use a humidiefier, or simply open the window and air the room,not if its raining though.if its plaster board replace it with a waterproof aqua board.by fixing it to the stud work or use dri-wall adhesive over block work. if the plaster manage's to stay when dry,coat it with a thick coat of unibond[pva based glue]to put a waterproof coat over the area to stop it happening again.goodluck
stu plasterer for 14years, July 2008