Are Flexible Fabric Ducts Really High-Resistance? Comparing Galvanized, Non-Metallic Composite and Fabric Air Ducts

Every HVAC project comes to the same question: how much resistance do flexible fabric ducts really have? Higher resistance means higher fan static pressure and higher electricity bills for years; lower resistance means a more efficient system. "Non-metallic composite duct" and "flexible fabric duct" are exactly the terms customers search before making a choice. This article answers with measured data from the report Friction Performance Comparison: Flexible Fabric Ducts vs. Galvanized Ducts.

First, clarify the duct categories

Based on 08K508-1, JG/T 258-2018 (Non-Metallic and Composite Air Ducts), GB 50736-2012 and ASHRAE data, the report divides ducts into three types:

• Galvanized metal ducts: spiral round and rectangular lock-form galvanized ducts with airtight walls.
• Rigid insulated non-metallic composite ducts: polyurethane, fiberglass and steel-faced phenolic composite ducts; walls are airtight with no air leakage.
• Flexible fabric ducts: further split into non-permeable (dense coated polyester, no wall air leakage) and permeable (porous fiber textile, uniformly permeable across the wall).

This distinction matters: permeable fabric ducts follow a completely different resistance calculation logic from the others, and most industry confusion comes from mixing them up.

Wall friction coefficients: the measured picture

Comparing wall friction only (measured at 6-9 m/s turbulent flow):

• Spiral round galvanized duct: roughness 0.05-0.10 mm, friction factor 0.018-0.020
• Steel-faced phenolic composite: 0.08-0.12 mm, 0.019-0.021 (up to +6% vs galvanized)
• Polyurethane composite: 0.20-0.30 mm, 0.021-0.025 (up to +20%)
• Non-permeable fabric duct: 0.22-0.35 mm, 0.022-0.026
• Permeable fabric duct: 0.30-0.48 mm, 0.026-0.030
• Fiberglass composite: 0.80-1.00 mm, 0.026-0.032 (+20% to +55%)

Conclusion: on wall friction alone, fabric ducts are not smoother than metal ducts. The raised fibers make them rougher, consistent with neutral fluid-dynamics measurements.

Specific friction loss at constant airflow

At a hydraulic diameter of 0.5 m, air velocity 7 m/s and no wall leakage (see chart above):

• Spiral galvanized duct: approx. 1.02 Pa/m
• Steel-faced phenolic composite: approx. 1.08 Pa/m (+6%)
• Polyurethane composite: approx. 1.26 Pa/m (+24%)
• Non-permeable fabric duct: approx. 1.35 Pa/m (+32%)
• Permeable fabric duct (theoretical, no leakage): approx. 1.52 Pa/m (+49%)
• Fiberglass composite: approx. 1.58 Pa/m (+55%)

GB 50736 recommends an economic specific friction range of 0.8-1.5 Pa/m. Fiberglass composite and permeable fabric ducts easily exceed this at 7 m/s, so engineers usually enlarge duct size and lower velocity to control resistance.

The "resistance puzzle" of permeable fabric ducts

If wall friction is not an advantage, why do some on-site measurements show very low resistance for fabric duct systems? The answer is permeability: while running, a permeable fabric duct releases 5%-15% of total airflow through the wall, so velocity decays continuously along the duct. Since resistance is proportional to the square of velocity, friction loss in the second half drops sharply. On a single long straight run with no branches, the full-length weighted average specific friction is only 0.35-0.6 Pa/m.

Three clarifications are essential:

• Weighted average resistance cannot be read directly from 08K508 tables, which assume airtight ducts at constant airflow.
• The low resistance comes from velocity decay, not a smoother fabric wall; wall friction alone is worse than galvanized.
• The advantage applies only to single long straight runs; with more elbows, reducers and tees, local resistance grows and the advantage shrinks sharply.

Whole-system resistance: what really drives fan selection

Fan static pressure is sized by total system resistance (friction + elbows + reducers + plenum + fittings):

• Galvanized steel duct system: 1.4-2.8 Pa/m (local losses 50%-70%)
• Rigid non-metallic composite duct system: 1.8-3.3 Pa/m
• Non-permeable fabric duct system: 1.6-3.0 Pa/m
• Permeable fabric straight-run system (few elbows, no multi-branch): 0.7-1.3 Pa/m

Design velocities and hydraulic calculation advice

• Round galvanized duct 7-10 m/s; rectangular galvanized up to 8 m/s;
• Fiberglass composite up to 8 m/s; resistance rises sharply at high velocity;
• Non-permeable fabric duct 7-9 m/s, designed like metal ducts;
• Permeable fabric duct inlet 7-9 m/s; mid and end velocities decay naturally with bleed air.

Key reminder: never apply a single specific friction value to the whole run of a permeable fabric duct. Calculate remaining airflow and velocity segment by segment, sum friction and local losses, and keep a 15%-20% fan static pressure margin. Asking the manufacturer for a hydraulic calculation report is the most direct way to avoid undersized fans and underperforming delivery.

Wrap-up

The resistance of a fabric air duct (bag duct, cloth duct) depends on whether it is permeable or not, how long the run is and how many fittings it has - there is no one-size-fits-all answer. RyboTex (Nantong) focuses on fiber fabric air ducts and insulation systems, providing CFD airflow simulation, anti-condensation calculation and standardized hydraulic design services.