We have a first floor property which we let out in a 1960s brick-walled, concrete floored, double-glazed building. There is a humidity-driven fan in the kitchen and bathroom. During both of the winters which we've let the property (to different tenants), black mould has very suddenly appeared during the particularly cold snap on the outside walls of one of the bedrooms, the lounge and the bathroom, plus the bathroom ceiling. Both sets of tenants have told us that there is water streaming down the walls at times. Clearly (?) this is condensation, and appears because of "life-style", ie bathing, showering, cooking, and breathing in a warm flat which has cold uninsulated walls. The mould appears at the top of the outside wall and the ceiling in the bathroom, and just above the skirting in the lounge and the bedroom. I believe the best solution would be to dry-line all of the outside walls, and/or to introduce a fan into the hallway to "dribble" air movement through the property - is this correct? But would an alternative (substantially easier and cheaper) option/solution be to put 1 or 2 air vents into each room - currently there are none in the property?