Most residential applications use flex duct these days anyway. Flex duct is much better insulated than box duct could ever hope to be as well so any losses in the systems efficiency through a not perfectly smooth interior surface are more than made up for by the insulated walls.
Yeah that's not even remotely true. Metal duct has much less friction per foot which increases the efficiency of the blower.
It can also be insulated or lined to provide the same or even higher R-Value as flex duct.
Metal duck work is much more robust and won't fail after being exposed to water once. It's also rodent proof. Somewhat antimicrobial, much easier to clean. I could go on and on.
Flex duct is a great solution when you have a specific problem, and it's much less labor intensive, but it's inferior to metal duct in just about every way.
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u/23370aviator 3h ago
Most residential applications use flex duct these days anyway. Flex duct is much better insulated than box duct could ever hope to be as well so any losses in the systems efficiency through a not perfectly smooth interior surface are more than made up for by the insulated walls.