3 said replace it, and yes insulate it. Are they also saying it is
improperly sized? Do you heat or cool unevenly? Properly sized
insulated ducting is going to get you the most efficiency out of a new
unit. Flex has the ridges that wont flow as much air as smooth duct so I
can see an efficiency issue switching as well as the kinks in the tube
and efficiency benefits. Also you posted 2.5 or 3 ton AC, Im no pro but
new ducting flowing more air might make the 2.5 ton the right option, 3
might oversize you leaving you humid. What size do you have now, If 2.5
ton you may be able to go smaller with new ducts if it cools well. You
need a load calculation done in writing before you proceed.