Radiant Barrier on Flex Duct

I came across flex duct that appeared translucent. I have heard that this indicates loss of radiant barrier on duct and reduced energy efficiency. How does radiant barrier deteriorate, where does foil-look go? House was built 2002.

The insulation does not deteriorate only the outer cover if plastic deteriorates due to excessive heat. If the insulation beneath the outer cover is missing someone removed it. All duct work supply and return in a non-condition space is required to be insulated