There are two possible reasons for the condensation buildup. First, the damper on the outside exhaust hood is either stuck open or missing. This allows freezing cold air into the duct, which chills the ceiling below. The second possibility is that cold air is coming into the ceiling area through the opening in the outside wall between the exhaust duct and the wall. You should seal this with foam in a can, not just fiberglass. On a brick veneer wall, the exhaust duct must be watertight to the brick and airtight to the sheathing behind the brick. Also ensure that it slopes to the outside to drain any condensation outdoors.