I live in south Mississippi. Most (but not all) of the ceiling registers in my home have developed a water stain around them this summer. I know this is caused by humidity in the attic condensing and coming into contact with the ductwork, but the only things that have changed since last summer are: 1) I had the condenser in the air handler cleaned, and 2) I had blown in insulation added to the attic. Originally, there was 6" fiberglass batting between the rafters and the insulated ductwork sat on top. We added 6" of blown insulation to help reduce the energy consumption. Our cooling bills went down but now the ceiling has to be repaired.

Typically, yhis occurs after a room has been painted. The grills are first removed prior to painting. When the paint is dry the grills are then replaced. The vapor seal on the grill at this time is usually brittle. This allows hot attic air to hit the cold surface of the grill causing it to sweat. To remedy the sweating. Remove the grill and caulk the open space around the register box.