I have a bathroom without a window and black mould growing in only one corner of this around the ceiling. The plasterboard walls simply do not dry off after showering, despite a fan and heater. Even if I don't take a shower, the walls simply do not dry out, it makes no difference. I have recently had the fan replaced with a humidistat fan, but there is no improvement. Two of the walls have tidemarks and are damp to the touch. I always leave the kitchen window open when I take a shower to ventilate/avoid condensation. I have lived in the same flat for 5 years and this problem is realtively recent, ie I never had damp walls previously. I did have a leak into the bathroom from the flat above me last year, which resulted in the bathroom being repainted.
I have yet to determine whether this is being caused by a leak from the flat above me. Please would you let me know if I should contact a surveyor to recommend steps I should take to rectify this or if there is anything I can do as I need to determine the cause of the problem before anything else.