What do I use to cover an insulation wrapped duct? (Silver foil wrap?)
You're looking for Duct Wrap.  It's actually an insulation in it's own right, being one side foil, and the other side fiber insulation.   I see no reason why you couldn't apply it over top of the existing insulation, provided everything is dry.



http://www.homedepot.ca/product/reflectix-duct-wrap-12x25/902353