/ Field knowledge · IMC 603.17
“Fabric duct” above a ceiling is the usual redline.
Air dispersion systems are not just “fabric duct.” IMC 603.17 treats them as a specific product: entirely exposed, positive-pressure only, no fire-resistance-rated penetrations, listed and labeled to UL 2518. Above a ceiling, on exhaust, or through a rated wall is the usual redline.
Educational 2024 International Mechanical Code (IMC) 603.17 from Daily Code Talk. Confirm the adopted edition. Distinct from flexible ducts and connectors (603.6) and registers (603.18). This is not a fabric-duct selection.
- Installed entirely in exposed locations — no above-ceiling or concealed runs.
- Positive-pressure systems only. Cannot be used in exhaust or negative-pressure applications.
- Cannot pass through or penetrate fire-resistant-rated construction. Transition to conventional duct before any rated assembly.
- Listed and labeled in compliance with UL 2518. Identify it as an air dispersion system, not just “fabric duct.”
On the set: exposed routing start-to-end, positive-pressure basis, UL 2518 listing, mounting/termination and transition details, and life-safety confirmation that no rated wall or floor is penetrated.
Fabric sock in a gym, drawn above the ceiling grid
Or used as exhaust. Or run through a rated wall like standard duct. Appearance, cost, and throw are why it gets selected; those four 603.17 limits are why it gets redlined.
Source: Daily Code Talk — IMC 603.17 · flexible: 603.6 · outlets: 603.18 · plenums: 602.
What to send
Where the fabric / air-dispersion run is shown (exposed vs above ceiling), whether it is supply or exhaust, and any rated wall it crosses.
If “fabric duct” is drawn above a ceiling or through a rated wall
Send the layout. Masterbuild will say whether 603.17 still has no UL 2518 / exposed / positive-pressure path.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. UL 2518, exposed-only, and positive-pressure-only are 2024 IMC 603.17 education from Daily Code Talk — not a fabric-duct specification. This page is not legal advice.