Insulated Ductwork

PatyO:

Typically a vapor "retarder" or barrier will be installed over the insulated ductwork for A/C. This is to prevent moisture in the air from contacting the cold duct, and condensing on the duct and in the adjacent insulation (if fiberglass or similar "wettable" material), which could damage the duct if it is steel, cause the insulation to become wet and lose its insulating ability, and also possibly result in the condensed water dripping onto whatever is below the ductwork. There are standard products made for this purpose, and often the vapor barrier is incorporated with the insulation itself for ease of installation. What the vapor "retardants" or barriers all have in common is that they prevent the passage of moisture through them, are strong enough to survive normal installation intact and allow their seams to be taped or otherwise sealed.

As far as the thickness guidelines you refer to, they are a factor of economics. The heavier insulation is justified on ducts carrying cooled air, but not on ducts for heating. But, that is only a general guide for typical ductwork, and any specific case should be decided on its own merits based in part on the, temperature differential of the air in the duct and ambient temperature surrounding it, length of run, etc.