Types of Duct Material: Metal, Flex and Fiberglass

At the same internal dimensions and airflow, a smoother air-side surface generally produces less straight-duct friction. Smooth rigid duct usually has lower resistance than rough or compressed flexible duct. However, “lowest friction” and “most durable” are separate questions: corrosion, moisture, damage, temperature and cleaning requirements can change the right choice.

In this calculator’s model, PVC has the lowest assigned roughness. That is not a recommendation to replace metal with plastic: flame and smoke performance, product listings, code and chemical compatibility must permit the application. Galvanized metal is a common general-purpose choice, not a universal winner for every air stream.

The assigned 3 mm and 6 mm flex roughness values are simplified resistance models. “70% extended” means 70% of full extended length in this model; it does not establish a tested pressure-loss curve for a particular flex product. Use manufacturer data for final flex sizing and installed compression.

Compare your own airflow and section with the duct velocity calculator, or select a material in the duct sizer for a preliminary friction-based size.

Here f is the Darcy friction factor, L and D use consistent length units, ρ is air density and V is velocity. For example, SI units give Δp in pascals. The calculator converts pressure loss per length to in. w.g./100 ft. Roughness affects f; it is not itself a friction rate.

The target design rate comes from the system’s available pressure, effective path length, noise and energy criteria. Choosing a lower target rate may yield a larger section, but no single target rate is required by a material name. See the duct friction rate guide and use the fan static pressure calculator to review the pressure budget.

Galvanized steel is widely used for supply and return distribution. Spiral and longitudinal seams have different modeled roughness here, but fittings, leakage and construction quality can matter more than the small difference between two smooth metal surfaces.

Choose aluminium or stainless steel only after checking compatibility with the environment and air stream. Stainless steel is not automatically suitable for every chemical. Kitchen grease exhaust requires compliant materials, thickness, joints, access, clearances and any required listed assembly—not merely a material selected from this table.

For a rectangular section, use its clear internal area for velocity. The rectangular-to-round converter compares friction-equivalent diameters; equal friction does not mean equal area or velocity.

Flex is useful for permitted terminal connections, but its installed resistance depends strongly on extension, bends and support. A straight, stretched laboratory sample does not reproduce a sagging or kinked field run. Do not apply a rigid-metal chart or add an arbitrary one or two standard sizes as a substitute for product data.

• Keep the route as short and direct as practical; remove unnecessary length and avoid sharp bends or kinks.
• Extend and support the duct according to the manufacturer’s instructions; use supports wide enough to avoid pinching or compressing it.
• Check permitted support spacing, bend radius, connections and maximum run length against the product instructions and applicable code.
• Account for fittings and terminals separately, using coefficients or pressure-loss data appropriate to the actual geometry.

Read the flex vs. rigid metal sizing guide for context, and use the equivalent length calculator only as a preliminary round-run fitting model. Manufacturer flex pressure-loss data takes precedence over the roughness approximation.

Duct board forms an insulated duct wall; liner adds an air-side surface inside a metal duct. They are not interchangeable construction systems. Use the specific product listing and assembly instructions for pressure class, velocity, erosion resistance, moisture exposure, fire and smoke performance and cleaning methods.

Internal liner reduces the clear airflow area. For example, a 24 × 12 in inside-metal section with 1 in liner on all four walls has a 22 × 10 in clear opening. At the same airflow, its velocity is higher than a calculation based on the unlined dimensions. Enter clear dimensions in the CFM, velocity and area calculator.

Do not apply a universal 2 in. w.g. pressure class or 4,000 FPM velocity limit to every fiberglass product. Wet, corrosive, grease-laden or high-temperature air streams need application-specific product and code review.

1. Identify the air stream and exposure: ordinary supply/return, grease, chemicals, outdoor exposure or moisture.
2. Confirm the permitted material and assembly, pressure class, fire/smoke listing and product limitations.
3. Use clear internal dimensions and product pressure-loss data where available; check velocity against the project acoustic criteria.
4. Total straight-run, fitting and equipment losses on the governing path before selecting the fan.

The duct velocity limits guide explains the calculator’s screening bands. The duct fitting loss calculator checks representative local losses.

• DOE: Support Flexible Ducts — support against sagging, pinching and compression, following manufacturer instructions.
• ASHRAE: duct sizing and flex compression — separate compression conditions and additional resistance from bends, kinks and excess length.
• ASHRAE Duct Fitting Database — geometry- and flow-dependent fitting data for a detailed loss analysis.

These sources support the design and installation guidance, not certification of the individual roughness values assigned in this calculator.