In regards to insulating "duct" work in the attic. As I've read and stated here, you risk creating moisture/mold by wrapping insulation around the duct board or flex duct. However, what about just draping fiberglass (un-faced or faced) insulation over the duct work? Wouldn't that be similar to insulating the roof line but instead you are placing the barrier directly over the duct work?
Just to be clear I am a fan of the radiant barrier but it is just one piece of the puzzle . You rarely will get hurt by adding more insulation . Had I kept the previous house/garage I would have added more insulation to that garage ceiling as 3 1/2 " R13 is just not enough . In my new garage I will go at least R30 or more PLUS the radiant barrier .