I was reminded of this tip by a colleague who asked a question about mold around the heat and AC registers especially ones that are located in the ceiling. This is usually caused by air leaking out around the vent boot that connects to the register. It is allowing hot and humid air from the attic to hit the cool metal register vent and form condensation. However, it is possible to have this condensation even if there is no air leak.  Hot humid air, even in an air conditioned house gathers at the ceiling. That means it is going to come in contact with the register cover that has been cooled by the AC. If this is your case, you may consider using a plastic register vent instead of the traditional metal vent. Because plastic does not conduct heat, like the metal one does, the condensation (“sweating”) goes away. Don’t use the plastic register vents on the floor since they can’t hold up to any weight. For the floor consider wooden register vents.