Flexible duct versus smooth metal at the same diameter

Flex has a rougher wall than smooth metal, so at the same diameter it carries less air — size it up one size, and never leave it sagging or compressed.

Two panels. On the left, a smooth straight galvanized metal duct with a full airflow arrow: a 6-inch duct carries the full 100 CFM, the baseline size straight off the table. On the right, flexible duct: pulled taut it still needs to be sized up one size because its corrugated wall roughly doubles the friction, and a sagging or compressed flex run is shown choked with a thin airflow arrow, cutting airflow in half. — Source: ACCA Manual D flexible-duct sizing

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Two panels. On the left, a smooth straight galvanized metal duct with a full airflow arrow: a 6-inch duct carries the full 100 CFM, the baseline size straight off the table. On the right, flexible duct: pulled taut it still needs to be sized up one size because its corrugated wall roughly doubles the friction, and a sagging or compressed flex run is shown choked with a thin airflow arrow, cutting airflow in half.

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