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heat loss issue on second floor

We live in a fantastic 100 year old house. We have a steam boiler with cast iron radiators throughout the house which do a great job keeping the first floor warm.

However, once you go to the second floor it becomes much cooler (approx 6-8 degrees F). We had all new Marvin windows installed throughout the home. Insulating the exterior walls is not an option (plaster over brick) but we insulated the attic (previously uninsulated) with R-38 fiberglass. However, the temp difference remains.

After an investigation, I made a discovery. We have an AC system with a compressor/condenser for each story. The second floor condenser is in the attic and the cold air is delivered to the ceiling of each room via flexible duct connected to a ceiling register. While repainting the ceiling, i noticed a significant draft coming from the diffuser. I balled up some fiberglass in a grocery bag and stuffed it in the ducting. Huge improvement, no draft and the rooms are warmer! We were clearly losing warm air through the ducting.

What can I do to fix this? I have seen dampers that attach to the diffusers, how well do these really work? The kinds I have seen are the butterfly dampers that attach directly to the diffuser and the inline dampers that would connect to the flexible duct right before it enters diffuser. Are either of these really that weather tight?

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