Condensation calculator
Put in a room’s temperature and humidity to see the dew point, below which surfaces get wet, the colder surfaces where mould risk starts, and how damp the air gets when the room cools overnight.
- Surfaces get wet below
- 12.0°C
- The dew point of air at 20°C and 60% humidity
- Mould risk below
- 15.4°C
- Where a surface reaches 80% humidity, a building-science rule of thumb for mould
- Humidity if the room cools to 16°C
- 77%
- Up from 60%, with the same water in the air
Any surface colder than 12.0°C in this room will get wet, such as window glass and frames on a cold morning, the corners of outside walls, and the wall behind a wardrobe. Mould can start on surfaces below about 15.4°C, even when you can’t see any water. If the room cools to 16°C overnight, the same air reaches about 77% humidity while the windows and walls get colder, which is why mornings are worst.
Read the humidity from a hygrometer in the room, away from the window and the kettle. The dew point uses the Magnus formula. The mould line is where a surface would sit at 80% humidity, a widely used building-science rule of thumb for when mould can grow. It isn’t a legal limit, and it marks where the risk starts: mould needs the damp to last, not just a few minutes after a shower.
What goes in
Put in the room’s temperature and its relative humidity, which a cheap digital hygrometer shows. Then say how cold the room gets overnight, when the heating’s off.
What the result means
The dew point is the temperature where the air can’t hold its water any more. Any surface colder than that gets wet: in a room at 20°C and 60% humidity, that’s anything below 12.0°C, which on a cold morning is often the window.
Mould doesn’t need water running down the wall. A widely used building-science rule of thumb puts the risk where a surface sits at about 80% humidity, which in the same room is any surface below about 15.4°C. It’s a rule of thumb, not a legal limit.
When the heating goes off, the room cools but the water stays in the air. The same 20°C, 60% air cooled to 16°C reads about 77% humidity, and about 82% at 15°C, while the windows and outside walls get colder too. That’s why condensation and mould are worst first thing.
The figures behind it
The dew point uses the Magnus formula with Alduchov and Eskridge’s coefficients, a = 17.625 and b = 243.04°C. The mould line is the temperature where the room air would be at 80% of what it could hold. More worked examples: 21°C at 50% gives a 10.2°C dew point and a 13.6°C mould line, and 18°C at 65% gives 11.3°C and 14.7°C.
Sources
- Alduchov and Eskridge, 1996: Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology
- UK Health Security Agency, DHSC and MHCLG: Understanding and addressing the health risks of damp and mould in the home, on where condensation forms