Also, insulating ducts can help to eliminate condensation problems in damp areas. The predominant insulation material for ducts is fiberglass. It can be applied in either a flexible or rigid form and comes in a variety of densities and thicknesses. The flexible blanket insulation is available in rolls for convenience in shipping and ease of application to round or rectangular ducting. Flexible insulation easily conforms to irregular surfaces.
Rigid insulation comes in pre-formed boards which are bonded with a thermosetting resin.
All duct insulation should have a foil or vinyl facing on the exterior side which serves as a vapor retarder. This prevents moisture from being absorbed into the fiberglass and thus maintains insulation ability. Kraft paper faced insulation should never be used on duct work because of flammability. If any existing insulation has become wet, it should be replaced. All duct work in attics, basements and crawlspaces should be insulated.