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.
Flex ducts work well if installed really well. Problem is that they are rarely installed really well.
Flex ducts are chosen in resi because they're less labor intensive and quicker to install, which is attractive to builders trying to make money on volume of houses built.
Losses due to airflow restriction impact more than energy efficiency, it's also noisier and harder on the blower motors.
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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.