If this is happening, it might be caused by a faulty roof jack allowing rain to come into the duct, or condensation from warm, humid air in the house striking the cold duct surface.

If your fan has a problem with condensation, two things may help. First, try operating the fan for longer periods of time to carry out more of the moisture, so the air is not humid when the fan stops running. Moisture will generally not form when the fan is running unless it is very cold, or the installation has a very long duct run in a cold attic. Second, you can insulate the duct to reduce the probability of condensation.

Also, if the fan does not appear to be clearing moisture from the room, check the ducting and run time of the fan. Poor duct design, installation, or damage may prevent the fan from overcoming the static pressure of the duct, which means that it will not be able to get the air and  therefore the moisture out.

Be sure to contact your local electrical and building officials to ensure local building and electrical codes are met before attempting to install ventilation fans in your home.