>>AIUI, if the temperature is lower then the air contains less water vapour. If a room is warmer, it follows the air could contain more water vapour. Wouldn't there be more chance of condensation forming on a cold surface?
Possibly. Heat is no guarantee that's true, the bathroom in the rental we had was well heated but mould grew in several places - ventilation comes in to it. But for a given amount of water the relative humidity will be higher in the air there is and the wall will presumably be colder.
It wasn't in my mind when I decided to put the MVHR* in here, but it does manage the humidity very well. It moves the equivalent of the house full of air every 2.4 hours. The bathroom humidity is usually 35-45%. It goes up to maybe 90% if someone has a long shower, and there's condensation on the windows, but in less than an hour it's gone and the RH is back below 50%.
*mechanical ventilation with heat recovery
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