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No moisture meter identifies mould, and a pinless meter on an exterior wall is reporting the dielectric response of everything within roughly 19 mm of its pad, which includes water but also salts, foil facings, mesh, framing, fixings and simple changes in material density. A high reading tells you that volume is electrically different from the dry area you compared it against, and nothing more. Scoping the cavity shows you what is actually there; the meter only tells you where to put the camera.
What a pinless meter is responding to on a wall
A pinless meter is a capacitance device. Two electrodes behind the rubber pad create a weak radio frequency field that penetrates the material, and the instrument measures how that field is loaded. Water loads it heavily, because liquid water has a relative permittivity around 80 while dry timber sits between about 2 and 5 and air is 1. That enormous contrast is what makes the technique work at all, and it is also why anything else with a high dielectric constant or any conductor produces the same deflection.
Two properties of the measurement follow directly and both matter on walls. It is a volume measurement, weighted towards the surface, so the reading is a blend rather than a value at a depth. And it requires continuous flat contact, because an air gap under part of the pad removes part of the field. On a textured render, a coarse brick or a wall with a slight bow, poor contact alone moves the number.
The published figures describe exactly this. The Proster pinless meter states detection up to 19 mm below the surface, with selectable material ranges of 5 to 70 percent for wall and masonry and 5 to 75 percent for softwood and hardwood, plus a three colour bar graph and rising audible tones to help find the most concentrated area. The General Tools MMD7NP is clearer still about what the scales mean: it states 0 to 53 percent for softwood, 0 to 35 percent for hardwood and relative readings for wallboard, drywall and masonry. An accuracy figure for those relative scales is not stated in the listing bullets of either unit, and no meaningful one could be, because a relative scale has no reference to be accurate against. That word relative is the single most important thing on either listing. On your exterior wall you are reading an arbitrary scale, so the only meaningful comparison is against another spot on the same material. Why the same number means different things on different substrates is covered in our page on moisture meter readings varying by material.
Why mould is outside the instrument’s reach
Mould is an organism. Identifying it means seeing hyphae and spores, or culturing a sample, or sending tape lifts and air samples to a laboratory. Nothing about that process is electrical, and no change in the dielectric constant of a wall distinguishes a sterile wet patch from a colonised one. What a meter can do is find the precondition: moulds need liquid water or a high enough surface water activity for long enough, and that is a moisture and temperature problem you can survey.
This is worth stating plainly because listings blur it. The General Tools bullets include a heading that reads MOLD DETECTOR, and the sentence underneath describes checking suspicious water stains around the house to determine whether an area is wet or has already dried. The sentence is accurate and the heading is not, and the gap between them is the whole subject of our pillar page on whether a moisture meter can detect mould. If you have scoped the cavity and seen discoloured growth, you have a visual identification, which the meter cannot give you and did not claim to.
The false positives that live in exterior wall assemblies
Exterior walls are the worst case for a capacitance survey because they contain more conductive and high permittivity material than any interior partition. Each of these produces a high reading on dry construction.
| What the pad is over | Why the reading climbs | How to rule it out |
|---|---|---|
| Foil faced insulation or a foil vapour layer | A continuous conductor loads the field completely | Reading pegs at maximum over a wide area with sharp edges at the board joints |
| Mesh or lath under render | Metal grid behaves as a conductive plane | High readings repeat on a regular grid pitch |
| Steel or timber framing, nail plates, screws | Local conductor directly under the pad | Narrow stripes at regular centres, unchanged over days |
| Embedded conduit or cable | Conductor plus a dielectric sheath | A line, not a patch, often running vertically to a socket |
| Salts and efflorescence on masonry | Hygroscopic salts hold water and conduct | Visible white bloom, readings high even when the surface feels dry |
| Denser material in the same wall | Dense substrates read higher on the same scale | Readings change at a material boundary, not a gradient |
| Surface condensation or recent wetting | Free water right at the pad, where weighting is strongest | Dries out within hours and returns on cold mornings |
Only the last of those is a moisture problem, and it is a different problem from a leak. That distinction is why a psychrometer earns its place in a damp survey: the Triplett MS475 states pin and pinless moisture measurement alongside a humidity and temperature probe reporting relative humidity, air temperature, mixing ratio, vapour pressure and dew point, plus an infrared thermometer with a laser pointer, an 8 to 1 distance to target ratio and a fixed emissivity of 0.95. With dew point and surface temperature in the same hand, condensation becomes a calculation instead of a guess: if the wall surface is colder than the room dew point, water will form there regardless of any leak. The 8 to 1 ratio also tells you how large the spot is, since at 600 mm from the wall the instrument is averaging a circle roughly 75 mm across, not reading a point.
Building a comparison instead of reading one number
A single reading is uninterpretable on masonry and plaster. A survey is interpretable, and it takes about twenty minutes.
- Select the correct material mode and keep it fixed for the whole survey. Switching ranges mid wall makes the numbers incomparable.
- Find a dry reference on the same material, same construction and same orientation, as far from the suspect area as the wall allows. That reference is your zero.
- Work a grid, roughly hand width spacing, and write the numbers on the wall in pencil or on a sketch. The pattern is the data, not any single value.
- Map the edges. A diffuse gradient with a centre suggests water. A sharp rectangle or a repeating stripe suggests the building.
- Repeat after rain, and again after a dry spell. Water responds to weather. Framing does not.
- Where timber is exposed, confirm with pins. A pin reading in a joist end gives an actual moisture content on a species calibrated scale, as described in our guide to using a moisture meter on wood.
Thermal imaging makes the mapping step much faster, because evaporative cooling and the different thermal mass of wet material show as patterns rather than as point readings. The FLIR MR160 builds that into the meter: it states an 80 by 60 thermal imager that guides you to the location, a pinless sensor for rapid scanning and an external pin probe for verification, so the image chooses where the probe goes. That order of operations is the correct one, and it is the same reason our comparison of pin and pinless meters treats them as a sequence rather than a choice: scan wide with the pinless pad, confirm narrow with pins, and use an image to decide where narrow means.
What a confirmed wet area needs next
Once the survey is repeatable and weather responsive, the question changes from whether the wall is wet to where the water comes from. Work from the outside in: roof edge and flashing, parapet, sill, cill drip, pointing, a blocked cavity tray, a downpipe discharging onto render, ground level above the damp proof course. Inside, check for the plumbing route and for any cold bridge that explains condensation in that exact spot.
If you have seen growth with the camera, the instrument question is already answered and the remaining work is identification and remediation, which means a sample rather than a reading. Keep your grid sketch and your dated numbers either way. A map showing the area drying after a repair is the only evidence that the repair worked, and it is evidence a single high reading on an exterior wall could never provide.
Frequently Asked Questions
Can any moisture meter detect mould?
No. Moisture meters respond to electrical properties, either resistance between two pins or the dielectric constant of a volume beneath a pad. Mould is a living organism identified by sight, by microscopy or by sampling and culture. A meter maps the sustained dampness that mould needs, which is a genuinely useful thing to map, but the instrument cannot tell a damp clean wall from a damp colonised one.
Why does one listing call the meter a mould detector then?
Because the phrase sells. Read the sentence under the heading and it usually describes checking suspicious water stains to see whether an area is still wet or has dried, which is a moisture question with a moisture answer. That is the honest claim; the heading above it is not.
What depth does a pinless meter actually read?
The field extends roughly 19 mm into the material on typical handheld units, and it is a weighted average rather than a measurement at a depth. Material nearest the pad dominates, so a damp surface skim over dry brick reads higher than a dry skim over damp brick, even though the second is the more serious condition.
Are the percentage figures on masonry real moisture content?
No. Scales for wood are calibrated against oven dry mass for particular species groups. Readings on drywall, plaster, render and masonry are relative indications on an arbitrary scale, which is why listings describe them as relative rather than quoting a percentage. Compare them with each other, never with a published wood threshold.
How do I tell metal from water without opening the wall?
Look at the shape of the high area. Framing, mesh and fixings give sharp edged, repeating patterns that line up with stud or batten spacing and do not change over days. Water gives a diffuse area with a gradient, a centre and an edge, and it moves after rain or heating. A thermal image sharpens this further because wet material has a different thermal signature from a metal fixing.
Does a high reading mean the wall needs opening up?
Not on its own. Establish a dry reference on the same material first, repeat the survey over several days and after rain, and check the exterior above the area for the plausible source. If the pattern is diffuse, repeatable and responds to weather, invasive investigation is justified. If it is sharp and static, you have most likely found the building, not a leak.