Had an HVAC inspection performed on a home built in '89 today. One of the comments was that the insulation on the duct work for the system was missing in spots. Looking at it the outer layer looks like plastic similar to what clothes shipped by mail come in. There still appears to be all the insulation around the inner duct it's just that the plastic is gone in places. Is replacing the duct work something that should be done at this point? Is this a major issue? I get new ducts will most likely perform better.