How to measure damp in a wall: moisture meter, hygrometer and what to record

A stain on a wall does not tell you whether the wall is wet now, only that it has been wet. A moisture meter helps you see how far the problem reaches and whether it is getting better, but it has to be read with care: the number on the screen is rarely a “percentage of water”. This guide explains the two kinds of meter, what the readings mean, how to measure the air in your home, a home foil test, and what to write down before asking for quotes.

Updated 4 November 2026. General information, not a substitute for a technician’s assessment.

The essentials

• There are two kinds of wall meter: pin (two metal pins touch the surface) and pinless (reads a few centimetres below the surface without drilling or scratching).
• The value is mainly for comparison: a suspect area against a dry area of the same wall and the same material. In different materials, the same number does not mean the same thing.
• Salts, metal and some materials (pipes, grilles, steel reinforcement) make the meter read higher than the water actually present.
• To find out about condensation, measure the relative humidity and temperature of the air with a thermo-hygrometer, several times a day.
• Measuring does not give the cause. It shows where it is damp and whether it is getting worse or better.

Two kinds of meter

	Pin (resistance)	Pinless (capacitance or radio frequency)
How it measures	Two pins touch or enter the surface	Reads below the surface without touching the water
Depth	Surface (or as far as the pins reach)	About 20 mm for one Protimeter model, as described by a reseller
Advantage	A value at an exact point	Quick, leaves no marks, reads through paint and paper
Care needed	Leaves small marks, especially on gloss paint	Does not replace a point confirmed with pins

What the numbers mean

There is no universal table saying “below X is dry”. Protimeter uses a reference scale, WME (wood moisture equivalent). The brand’s technical paper (2011) says any reading above 20% WME, in any material, is in the red zone and must be investigated. A reseller of the brand explains that the scale is a relative reference and that the display usually has three colour bands: green (dry), amber (borderline) and red (damp).

In pinless modes the reading is relative, on a scale from 0 to 999. The data sheet for one Protimeter model gives bands of dry 60 to 169, at risk 170 to 199 and wet 200 or more. Those bands belong to that device. Another brand uses other numbers, so always read the manual for your own meter.

Why not measure in “percentage of water”: Protimeter explains that the same amount of water gives different percentages depending on the weight of the material. Its examples: lime mortar is dry at 5%, cement mortar is already wet at 5%, and most plasters are wet at 1%. A percentage for a brick wall cannot be compared with that of the skirting board timber.

Practical summary:

1. Measure an area you know is dry (an interior wall, away from bathrooms) and write the value down. This is your reference.
2. Measure the suspect area in the same material and compare.
3. Repeat on different days. A value that falls over time is good news. One that rises or does not fall means the water source is still there.

When the reading misleads

• Salts. Salts left by rising damp, by old water ingress or by decades of leaks make electrical meters read high (Protimeter, 2011). Historic England researchers say the same: electrical resistance is also affected by salts, which creates ambiguity. See salt on walls.
• Metal and carbon. Pipes, nails, metal mesh, wires and the black in some wallpapers or blocks conduct electricity. Protimeter says that if you get high readings all over a wall, or only on the black areas of a wallpaper, the meter is not measuring water.
• Different materials. A paper by Historic England researchers (ICMB21 conference, 2021) advises against directly comparing readings from different materials, and another Historic England text says the meter is most reliable as a comparative technique.
• Shallow reach. A pinless meter can show zero on water that sits deeper. A low surface value does not guarantee a dry interior.

Measuring the air: thermo-hygrometer

Condensation comes from the air. A thermo-hygrometer measures relative humidity (RH) and temperature. It is cheap: the US EPA mentions devices at 10 to 50 dollars in hardware stores and recommends keeping relative humidity below 60%, ideally between 30% and 50%. The Swiss FOPH recommends 30% to 50% in the heating season.

How to use it:

1. Place it at mid-height, away from open windows, radiators and the shower.
2. Write down date, time, room, RH and temperature (Protimeter suggests a simple spreadsheet).
3. Do this morning, afternoon and night for several days. Condensation comes and goes, mostly at night and in winter.
4. When RH is high, check the surface temperature (a small probe or an infrared thermometer). If the wall is colder than the air’s dew point, condensation forms. The dew point is the temperature below which air releases its excess water vapour.

The same document says readings above 70% RH damage natural materials. For the link with mould, see bedroom condensation and ventilation and mould on walls.

The foil test

This is a 48-hour test that helps separate condensation from water coming through the wall. The summary by Selectra (UK, 4 November 2026), an energy comparison site and not a technical source, describes it like this:

1. Tape a square of kitchen foil about 30 cm across over the worst part of the stain, sealing all four edges.
2. For 48 hours, do not change your heating, ventilation and shower habits.
3. Peel back a corner and look at both faces of the foil and the wall.

Drops on the room side and a dry wall: condensation from indoor air. Drops behind the foil and a dark wall: the water comes from the wall (penetrating damp, rising damp or a leak). The test cannot tell these three apart. For pipe leaks see how to find a water leak in a wall. A dry result in mild weather is not conclusive.

What to record before asking for quotes

A technician can quote better when you have:

• dated photographs, close and wide, with a ruler beside the stain;
• a list of affected rooms and walls (outside, inside, next to a bathroom) and the height of the stain;
• the meter value (and model) at the suspect spot and at the reference spot, on different days;
• RH and air temperature of the room over a few days;
• whether it appears after rain, showers or heating;
• whether there is white salt, blistering paint or a musty smell;

For the types of problem, see damp walls: the four types. To decide whether a dehumidifier is worth it, see dehumidifier: when it helps. To estimate paint once the cause is fixed, use the calculator.

What we could not confirm

• A Portuguese standard with wall moisture limits for painting or for calling a wall “dry”. We did not find one.
• The values of your own meter. Each brand has its own scale. Use the manual.
• The foil test comes from an energy comparison site. We found no official study that validates it.

Sources (read on 4 November 2026)

• GE Measurement & Control / Protimeter, How to Measure Moisture in Buildings, document BR-200 Rev. A, May 2011, PDF: link. Read in full (WME scale, salts, different materials).
• Survey Express Services, Using a Protimeter Moisture Meter To Check Damp within a Building, 2015 (reseller), PDF. Read in full (pins, pinless, bands, RH).
• Protimeter Surveymaster data sheet, PDF, quoted through a search summary, not read directly (bands 60 to 169, 170 to 199, 200 or more).
• Historic England, A Comparative Evaluation of Methods to Monitor Moisture in Historic Porous Masonry Materials, report 2-2022, page, the IHBC Context article on moisture in porous materials and the ICMB21 paper (2021): quoted through search results, not read directly (the page returned a 403 error).
• U.S. EPA, A Brief Guide to Mold, Moisture and Your Home: relative humidity and meters.
• FOPH (Switzerland), Mould in living spaces, February 2021, quoted through our other guides, not re-read today (30% to 50%).
• Selectra, Damp walls, condensation and windows: the foil test, updated 4 November 2026.