Thank you, Mikemeier. The molds in one of the bathroom ceiling are dots about 2 mm in diameter. In another bathroom, I have a skylight. I notice that in the winter, cold air leaks in at one corner and the vertical drywall have a crack of about 8 inches and the paint on this wall peels in large piece. I guess the resulting moisture condensation on the wall and drying caused the crack. I have planned to caulk the wooden base frame of the skylight first to seal off the leak. I do not have hole in the ceiling at present but when I replace the exhaust fan, I guess I would have holes.