Tool · moisture register · air to surface
Dew point and humidity calculator. Find the cold spot before the mould does.
Condensation and mould aren’t about how damp a room feels — they’re about how cold its coldest surface is. Set the room’s temperature and humidity, and the weather outside, and see the dew point and whether your windows or walls are likely to get wet or grow mould.
Calculate dew point and condensation risk
Dew point chart
Dew point at everyday room conditions.
| Room temperature | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 16°C | 2.4°C | 5.6°C | 8.2°C | 10.5°C | 12.5°C |
| 18°C | 4.2°C | 7.4°C | 10.1°C | 12.4°C | 14.5°C |
| 20°C | 6.0°C | 9.3°C | 12.0°C | 14.4°C | 16.4°C |
| 22°C | 7.8°C | 11.1°C | 13.9°C | 16.3°C | 18.4°C |
Read across: in a 20°C room at 60% humidity the dew point is about 12°C, so any surface colder than that will get wet. An old single-glazed window on a cold night can be far colder — which is why it streams while the walls stay dry.
Ideal humidity in a house
What humidity should your house be?
Most UK guidance suggests keeping indoor relative humidity roughly between 40% and 60%. Within that range, most homes stay comfortable and cold surfaces stay dry. Above it, the risk of condensation and mould rises quickly — not because the air is “wet”, but because its dew point climbs towards the temperature of your coldest walls and windows.
- Living rooms and bedrooms: aim for the 40–60% band, and watch bedrooms overnight, when breathing raises humidity with the door shut.
- Bathrooms and kitchens: short spikes well above 70% are normal while cooking or showering. What matters is that an extractor fan brings it back down within the hour.
- Persistently over 70%: too much moisture is being made or too little removed — or water is getting into the building.
Mould itself starts on surfaces where the air stays above about 80% relative humidityfor long periods — the criterion used by the international standard for assessing surface mould risk. That’s the “humidity at the surface” the calculator shows.
Next steps: how to stop condensation on windows, how to get rid of black mould, mould in the bathroom, or work out the cause with the damp checker.
Sources
Common questions
Humidity, dew point and mould.
What is the dew point?
The temperature at which the air can no longer hold its water vapour, so it condenses as liquid water. Any surface colder than the dew point of the room air — a single-glazed window, a cold corner, an uninsulated wall — will get wet. The more moisture in the air, the higher the dew point.
What humidity should my house be?
Most UK guidance suggests keeping indoor relative humidity roughly between 40% and 60%. Above 60%, cold surfaces start to attract condensation and mould; persistently above 70% usually means too much moisture is being produced or not enough is being extracted. Bathrooms and kitchens spike higher for a while after use — what matters is how quickly they recover.
Why do I get mould even when the walls look dry?
Mould does not need running water. It can grow where the air right next to a surface stays above about 80% relative humidity for long periods, which happens on cold walls well before you see droplets. That is why mould appears in corners and behind wardrobes, where the wall is coldest and air moves least.
How do I lower the humidity in my house?
Reduce moisture at source — lids on pans, extractor fans on and left running after cooking and showers, washing dried outside or in a vented or condensing dryer — keep trickle vents open, heat steadily rather than in short bursts, and use a dehumidifier in problem rooms. If humidity stays high despite all that, look for a leak or water getting into the building.
Is this calculator accurate?
The dew point uses a standard formula and is accurate to a fraction of a degree. The surface temperatures are typical values for each kind of window or wall, so treat the condensation and mould result as a guide. Real walls vary, and cold bridges such as lintels and wall-ceiling junctions can be colder still.
Mould that keeps coming back?
Find the cold spot, or the leak.
If humidity is under control and mould still returns, the cause is usually in the building — a cold bridge, missing insulation or water getting in. We investigate independently and sell no treatment.
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