If you compare internally insulated rectangular ducts versus internally insulated flat oval ducts, you will usually see a big price advantage for the rectangular duct. That’s because you insulate rectangular ducts – at least 25 years ago – with glued and pinned duct liner. Round and flat oval ducts had to be double-wall construction. We’ve had a huge change in the industry since 1990 because we now see the industry move away from duct liner and towards external insulation wrap.