Duct Material Types: Metal, Flex & Fiberglass — Friction Rates and Best Uses
Every duct material has its own wall roughness, and that roughness decides how big the duct must be for a given airflow. This guide compares the common commercial materials — galvanized metal, flexible duct, fiberglass duct board and duct liner — gives the friction-rate range each is designed at, and explains where each one belongs.
Last updated September 2026
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Why material changes the duct size
In the Darcy-Weisbach method, the friction the air loses to the duct wall depends on the wall's absolute roughness (ε), usually quoted in millimetres. A rougher wall makes the air's boundary layer more turbulent, raises the friction factor, and means the same airflow needs a larger duct to hit the same friction rate.
The spread between materials is enormous: flexible duct compressed to 70% extension has a published roughness around 6 mm, while PVC is near 0.01 mm — a six-hundred-fold range. That is why the single most common field error is sizing a flex runout with a metal-duct friction rate.
Material comparison: roughness and design friction rates
| Material | Roughness ε (mm) | Common design rate (in. w.g./100 ft) | Where it belongs |
|---|---|---|---|
| PVC / plastic | 0.01 | 0.10 – 0.15 | Corrosive exhaust, lab fume, wastewater off-gas where metal would corrode. |
| Aluminium | 0.05 | 0.10 – 0.15 | Coastal and corrosion-prone environments; round duct where weight matters. |
| Stainless steel | 0.05 | 0.10 – 0.15 | Kitchen exhaust, grease duct, process exhaust, wash-down environments. |
| Galvanized, spiral seam | 0.09 | 0.08 – 0.15 | The commercial baseline — supply and return trunks, exposed ductwork. |
| Galvanized, longitudinal seam | 0.15 | 0.08 – 0.15 | Rectangular trunks; slightly rougher seam, otherwise used like spiral. |
| Fiberglass duct board | 0.9 | 0.06 – 0.10 | Low-pressure residential / light-commercial supply and return, insulated by design. |
| Metal duct, fibrous glass liner | 1.5 | 0.06 – 0.10 | Acoustically sensitive trunks — offices, classrooms, healthcare — needing lined metal. |
| Flexible duct, fully extended | 3 | 0.05 – 0.08 | Short branch runouts and final connections to diffusers and terminals. |
| Flexible duct, 70% extended | 6 | 0.05 – 0.08 | Avoid — compression multiplies friction loss; size and install fully stretched. |
Roughness values as used in ASHRAE-based duct sizing; design rates are common practice, not code values.
Two patterns stand out. First, all the smooth-wall metals — spiral, longitudinal, aluminium, stainless — share one design range, because their roughness differences are small compared with the effect of fittings and layout. Second, everything with an insulation or fabric component — duct board, liner, flex — needs a distinctly lower friction rate to make up for its rougher air side.
Metal duct: the commercial baseline
Galvanized steel is the default for commercial supply and return. Spiral-seam round duct (ε ≈ 0.09 mm) is the smoothest commonly installed metal and the reference material in most duct-sizing charts; longitudinal-seam rectangular duct (ε ≈ 0.15 mm) is slightly rougher at the seams but is sized at the same 0.08–0.15 in. w.g./100 ft range in practice.
Choose aluminium (ε ≈ 0.05 mm) near coastlines and in corrosive atmospheres, and stainless steel (ε ≈ 0.05 mm) for grease duct, kitchen exhaust and wash-down or process applications. Their sizing penalty versus galvanized is negligible — the choice is about durability and code, not friction.
Best suited to: supply and return trunks, exposed architectural ductwork, high-velocity systems, and any run longer than about 30 ft where the smooth wall pays for itself in fan energy.
Size it in the duct sizer with the material set to spiral or longitudinal galvanized, and check the resulting velocity against the application limits in the velocity calculator.
Flexible duct: rougher, and priced for it
Flex duct's wire-reinforced fabric wall is far rougher than metal: roughly 3 mm of roughness fully extended, and up to 6 mm when compressed to 70% extension. Its friction loss also rises sharply with compression, sag and kinked bends — field studies repeatedly show flex runs losing several times the pressure their catalog friction chart promised.
Because of that, flex is designed at a lower friction rate than metal — commonly 0.05–0.08 in. w.g. per 100 ft — and only for short runs. Keep runouts under about 15 ft, pull the duct taut, support it on narrow hangers without crushing it, and use a metal elbow at the collar and the diffuser rather than bending the flex itself.
Best suited to: final branch runouts and terminal connections — not trunks. If a flex run has to be long, size it one or two standard sizes up from what the metal equivalent would be. The flex vs. rigid metal guide covers the friction-rate adjustments in full.
Fiberglass: duct board and duct liner
Fibrous glass duct board (ε ≈ 0.9 mm) is a structural panel that combines duct, insulation and acoustic lining in one material. Its rougher air side pairs with a conservative design range of roughly 0.06–0.10 in. w.g./100 ft, and its use is limited to low-pressure systems — typically up to 2 in. w.g. static class, residential and light-commercial supply and return.
Fibrous glass liner applied inside metal duct (ε ≈ 1.5 mm, measured on the air side) exists to control noise in offices, classrooms and healthcare spaces. The acoustics are real, but so is the sizing consequence: a lined trunk should be computed with the liner's roughness, not the steel's, or the section will come out undersized and the liner will erode at high velocity. Keep liner face velocity below about 4,000 fpm where the liner is not coated.
Best suited to: duct board for short, low-pressure residential and light-commercial runs where installed insulation and acoustics outweigh durability; liner for metal trunks in noise-critical spaces. Neither suits kitchen exhaust, high-velocity systems, or anywhere the air side can erode or stay wet.
Choosing a material by application
| Application | Typical choice | Design rate (in. w.g./100 ft) |
|---|---|---|
| Commercial supply / return trunk | Spiral or rectangular galvanized | 0.08 – 0.15 |
| Exposed architectural duct | Spiral galvanized, painted | 0.08 – 0.12 |
| Kitchen grease exhaust | Stainless or black steel per code | 0.10 – 0.15 |
| Corrosive / fume exhaust | PVC or FRP | 0.10 – 0.15 |
| Noise-critical office trunk | Galvanized with fibrous glass liner | 0.06 – 0.10 |
| Residential / light-commercial supply | Duct board or galvanized | 0.06 – 0.10 |
| Branch runout to diffuser | Flexible duct, fully extended | 0.05 – 0.08 |
Common practice. Project specifications, energy codes and the AHJ override any value here.
Method and assumptions
- Method
- Darcy-Weisbach friction with the Altshul-Tsal friction factor; roughness ε from ASHRAE-based tables.
- Air properties
- 70 °F at sea level, standard air density.
- Design rates
- Common industry practice ranges, not code requirements.
- Flex condition
- Values assume the flex is fully extended; 70% extension roughly doubles the roughness again.
Limitations of this method
What the calculation on this page does not account for. Read these before using a number on a drawing or a submittal.
- Material roughness varies with manufacture, seam type, lining and installation quality; the tabulated ε values are representative, not guarantees.
- Flex duct performance depends heavily on installation: compression, sag, crushed sections and tight bends can multiply friction loss well beyond any tabulated value.
- Duct board and liner are limited by pressure class, velocity, erosion and moisture rules that vary by product listing and code — this page does not replace the manufacturer's listing or the mechanical code.
- Material selection also depends on fire and smoke rating, corrosion, cleanliness class (e.g. healthcare), and cost — friction is only one axis of the decision.
- Applying any rate here to a real fan requires totalling the critical path; see the guide on identifying the critical duct path.
Frequently asked questions
Which duct material has the lowest friction loss?
Smooth non-metallic materials such as PVC are smoothest (ε ≈ 0.01 mm), followed by aluminium and stainless steel (ε ≈ 0.05 mm), then spiral galvanized steel (ε ≈ 0.09 mm). In everyday commercial work, spiral galvanized is the smooth-wall baseline — its roughness is close enough to aluminium that the sizing difference is negligible for most runs.
Can I size flexible duct with the same friction rate as metal duct?
No. Fully extended flex has a roughness around 3 mm — more than thirty times spiral galvanized steel — and 70% stretched flex is worse still. Sizing flex at a metal-duct friction rate undersizes the run badly. Design flex at a lower rate (commonly 0.05–0.08 in. w.g. per 100 ft), keep runs short and fully extended, and use a full-radius bend at each end.
When is fiberglass duct board the right choice?
Duct board suits low-pressure residential and light-commercial supply and return where thermal insulation and acoustic attenuation are built in and first cost matters. It is not suitable for high-velocity systems, kitchen exhaust, outdoor air intakes without special treatment, or anywhere the surface can erode or the board can be damaged.
Does duct liner affect duct sizing?
Yes. Internally lined metal duct has the roughness of the liner facing (ε ≈ 1.5 mm), not the steel behind it. A lined trunk behaves closer to a rough flex run than to bare metal, so the section should be sized with the liner's roughness and checked for the resulting velocity and noise.