HVAC Engineering 101 - Question about heat loss through ductwork

Heat engineering question:
Background: I have some insulted flex hot air ducts in my attic supplying heat to the second story from the ceiling. They currently lose a fair amount of heat, as evidenced by an ice dam problem above each duct location. I heat mostly by wood so the hot air runs only few times a day, usually twice in the morning to bring the temps up and a couple runs overnight when the wood stove is burning low.
I know of a variety of alternative ways to deal with the dams from the carpentry, roof, and venting disciplines, but I had another idea that seemed intuitively like it might be a better choice (but maybe not, intuition isn't engineering :-).
If I was to insulate the ducts heavily ( they currently have the thin built-in flex duct insulation only and sit on 6" of fiberglass ) would this reduce the loss of heat to the attic appreciably? Or would it just slow the loss so that it happens over a longer time period? I'm guessing that having more insulation between the ducts and the zero to 10 degree air above (when the dams form) would reduce loss while running, but most of the time the heat in there is just residual... or perhaps back-feeding from the room. Would insulation cut the loss to the attic itself?
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