Condensation problem in AC ducts in winter
I have a dedicated AC system with the air handler in unconditioned attic (heat is provided by hydronic baseboard). When outside temps drop down below about 40 (pretty much Nov thru March) condensation starts forming in the ducts and drips from some of the ceiling registers. First winter after moving into this home I just tried closing the dampers on the registers, but that had limited if any improvement since air still passes thru the vents. As a temporary fix each year since then (4 years now) I tape plastic over all the registers each winter. This solves the problem, but looks horrible. I have worked with 2 HVAC contractors but neither of them have been able to completely resolve the issue yet. The ducts are all now insulated to code (replaced some to bring up to code) and the metal registers in attic are all low-e wrapped and spray foamed.
This is not just a little condensation on the vents, the ducts will actually fill with gallons of water. The warmer I set the heat the worse the condensation. My theory is that there is a slight and slow circulation occurring between different floors in the home to balance out minor temp/pressure differences. The slow movement of air gives it time to condense in the ducts, regardless of how well insulated they are. RH is about 40% and we keep the thermostat set at about 68 all winter, so there are no environmental extremes. This is a 60's era split level home located in mid-Atlantic region.
Since sealing the vents works, I'm looking for a more attractive solution. Does anyone know if someone makes (reasonably priced) ceiling registers that will completely seal tight when the damper is closed?
Or any other expert advice?
Another odd thing is that there are many other homes in our neighborhood with same floor plan and similar AC systems installed about the same time, yet our home seems to be the only one with this problem.