We had this.
The roof space between the roof and the ceiling should be vented.
This is doneusually with a patterned air brick, some times with wire gauze to keep out the wasps nesting
in the roof space.
Cover these air vents in such a way, that they let in air, but not Rain.
Guttering is important and make sure the gutter water is dispersed well away from the house.
When a friend had this problem they simply left the wardrobe doors ajar and it didn't occur again.
Don't seal up the bedroom by keeping the doors shut either as there needs to be air circulation to stop the mildew forming.