Mould risk temperature at a glance
Each figure is the surface temperature below which mould can grow, for a room at that temperature and humidity. If your coldest wall, window reveal or corner is below it, that is where mould will appear first.
| Humidity | Room at 16°C | Room at 18°C | Room at 20°C | Room at 22°C |
|---|---|---|---|---|
| 50% | 8.8°C | 10.7°C | 12.6°C | 14.5°C |
| 55% | 10.3°C | 12.2°C | 14.1°C | 16.0°C |
| 60% | 11.6°C | 13.5°C | 15.4°C | 17.4°C |
| 65% | 12.8°C | 14.7°C | 16.7°C | 18.6°C |
| 70% | 13.9°C | 15.9°C | 17.9°C | 19.8°C |
| 75% | 15.0°C | 17.0°C | 19.0°C | 20.9°C |
| 80% | The whole room | The whole room | The whole room | The whole room |
Why the wall temperature matters more than the room
Mould doesn't need the whole room to be wet. It needs a surface where the air stays very humid for days at a time. The air right next to a cold outside wall, a window reveal or the back of a wardrobe is cooler than the middle of the room, and cooler air at the same moisture level has a higher relative humidity.
So a room that reads 65% on a hygrometer, only a little above the comfortable range, can still have a corner sitting above 80%, which is where mould starts. The checker shows the surface temperature at which that happens. If your coldest walls are below it, that is where mould will appear first.
Bringing the humidity down
- Ventilate after the things that add the most water: showers, cooking and drying washing.
- Keep a low, steady background heat rather than letting rooms go very cold between bursts of heating.
- Move furniture a few centimetres off outside walls so air can reach them.
- If it stays above 60% with all of that in place, a dehumidifier is the next step.
More on the ideal humidity for a UK home, and on choosing a hygrometer to measure it.