Why flexible duct requires a larger diameter than sheet metal duct for the same airflow and friction rate, what causes the difference, and how installation quality affects it further.
Why Duct Material Isn't Just a Cost or Installation Choice
Choosing between sheet metal, flexible duct, and fiberglass duct board is often framed primarily as a cost, installation-speed, or space-constraint decision — but each material has a genuinely different interior surface roughness, and roughness directly affects friction loss for a given airflow and duct size, exactly the same underlying physics covered in this site's chemical process content on pipe roughness values (see the companion cluster there for the general principle). This means duct material choice isn't purely a non-hydraulic decision — it has a direct, calculable effect on required duct size.
Why Flexible Duct Has Meaningfully Higher Friction Than Metal
Sheet metal duct has a comparatively smooth interior surface, producing relatively low friction loss for its size. Flexible duct's interior surface — a corrugated or spiral-wire-reinforced plastic liner — is inherently rougher than smooth sheet metal, producing meaningfully higher friction loss for the same nominal diameter and airflow. This is a real, well-documented hydraulic difference, not a minor manufacturing quality concern, which is why duct sizing calculations that don't account for material-specific friction differences will under-predict actual friction loss for a flex-duct system sized as if it were smooth metal duct.
How This Site's Calculator Represents the Material Difference
This site's Duct Sizing Calculator applies a friction adjustment multiplier by material — metal at a baseline of 1.0, flexible duct at 1.35, and fiberglass duct board at 1.2 — effectively requiring flex duct to be sized larger than an equivalent metal duct to achieve the same actual friction rate at the same airflow, since a straightforward same-diameter substitution would leave a flex-duct system with meaningfully higher real friction loss than the design friction rate intended.
Why Flex Duct Installation Quality Compounds the Material Difference
Beyond the inherent surface-roughness difference, flexible duct's actual installed friction performance is also highly sensitive to installation quality in a way rigid metal duct generally isn't — flex duct that's compressed, kinked, or installed with excessive sag between support points can show dramatically higher real-world friction loss than its rated material friction factor alone would predict, since these installation defects effectively reduce the duct's usable cross-sectional area and disrupt smooth airflow at the defect location. This is why flex duct installation guidance emphasizes proper support spacing, avoiding sharp bends, and pulling the duct reasonably taut (without over-stretching, which can also cause problems) — these installation practices aren't purely cosmetic, they directly affect whether the duct actually delivers the friction performance its material factor assumes.
Why This Matters for Choosing Between Materials
Flexible duct's real installation-speed and cost advantages over rigid metal duct are genuine and often make it the practical choice for many residential branch runs — but a designer choosing flex duct should size it using the appropriate higher friction factor from the start, rather than sizing as if for metal duct and only later discovering the installed system's actual friction loss (and resulting reduced airflow or increased blower strain) exceeds the design intent. This is exactly why duct material selection happens before, not after, final duct sizing in a proper design sequence — the material choice is a direct input to the sizing calculation, not an independent downstream decision.
Why Fiberglass Duct Board Sits Between Metal and Flex
Fiberglass duct board's friction characteristics fall between smooth metal and flexible duct — its interior surface is smoother than flex duct's corrugated liner but not as smooth as bare sheet metal, which is why it carries an intermediate friction adjustment factor. This reflects fiberglass duct board's real, distinct hydraulic performance profile as its own material category, not simply an average or approximation between the other two options.