How-to · 4 min read · Updated 6 November 2026
What humidity causes mould? Ideal indoor levels and how to measure
Relative humidity in plain terms
Relative humidity (RH) is how much water vapour the air holds compared with the most it could hold at that temperature. Warm air can hold more than cold air, so the same amount of moisture gives a lower RH in a warm room and a higher RH in a cold one. That is why a bedroom can read 55 percent in the evening with the heating on and 75 percent at dawn once it cools, without any extra moisture being added.
Mould cares about the conditions right at the surface, not the middle of the room. A cold external wall corner can be close to 100 percent at its surface while the room reads a comfortable 60 percent. That is why mould often grows in corners, behind furniture and on window reveals first.
Humidity levels and what they mean
| Indoor RH | What it usually means |
|---|---|
| Below 35 percent | Dry air; can cause dry skin, static and irritated eyes |
| 40 to 60 percent | Comfortable and low risk for most homes |
| 60 to 70 percent | Raised; condensation likely on cold glass and walls in winter |
| Above 70 percent for long periods | High risk of mould growth and dust mites |
| Above 80 percent | Serious moisture problem; mould likely within weeks |
These are general guide bands. Short spikes after a shower or while cooking are normal; what matters is how high humidity stays and for how long.
How to measure it properly
- Buy a digital hygrometer, usually £10 to £25, or a smart sensor that logs readings
- Place it at head height away from radiators, windows and kettles
- Leave it for 30 minutes before trusting the first reading
- Check bedrooms first thing in the morning, when humidity peaks
- Compare rooms on the same day to find the dampest spaces
- Note readings over a week, not just a single day
- Use a surface thermometer or an infrared thermometer to find the coldest walls
Smart sensors that log humidity over time are especially useful for landlords, because they show exactly when and where moisture builds up and help separate condensation from leaks.
How to bring humidity down
- Run extractor fans during and after cooking and showers
- Keep trickle vents open and air bricks clear
- Open bedroom windows for 10 to 15 minutes each morning
- Dry washing outside, in a dryer or in a closed, ventilated room
- Keep rooms evenly heated so surfaces stay warmer
- Use a dehumidifier in persistently damp rooms
- Put lids on pans and avoid leaving kettles or steamers running
- Fix leaks, overflowing gutters and damaged seals
If humidity is consistently high despite all of this, the home may need better ventilation, such as a continuous extract fan or positive input ventilation, or there may be a hidden source of water such as a leak under a floor. Flats are a common example: a leaking shower tray or a pipe in the flat above can keep a ceiling damp for months, and no amount of ventilation in the room below will bring humidity down until the leak is found and fixed.
Dew point and cold surfaces
The dew point is the temperature at which air of a given humidity starts to condense. At a typical indoor temperature of around 20 degrees with 60 percent RH, the dew point is roughly 12 degrees. Any surface colder than that, such as single glazing, a metal window frame or a poorly insulated corner, will get wet. Raising the surface temperature with insulation or steady heating is as important as lowering humidity.
When to get help
If readings are regularly above 70 percent in more than one room, or mould keeps returning within weeks of cleaning, it is worth having the home assessed. Moisture readings in walls, checks on fans and vents and a look at the outside of the building usually show whether the cause is condensation, penetrating damp or a leak.
Typical readings by room
| Room | Typical healthy reading | Warning sign |
|---|---|---|
| Bedroom, morning | 50 to 60 percent | Above 70 percent most mornings |
| Bathroom, an hour after a shower | Back below 65 percent | Still above 75 percent with mirror steamed |
| Kitchen, an hour after cooking | Back below 60 percent | Condensation running down windows |
| Living room | 40 to 55 percent | Above 65 percent with no washing drying |
| Cupboard on external wall | Similar to the room | Much higher than the room, or musty smell |
These are rough guide figures. If one room is consistently much higher than the rest of the house, focus on that room first, and look for a specific source such as a leak, a blocked fan or washing being dried there.
Frequently asked questions
What humidity should a bedroom be in winter?
Aim for roughly 40 to 60 percent relative humidity in a bedroom. Readings often rise overnight because people breathe out moisture in a closed room, so check first thing in the morning. If the bedroom regularly reads above 70 percent, open the window briefly each morning, keep trickle vents open and consider a dehumidifier.
Are cheap hygrometers accurate?
Most cheap digital hygrometers are accurate to within a few percentage points, which is good enough to spot a damp room. Compare two units side by side if you want to check them. Their real value is in showing trends over days and between rooms, rather than giving a precise figure.
Can humidity be too low?
Yes. Very dry air, below about 35 percent, can cause dry skin, irritated eyes and static. It is uncommon in UK homes except in very well heated flats in cold, dry weather. If it happens, reduce heating slightly or let in less cold air rather than using a humidifier near cold walls.
humidity hygrometer condensation prevention
