how to measure humidity home walls limit values

How to measure humidity at home and in the walls

Air relative humidity and moisture in the wall are not the same number. The EN ISO 13788 mould threshold (about 80% at the surface) and how to use a hygrometer and a moisture meter.

Updated 2026-09-25 · 9 min read

Two different measurements are both called “humidity”. A hygrometer reads relative humidity of the air. A contact meter reads, roughly, how wet the material is. Mixing them up is the usual way to diagnose badly.

Air: relative humidity and dew point

Measure in the middle of the room, away from a radiator, an open window and a shower just taken. A thermo-hygrometer gives temperature and RH together: dew point needs both. CIBSE Guide A uses design comfort ranges, not a legal duty: a band often watched in occupied rooms is about 40–60% RH. If the air stays wetter for long, cold surfaces (glass, reveal, behind a wardrobe) move toward condensation.

Wall: a comparison, not a universal percentage

A pinless meter does not measure air RH and does not have a “12% = wet” that holds for every brick and every plaster. Use it like this: same wall, low and a metre up; stained versus sound; after rain and days later. If the bottom is clearly higher than the rest, and there are salts or crumbling, the track is rising damp or a leak, not the glass. Absolute numbers stay rough.

What not to mix up

60% air RH does not mean the wall is 60%.
The ISO 13788 80% is on the cold surface in the calculation, not on the hygrometer in the middle of the room.
Dew point is a temperature (°C), not a percentage.
None of these numbers certifies a building or replaces a surveyor if plaster crumbles or the stain follows rain.

Calculate dew point with your numbers Compare the surface with the 80% threshold

FAQ

What is the “right” indoor relative humidity?+

There is no domestic legal limit IgroCasa can cite as an obligation. As a design comfort reference, CIBSE Guide A discusses humidity ranges: in practice a band around 40–60% RH is often watched. Above that, for long, cold surfaces are more likely to condense. A reference, not a certificate for your house.

Is 80% the air or the wall?+

In EN ISO 13788, 80% is a conventional surface relative-humidity threshold used in design calculations for mould risk. It is not the hygrometer in the middle of the room and not the percentage on a contact meter.

Does a wall meter prove rising damp?+

It compares zones: skirting versus a metre up, stain versus sound plaster. The absolute number depends on the material and the instrument scale. It is not a survey.

What may help (after diagnosis)

Some links go to Amazon.it and are affiliate links: we may earn a commission, at no extra cost. They are category examples, not lab reviews. Amazon.com is not connected on the English edition.

12–40 €

TempPro TP53

Thermo-hygrometer (T + RH together), category example. Dew point needs both numbers. Not a lab review.

When: When you want the calculators on your numbers, not a guess.

Not when: It does not measure moisture in the wall.

View on Amazon.it

Amazon.it affiliate link (tag qordac21-21): we may earn a commission, at no extra cost to you. A category example, not a lab review. Amazon.com is not used.

25–45 €

Pinless wall moisture meter (e.g. RDINSCOS)

Example material moisture meter, no drilling. It helps compare “damp air” and “damp wall”: low vs high, stained vs sound. Absolute numbers are rough. Not a survey instrument.

When: Suspected rising damp or a leak; you want to see whether it starts at the skirting.

Not when: It does not certify a building, does not measure air RH, and does not replace a surveyor if there are salts, crumbling plaster or stains after rain.