Insulated fiberglass duct performs best in terms of thermal efficiency, noise reduction, reduced condensation and air quality. It provides the correct R-values (a measure of capacity to resist heat flow) to maintain room temperature. It is the most efficient and most expensive duct.
Flexible spiral duct is most common in residential construction. It is relatively inexpensive and easy to install. However, it suffers from condensation and poor thermal efficiency. As a duct passes through unconditioned air, it loses heat, especially if it is not properly sealed. Fiberglass insulation improves its performance.
Uninsulated round or rectangular sheet metal duct is cheapest to install, but is prone to condensation, air leakage and noise. Metal duct often loses up to 40 percent of the heating or cooling energy passing through it. Metal duct also conducts household sound more than other material and makes popping and squeaking sounds as it expands and contracts. Wrapping it with insulation improves its performance some, but lining it with insulation greatly improves its performance.