IMC 607.7 closes the duct and transfer opening section with one categorical rule: flexible air ducts and flexible air connectors are not permitted to pass through any fire-resistance-rated assembly. There is no gage substitution and no ‘short run’ allowance — if the boundary is rated, the flex stops before it and listed sheet steel duct with the correct penetration protection carries through.
🧠 Plain-English Highlights
• 607.7 Flexible Ducts and Air Connectors
- Flexible air ducts and air connectors shall not pass through any fire-resistance-rated assembly. This is a prohibition, not a damper or protection option — the flex simply cannot be the element that crosses the rated line.
- ‘Any rated assembly’ is broad on purpose: rated walls, rated floor/ceiling and roof/ceiling assemblies, rated ceiling membranes, shaft enclosures, fire barriers, fire partitions, fire walls, smoke barriers, and smoke partitions all qualify.
- The rated assembly must still be crossed by a compliant penetration: listed sheet steel duct with the damper, sleeve, firestop, or shaft protection the assembly type requires under 607.5 and 607.6.
- Air connectors — the listed flexible connectors used at equipment — carry the same restriction. They are a terminal connection device, not a rated-penetration component.
🏗️ Why it matters
This is one of the most common field and coordination failures in the entire 607 series, precisely because it looks minor. Flex is the default for the last few feet to a diffuser, a VAV box, or a piece of equipment. When a ceiling gets tight or a wall lands in an awkward spot, the path of least resistance is to push the flex through — and a rated boundary is not always obvious in the field. A short ‘final connection’ that crosses a rated wall, shaft, or ceiling membrane is a life-safety breach and a guaranteed correction, whether it is caught at plan review or at inspection.
🗺️ Where to show it
• M-001: IMC 607.7 flexible-duct limitation note — state plainly that flex and air connectors do not cross rated assemblies
• M-101: duct routing near rated walls, shafts, and rated ceilings — show where flex stops and hard duct begins
• M-501: rated penetration and flex-to-hard-duct transition details
• A / LS sheets: rated assembly locations and required separations the mechanical routing must respect
✅ Do
• Stop flexible duct and air connectors before the rated assembly
• Carry listed sheet steel duct through the rated boundary with the correct 607.5/607.6 penetration protection
• Coordinate rated wall, shaft, floor, and ceiling-membrane locations onto the mechanical plan before routing
• Show the flex-to-hard-duct transition point clearly on plans and details
⛔ Don’t
• Don’t route flex through rated walls, fire/smoke barriers, or fire partitions
• Don’t pass flex through rated floor/ceiling or roof/ceiling membranes
• Don’t treat flex like sheet metal duct at a fire-rated penetration
• Don’t leave the rated-boundary conflict to field judgment — resolve it on the drawings
🔧 Field tip
When you review diffuser and equipment connections, follow every flex run back to the hard duct. If the flex crosses a rated wall line, a shaft wall, a rated ceiling membrane, or a floor assembly, it has to be corrected before permit and before the boundary is concealed. The cheapest place to find it is the drawing; the most expensive is after the ceiling is closed.
**Masterbuild QA Lens**
607.7 is a routing-discipline check, not a product-selection check. Overlay the rated-assembly locations from the architectural life-safety plan onto the mechanical plan, then trace every flexible connection. The compliance question is binary: does any flex or air connector cross a rated line? If yes, the fix is always the same — terminate the flex on the occupied side, carry listed sheet steel duct through the assembly with the required 607.5/607.6 protection, and transition back to flex only after the boundary is cleared.
**Drawing / Submittal Check**
(1) Confirm the rated-assembly locations are shown or referenced on the mechanical background. (2) For each diffuser, VAV, or equipment connection near a rated boundary, show where flex ends and hard duct begins. (3) Detail the rated penetration — sleeve, damper, firestop, or shaft — on M-501; the flex limitation is meaningless without the compliant hard-duct penetration beside it. (4) Add the 607.7 note to M-001 so the prohibition is on the record, not just implied.
**Common Review Risk**
The recurring pattern: a diffuser shown hard against a rated corridor or shaft wall with a flex tail crossing the boundary and no transition or penetration detail. Reviewers and inspectors both look for it because it is so common. A close second: a flex run penetrating a rated floor/ceiling membrane at a transfer or return opening with no CRD-protected hard-duct penetration.
**When To Escalate**
Escalate when the architecture puts a required diffuser, return, or equipment connection on the wrong side of a rated assembly with no room for a compliant hard-duct penetration — that is a coordination conflict to resolve with the architect, not a detail to fudge with flex. Also escalate where a rated ceiling membrane forces a CRD-protected hard penetration the ceiling type cannot easily accommodate.
**Rated Assembly Coordination**
607.7 is the rule that ties the whole 607 series back to the architectural life-safety plan. Flex is allowed everywhere except across the rated line — so the only way to apply it correctly is to know exactly where the rated lines are. Get the rated wall, shaft, floor/ceiling, and smoke-assembly locations onto the mechanical background early, and the flex-routing question answers itself.