Damp diagnosis
How to find the cause of excess humidity
Misted windows, dark corners and a musty smell have different causes. Work out where the moisture comes from first; then you will know what to fix and whether you need a dehumidifier.
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The short answer
For a week, note humidity, temperature and events: showers, cooking, drying laundry, rain. Then check the five usual sources: cold glass, cold corners, leaks, household steam and extraction. A dehumidifier removes water from the air but will not fix a pipe, a roof or a cold bridge. Deal with the cause first, then check the same spots again.
Five places to look for moisture
Check these areas to help identify the source of moisture and decide what to investigate first.
- Cold glass
- Droplets on the glass and wet reveals in the morning mean condensation. Ventilate, keep curtains off the radiator, wipe the water away.
- Cold corner
- A darkening external corner or wall behind a wardrobe suggests a cold bridge. Move furniture; persistent damp needs insulation work.
- Ceiling or pipe stain
- Grows after rain or plumbing use — suspect a leak. Photograph it and call a professional.
- Household steam
- Showers, cooking, washing and laundry add moisture. Pan lids, extraction and airing after each activity help.
- Extract grille
- Extraction only works with an air supply. Do not tape over grilles; a fan that hums is not proof of airflow.
If the damp involves wiring, the roof or hidden pipes, call a professional.
A week of notes instead of one number
A single hygrometer reading proves nothing. For a week, write down humidity and temperature morning and evening, together with what happened: showers, cooking, washing, drying laundry, rain or a thaw. Keep the hygrometer away from windows, radiators, the kettle and any humidifier. Note which surfaces get wet and how long they take to dry. A repeating link, such as a patch darkening after every rainfall, tells you what to check next.
Condensation: moisture settles on cold surfaces
Droplets on the glass in the morning, wet window reveals and dark corners behind a wardrobe are typical signs of condensation. It forms where warm, moist air meets a cold surface: glass, an external corner, the lintel above a window. These spots are called cold bridges. Move furniture away from external walls, do not cover radiators with heavy curtains, and ventilate after showers and cooking. If the same spot keeps getting wet, the insulation needs assessing; paint or wallpaper will only hide the problem.
Leaks: a patch that grows
A patch that spreads after rain, snowmelt or plumbing use usually means water is getting in from outside or from pipes. Photograph it with the date and mark the edge in pencil so you can see whether it grows. Only inspect what you can reach without taking anything apart. Do not open walls or touch wiring near a wet area; contact the building manager, the landlord or a plumber. A dehumidifier can help dry the room during repairs, but a surface that looks dry does not prove the leak has stopped.
Household steam and extraction
Showers, cooking, wet cleaning and drying laundry add a noticeable amount of water to the air every day. Check that the bathroom and kitchen extractors work and that the grilles are clean. A running fan does not prove air is moving: it needs a supply, such as a gap under the door, trickle ventilation or an air inlet. Use pan lids, leave the extractor on after a shower and never tape over grilles. Do not alter shared ventilation ducts yourself.
When to use a dehumidifier, and what to do about mould
A dehumidifier makes sense once the cause has been found and fixed, or repairs are under way, and the air is still damp: a windowless bathroom, a basement, laundry drying indoors. Choose a model with the dehumidifier sizing guide and plan where the water will go. Mould is a separate job: stop the wetting first, then clean or replace the affected material; large, hidden or recurring mould should be assessed by a professional. A dehumidifier does not remove mould or repair plaster.
Different hygrometer readings: a fair comparison
First place both together in an ordinary room location, away from direct mist, sunlight, draughts, radiators and cold walls. Let readings settle as the sensors’ instructions require and record temperature too. Then swap positions: a location-dependent difference points to different conditions; a persistent difference calls for checking specifications or the sensor. This compares readings, not calibration: two numbers alone cannot tell you which is absolutely accurate. A display beside a humidifier outlet and a hygrometer across the room are not an equivalent pair. Judge trends alongside condensation and surface conditions instead of switching appliances after every fluctuation.
RH rises again after the dehumidifier stops
RH depends on temperature: air cooling after operation can show a higher percentage without a matching increase in water vapour. Moisture may also continue entering from household activities, air exchange and damp materials. Record RH and temperature before switching off and several times afterwards, using the same sensor in the same place. Note showers, cooking and ventilation. A quick rebound alone proves neither a fault nor a leak; a short dip in RH does not mean walls have dried.
Why can it be humid outdoors but dry in a heated room?
Relative humidity (RH) depends on temperature. Heating cold air without adding water vapour lowers its RH; water need not disappear for the percentage to fall. A high outdoor winter reading therefore does not imply equally humid indoor air.
| Process | What changes |
|---|---|
| Heating | Temperature rises; without added vapour, RH falls. |
| Dehumidifying | The appliance removes water from the air. |
| Comparing readings | Record temperature alongside RH in each location. |
Step by step
- Log humidity, temperature and household events for a week.
- Mark exactly where droplets, patches or smells appear.
- Inspect windows, external corners and walls behind furniture.
- Check the extractors, grille cleanliness and air supply.
- Fix leaks or cold spots first, then decide about a dehumidifier.
- Keep a joint RH, temperature and events log.
- Compare several similar cycles without moving the sensor.
- Investigate and fix moisture sources where wet patches appear.
What gets in the way
- Drawing conclusions from one hygrometer reading.
- Calling every patch condensation without checking for a leak.
- Painting over a wet area or hiding it behind furniture.
- Taping over ventilation grilles to "keep the heat in".
- Judging drying only by the first reading after shutdown.
- Promising structures will dry fully within a fixed number of days.
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WetAir manuals and support
Check the exact model requirements
Once the cause is fixed and the air is still damp, go to the dehumidifier sizing guide: it compares three models and shows how to drain the water.
Sources and limits of this guidance
The line between drying the air and repairing the moisture source follows the US Environmental Protection Agency (EPA). The approach to condensation, leaks and building causes follows UK government guidance on damp and mould; Approved Document F on ventilation is used only for context, not for local legal requirements. Dehumidifier data comes from WetAir manuals. The sensor-comparison workflow is editorial observation guidance, not laboratory accuracy testing.
- US EPA: A Brief Guide to Mold, Moisture and Your Home
- GOV.UK: Understanding and addressing damp and mould in the home
- GOV.UK: Ventilation: Approved Document F
- CEE: Portable dehumidification in single-family homes
- National Weather Service — Discussion on Humidity
- Australian Government — Heating and cooling
Frequently asked questions
Does a misted window mean a leak?
Usually not: it is condensation on cold glass. Suspect a leak when a patch appears after rain, near a pipe or on the ceiling and slowly grows. Check each spot separately.
Can a dehumidifier replace ventilation?
No. A dehumidifier only removes water from the air, while ventilation also replaces the air itself. If an extractor is not pulling air, find out why separately.
When will a dehumidifier really help?
When the source is known and fixed, or repairs are under way, and the air stays damp. Judge the result by a week of notes, not by one full tank.
What should I do about mould?
Stop the wetting first, then clean or replace the affected material. Large, hidden or recurring mould needs a professional assessment; a dehumidifier does not remove it.
Does a difference between sensors prove a fault?
No. First exclude different locations, temperatures and unsettled readings. Look up permitted accuracy in each sensor’s own documentation; there is no universal deviation rule for every appliance.
Should I select the lowest humidity every time?
Not necessarily. Check your target, room conditions and the model's automatic mode. First establish whether temperature is changing and moisture is still entering.