She lives 50 meters high next to the atlantic in a flat with shit portuguese 80s isolation and windows, and minimum sun exposure. It's a very humid house and was starting to get black mould all over. 4 days of dehumidifiers cleared the air and after a week the mould was gone. If you switch them off for 12 hours the humidity goes to a steady 92º in winter, and after a week the walls would be in the same state.
Granted it's more rain there but that just means you have to be intensive and constant in taking humidity off.