Vapour condenses into water as soon as a the surface temperature drops below the dew point.
The humidity was measured and found to be 60.2%RH (which is normal), with a temperature of 18.7°C. The calculated dew point is 10.8°C (temperature below which condensation will form – see mouldpoint.co.uk).
The measured temperature of the damp wall was 13.1°C, 2.5°C over the dew point.
Reversing the calculation this give us a surface relative humidity of 86%RH.
Electrical sockets are made from cutting a section of wall out, and installing a metal box and metal wiring, which is often chased into the plaster close to the surface.
The wiring often comes into contact with cold external temperature, either from the sub-floor void or outside walls.
At night it is highly likely that the wall temperature of this exposed wall, with no heating, ventilation and shaded on the outside, will drop below the dew point.
Tips for dealing with metal sockets
- Improve ventilation.
- Improve heating, especially at night and in cold rooms with cold external walls.
- Consider chasing wring further into the wall.
- Consider insulating wiring.
- Consider replacing the metal box for plastic.