem, as it appears

A dark patch, a musty smell, or mould in a corner. A cheap meter says the wall is wet. It is not obvious whether that means a leak, rising damp, or simply a cold wall in a flat that does not ventilate.

Why the obvious assumption disappoints

The number is not what it looks like. A meter showing a high figure on plaster is telling you the surface conducts. Salts conduct. Foil-backed plasterboard conducts. Some paints conduct.

And the most common cause is not damp at all. In flats, the overwhelming majority of what gets called damp is condensation — warm moist air meeting a cold surface. That is not water coming in; it is water already in the room.

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What actually matters

1. Measure the air first

Relative humidity is the variable that drives condensation, and it is the one you can act on. If the room sits consistently above 60%, the wall is a symptom.

2. The foil test, which costs nothing and settles it

Tape foil or plastic tightly to the affected area and leave it several days.

• Moisture on the room side — condensation. The water is coming out of the air.
• Moisture underneath, against the plaster — the water is coming through the wall.

This answers the question a meter cannot, and it is free.

3. What a meter is genuinely good for

Comparing. Not an absolute figure, but the difference between an affected area and a known-dry one on the same wall, taken the same way. A relative survey finds the wettest point and the direction the problem runs in.

On timber — a floor, a window frame, a joist — it does what it was designed for.

4. Reading the pattern, which is better evidence than any instrument

Condensation: worst in winter, on the coldest surfaces, in corners and behind furniture, on outside walls, often with black spot mould.

Penetrating damp: tracks a defect — a leaking gutter, cracked render, a bridged cavity. Often persists in dry spells and is worst after rain.

Rising damp: a tide mark at a consistent height, salts on the surface, at the base of a wall. Far rarer than it is diagnosed.

If it turns out to be condensation

The fix is the moisture balance in the room, not the wall.

The full argument, including why warming the glass without reducing the moisture makes things worse, is in window condensation in a flat.

Where the free option is genuinely enough

The foil test. More diagnostic value than a meter, at no cost.

Looking outside after rain. A blocked gutter, a cracked downpipe or ground level above the damp course explains a great many internal patches, and all of them are visible from the garden.

Moving furniture off the wall and ventilating, then waiting a fortnight. If it improves, it was condensation.

The mistake to avoid

Treating a high meter reading as proof of damp and buying a fix for it. The instrument is measuring conductivity, the salts in an old wall conduct, and people spend serious money on the strength of a number that was never a moisture percentage.

The second mistake is running a dehumidifier against a leak. It will hold the symptom down indefinitely and the water will keep arriving. The same measure-before-you-buy argument, applied to a garage, is in how damp is your garage — measure first.