The "Code-Compliant Air Terminal" of Public Architecture: How Fabric Ducts Respect the Fire Safety Red Line

In public buildings, the sentence HVAC engineers fear most is: "The duct looks nice, but it won't pass fire inspection." Traditional metal ducts themselves are non-combustible, yet the external rubber/plastic insulation is often only B1-rated. If fire dampers and fire-stop seals are not properly installed at fire compartment penetrations, the system becomes a hidden path for smoke to travel between floors. Fabric ducts are often mistakenly seen as "cloth-made and therefore flammable." In reality, with compliant design, a fabric duct is a "code-compliant air terminal" that clearly separates four issues: material flame retardancy, no cross-compartment routing, positive-pressure supply, and independent smoke control.

First, the material: for exposed areas in above-ground public zones, permanently flame-retardant polyester fiber (B1 class, e.g., 《GB 8624》 B-s1,d0)​ can be used. For strict-fire locations such as high-rise buildings, underground spaces, and transit concourses, fluorosilicone fabric (A2 non-combustible)​ is available. The fabric itself does not replace the smoke control system—during a fire, the HVAC damper closes and mechanical smoke extraction activates as designed; the fabric duct is only responsible for normal-condition air supply at the terminal.

The key principle is "don't let the duct become a smoke channel":

• No fire-compartment penetration:​ Fabric ducts are suspended and exposed within a single fire compartment; they do not penetrate fire walls or floor slabs. If the main duct must cross a wall, the fire damper and fire-stop sealing are installed on the metal main duct—not forced through at the fabric terminal.
• Positive-pressure supply only:​ Following the spirit of NFPA 90A for air diffusion systems, fabric ducts are typically used for positive-pressure, exposed, non-penetrating installations, preventing fire and smoke from being carried into other rooms via the fabric tube.
• Low smoke, low toxicity:​ Qualified fabrics have low flame-spread indices and low smoke density. Even under extreme heat, they do not release highly toxic smoke, buying time for evacuation.
• Verifiable and documentable:​ Flame-retardant reports, smoke/toxicity tests, and third-party listings (e.g., UL 2518 / ASTM E84 concepts) must be archived. Owner acceptance should never rely on "it looks like cloth."

Therefore, in public building projects, the fabric duct is not a "fire-safety compromise." Used in the right location, it is a safer terminal than "metal pipe + inferior insulation": shopping mall atriums, stadium seating, library reading rooms, and exhibition hall clear-height zones—as long as it does not cross the fire red line or pretend to be a smoke-extraction duct, it performs reliably.