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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.

you need an airbrick fitted through the wall and an internal vent left open all the time to dry the room out
fit it as high to the ceiling as possible.Also hire an industrial de humidifyer to dry the damp already in the room.Remove any mould by washing it with neat bleach.
Do not heat the room untill the airbrick is fitted this will only lead to condensation

The fan is connected to ducting which leads to an outside vent on the exterior of the building, which should have the same effect as an airbrick.

The mould is only around one corner of the bathroom ceiling, elsewhere is mould free. The condensation/damp is around the same wall, elsewhere in the bathroom dries off after a shower due to the fan and the walls are continually dry.

I was told by the electrician who fitted the humidstat fan that this should be sufficient for a small bathroom but that he can fit a larger, more powerful fan if the problem was not resolved.

My question: should I get a plumber into the bathroom of the flat above me to check for leakage/seapage, or should I install a more powerful fan?

I have successfully removed the black mould and a dehumidifier does help to dry out the bathroom after showering, however I am still experiencing a lot of condensation when I take a shower, despite a 4" humidistat fan. My electrician advised this size fan should be adequate, however he did mention that a 6" fan would be more powerful. In your opinion, would fitting a 6" humidistat fan be more successful in removing the amount of condensation or would this be a total waste of money?