I have a network of insulated flexable heat lines sitting on blown in insulation. In the winter, I get an ice dam in a gutter. I suspect the duct work is letting off enough residual heat to cause a slight roof melt leading to ice. I decided to buy that insulation that resembles foil faced bubble wrap and cover the flexable duct work. My question, will covering those lines cause some sort of barrier that might cause mold in the future?
Ice dams are a result of bulk heat loss through any number of sources. Unsealed ductwork is certainly one of those but not the most significant in most homes cases.
You need to air seal, add insulation, inspect venting, and make sure the envelope layer is tight.
If the ductwork is tight, drop them down into the insulation layer and blow additional insulation over the top of them. This will help with the radiant heat loss 100X more than the radiant heat barriers.
Radiant barriers are mostly crap in about 99% of the applications that homeowners and contracts try to use them for. They only work with an air space so in this application, they will do nothing for you but, as you theorized, potentially create a double vapor retarder layer and mildew/mold issues.