Most advice for UK homes lands on 40 to 60% relative humidity. Above that, damp problems become more likely. You can only know where you stand by measuring.
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Size my dehumidifierWhat relative humidity means
Relative humidity (RH) is how much water vapour the air holds compared with the most it could hold at that temperature. Warm air holds more than cold air, so the same amount of moisture gives a higher RH in a cold room than in a warm one. That is why cold rooms and cold walls are where damp shows up first.
The ranges people quote
| Reading | What it usually means |
|---|---|
| Below 40% | Dry air. Some people notice dry skin or throats. Not a damp risk. |
| 40 to 60% | The range most guidance treats as fine for a home. |
| 60 to 70% | Mould and dust mites become more likely, especially in rooms above about 10 °C. |
| Above 70% for long periods | Condensation is likely to form, and black mould can follow. |
Experts do not agree on a single number. Meaco's managing director has said 40 to 60% is fine, with 50% ideal, and that mould thrives above 60% when it is warmer than 10 °C. A mould specialist quoted in the same Ideal Home piece also gave 40 to 60%, while an AO.com appliances expert suggested 30 to 50%. A Scottish council's condensation guide says condensation will form if humidity stays above 70%, and recommends keeping the home between 40 and 60%. Treat the bands above as a guide, not a rule.
How to measure it
- Buy a digital thermo-hygrometer. Basic ones are cheap and show temperature and RH together.
- Put it in the room you are worried about, away from windows, radiators and the kitchen or bathroom door.
- Read it at different times: morning, evening, and after cooking, showering or drying laundry. One reading tells you little.
- Note the pattern for a week. Persistent readings above 60% are the signal to act.
Many dehumidifiers include a built-in humidistat, so you set a target and the unit switches off when it reaches it. A separate hygrometer is still useful because it shows what the rest of the room is doing.
If the number is too high
- Find out where the moisture comes from. Read Why are my windows wet in the morning?
- If it looks like a leak or damp coming through the walls, read when damp isn't a dehumidifier problem.
- If it is everyday moisture, work out the capacity you need with our dehumidifier sizer.
Sources: Ideal Home, What should the humidity be in a room?; Fife Council, Landlord Guide to Condensation Control. The bands in the table are our summary of these sources. This is general information, not building advice.