Flex duct vs sheet metal duct comes down to one trade: cost against lifespan. Flex costs about a third as much per foot and installs in minutes, no fabrication needed.
Sheet metal moves more air over long runs with less friction and lasts up to twice as long before it needs replacing.
Flex Duct vs Sheet Metal Duct: Key Differences at a Glance
Material cost per linear foot only. Installed pricing runs higher for both and depends on region and access.
Flex duct and sheet metal duct, side by side:
| Flex Duct | Sheet Metal Duct | |
|---|---|---|
| Material & construction | Wire coil in a plastic sheath, wrapped in fiberglass | Galvanized steel or aluminum, cut and formed rigid |
| Cost per linear ft | $1 to $4, material only (Angi, 2026) | $7 to $13, material only (Angi, 2026) |
| Installation difficulty | Cut, bend, strap in place. No fabrication | Cut, form and seal every joint. Two person job |
| Airflow / friction performance | Corrugated interior adds turbulence, cuts CFM on long runs | Smooth interior moves more air, especially past 15 ft |
| Noise | Insulation dampens sound, but sagging causes whistling | Can boom or tick as it expands. Needs lining to quiet |
| Durability / lifespan | 10 to 20 years (HVAC.com, 2024). Tears, sags, crushes over time | Up to 25+ years (HVAC.com, 2024). Resists pests and crushing |
| Best use case | Short branch runs, tight attic and crawlspace turns | Main trunk lines, runs over 15 to 20 ft |
Which One Should You Choose
Choose Flex Duct if…
- Running short branch lines to tight attics or crawlspaces
- Budget and install speed matter more than extra lifespan
- The run is straight and fully supported not snaking around obstacles
Choose Sheet Metal Duct if…
- Sizing the main trunk line or any run over 15 to 20 feet
- The space stays exposed to pests, heat or moisture
- Lowest long term friction loss and fewest future repairs matter most
Bottom Line
Both in most homes. Sheet metal handles the trunk, flex handles the short branches to registers. Treat flex duct vs sheet metal duct as a per run decision not a single pick for the whole house.
Flex duct's friction loss cuts CFM more than sheet metal's does so the same trunk size won't move the same air in both materials. The calculator converts your CFM and friction rate into the exact duct dimensions for whichever material you pick. HVAC duct sizing and airflow calculator
Other Common HVAC Questions: How to Size Ductwork for a Room Addition: A 6 Step Guide
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