/ Field knowledge · IMC 603
Flex through a wall is not a connector.
IMC 603: most comments are not airflow comments. They are detailing comments — wrong flex, no pressure class, no seal class, no balancing access, underground duct with no test path.
Educational 2024 IMC 603. Confirm adopted edition and SMACNA edition. This is not a duct design or a leakage test.
- Residential sizing: ACCA Manual D. Other buildings: ASHRAE Fundamentals or equivalent calculations. Metallic ducts: SMACNA 006. Nonmetallic: UL 181, SMACNA, NAIMA, plus gypsum/phenolic limits.
- Flex duct is part of the run. A flexible connector is a short connection piece. Connectors are limited to 14 feet and cannot pass through walls, floors, or ceilings. Both still need UL 181, temperature, and appliance-clearance compliance.
- Penetrations per 607. Dwelling-to-garage: special continuity, gauge, and no-opening rules. Underground: approved, sloped, sealed, tested, sometimes concrete-encased.
- Air dispersion systems: fully exposed, positive pressure only, out of rated construction, listed to UL 2518. Registers/grilles/diffusers: manufacturer instructions, accessible balancing means, floor-register and wet-room limits.
“Per SMACNA” with no pressure class
M-001 must name the SMACNA standard and edition and the pressure class for each system. Shop drawings after fabrication is already in the building is the expensive path.
Source: DCT 603 · insulation: 604 · plenums: 602 · calc: calculation support.
What to send
The duct schedule if one exists, whether flex is shown through rated walls, and the pressure class if anyone wrote it down.
If flex is drawn through the wall
Send the M-sheet. Masterbuild will say whether 603.6 is even on the path — or the pressure class was never stated.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
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