rooms, the house generally, must be well-ventilated of course.
some younger friends of mine were having problems with mould in their rented flat.
it was modern, double-glazed, warm.
i was puzzled, until it came to light that they never opened any windows, ever, except in hot summer.
they knew nothing about trickle vents in windows.
i explained and no further problem.
this should have been explained when they took the flat. they come from a tropical climate and things are totally different there.
i often hear of mould in flats and wonder if younger people do not understand about ventilation. seems obvious to me. but guess it depends on experience.