Two rules apply to different product and boundary conditions. Under 2024 IMC 603.6.2.2, a flexible air connector may not pass through any wall, floor or ceiling, even if unrated. Under 607.7, flexible ducts and air connectors may not pass through a fire-resistance-rated assembly. Identify the UL 181 product and actual boundary before selecting a rigid-duct penetration detail.
Plain English: Plain-English Highlights
603.6.2.2 Flexible Air Connectors
A flexible air connector cannot pass through any wall, floor or ceiling, regardless of its fire-resistance rating. A short run or damper does not waive that rule.
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, and fire walls qualify when fire-resistance-rated. Under the model IBC, a smoke barrier generally requires a fire-resistance rating, while a smoke partition may be unrated unless another provision requires a rating. Verify the adopted assembly and actual rating before applying 607.7. Section 607.5.7 separately addresses air-transfer openings through smoke partitions.
For an eligible rigid duct crossing, document material and the damper, sleeve, firestop or shaft protection required by the exact assembly under 607.5-607.6. A smoke designation alone does not prove a 607.7 fire-resistance rating; smoke protection needs its own review.
Verify the UL 181 label: a flexible air connector has the additional any-wall/floor/ceiling prohibition under 603.6.2.2. A flexible air duct has the 607.7 rated-assembly prohibition. Both require applicable listing and installation review.
On Plans: 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.
Code Path: Mark the UL 181 duct-versus-connector choice and 603.6.2.2/607.7 boundaries on M-001 and M-101. Show the transition and eligible rigid-duct penetration protection on M-501; confirm ratings and smoke designations against A/LS sheets.
Check: 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
Review Risk: Do not route a connector through an unrated wall, floor or ceiling. Do not route either flexible product through a rated assembly. Do not equate every smoke partition with a rated assembly or skip the separate smoke-damper review.
Field Tip: 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
First read the UL 181 product label. A connector crossing any wall, floor or ceiling is barred by 603.6.2.2. For a duct or connector, verify the actual fire-resistance-rated assembly under 607.7. Then detail an eligible rigid-duct crossing and independently review smoke protection. Neither a short flex run nor a damper converts a prohibited flex crossing into a permitted one.
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
Overlay fire-resistance ratings and smoke designations from the A/LS plan onto the mechanical routing. The connector rule applies even where an assembly is unrated; the 607.7 duct/connector rule applies when it is fire-resistance-rated. Smoke barriers and smoke partitions need their own classification and damper review.
Model-code source: 2024 IMC chapter text. Verify the locally adopted edition, amendments, assembly, listing, and approved exception for a project.