I have posted this question in the past, but never received a satisfactory answer. Perhaps this time someone smarter than I can come up with a good solution...
Question: How to stop AC ducts in the attic from accumulating water during the winter
Environment: Ranch house, baseboard hot water heating, whole house air conditioner with AC registers in the ceiling. Floor of attic insulated to at least R40. Ceiling of attic not insulated. AC air handler in attic attached to rigid duct surrounded by 4 inches of insulation. Rigid duct traverses the center of the attic for about
2/3 the length of the house. Flexible, insulated 6 inch duct rated R4.2 attaches the ceiling registers to the rigid duct. AC ceiling registers have internal dampers that can be closed in the winter.Problem: When the AC ceiling registers are closed for the winter, the internal dampers do not affect a perfect seal. Some warm, moist air from the house will leak through the ceiling registers into the flexible duct and into the rigid duct. Even though the rigid duct is insulated, on very cold winter days, the internal temperature of the flexible and rigid ducts will be low enough to condense the water vapor in the warm air that leaked from the house into the ducts. This condensed water accumulates in the rigid duct, causing rusting and if there is enough water, leaking back into the house through the ceiling registers.
My solution: Not perfect. Every fall, I enter the attic and disconnect the flexible ducts from the ceiling registers. I cover the ceiling registers with lots of insulation. In the late spring, I go into the attic and reverse this process by reconnecting the flexible ducts to the ceiling registers. This process is a royal pain. I would like a more permanent solution.