We had it real bad in the bathroom and bedrooms, got a compressor one and it filled up with water but that never solved the problem. And open windows don’t help either.
What we did in order of effectiveness:
The correct bathroom (shower) extractor fan and keeping the door shut.
A low watt electric radiator on a timer in the bathroom.
Wickes anti-mould paint.
Keeping the kitchen door shut, windows open when cooking or the dryer is on.
Wiping down the inside of the windows in the morning.
Dehumidifier
Opening windows an hour a day.
Train the children to put towels in the freakin washing basket.
Edit – reckon there was a leak in the bath overflow too sometimes its hard to tell, replaced the pipes anyway.