The picture below is an air-conditioning and heating duct system installed by us. The ducts are installed high enough for roofers to do work and low enough for curb appeal. The actual duct that conveys the air is the inner part then R8 insulation wraps it. A foil back cover wraps the insulation then the sheet metal covers everything. All of the duct work is sealed weather tight.
We took this picture of the home next door. They installed the duct system then wrapped it with insulation. The insulation is exposed to the elements.