What you need to know
- A hygrometer measures air humidity; a material meter estimates moisture in compatible surfaces.
- Compare readings across similar areas instead of relying on one universal number.
- Persistent damp, mould or deteriorating finishes should be investigated before decoration.
Use your own measurements
Use the linked BricoDato resource to check these inputs in your own situation. Relative humidity changes with air temperature, while damp patches on a wall can also arise from leaks or cold surfaces.
Air humidity and wall moisture are different
A room can have normal relative humidity while one wall remains wet because of a leak or water ingress. Use the right measurement for the question you are asking.
Look for the cause
Note whether marks appear after rain, near plumbing, on cold corners or behind furniture. Condensation behaves differently from a plumbing leak or façade penetration.
Use measurements comparatively
Material meters vary by substrate. Take several readings and compare the suspect area with a dry area of the same material.
Know when to escalate
Large mould growth, repeated damage, salts, crumbling plaster or damp near electrical installations deserve professional diagnosis.
Use a short humidity log to identify patterns
Relative humidity changes with air temperature, while damp patches on a wall can also arise from leaks or cold surfaces. One humidity reading does not diagnose the source of moisture.
Example to adapt
Suppose a room reads 70% relative humidity on cold mornings but drops near 50% after ventilation and warming. Record humidity and temperature at the same hours for several days and note whether any wall patch stays wet. A persistent isolated wet area after rain suggests a different investigation from general window condensation.
| Observation | What to compare | Possible next action |
|---|---|---|
| Windows wet after cold nights | Room temperature and RH | Ventilation and thermal bridge review |
| Patch follows heavy rain | Weather and location | Inspect external water entry |
| Patch near pipe or fitting | Meter movement and plumbing | Check for leakage safely |
Checks before acting
- Place sensors away from direct steam, radiators and strong sunlight.
- Ventilate appropriately and control indoor moisture sources rather than relying only on a dehumidifier.
- Check whether plaster or timber is actually damp with suitable professional measurement when needed.
- Do not paint over persistent moisture or mould without resolving the cause.
Final check
Relative humidity is affected by temperature: a cooler room can show higher relative humidity without adding moisture. Make readings at repeatable times, away from showers or open windows, and compare suspected areas with unaffected ones. A damp wall reading may be influenced by material density, salts and the instrument method; it does not by itself distinguish condensation from a leak.
- Record date, location and conditions with each reading.
- Compare like-for-like areas.
- Identify the likely moisture source before redecorating.
- Seek professional help if damage persists or safety is involved.
Keep a small job sheet
Log RH, temperature, time, weather, room use and photographs of any visible moisture.
Turn the guide into a decision
Keep dated photos and readings with notes about rain, heating and ventilation. If the patch spreads after rainfall or plumbing use, investigate that cause before sealing the wall or buying a dehumidifier; call an appropriate professional for active ingress or electrical risk.