IMC 603.7 through 603.10 is where duct detailing becomes inspection-critical. These sections deal with penetrations, underground conditions, closure systems, and supports.
Plain English:
603.7 Rigid Duct Penetrations
Duct penetrations of walls, floors, ceilings, roofs, and air transfer openings must be protected per Section 607.
At a dwelling-to-private-garage separation, ducts must be continuous, minimum 26 gage sheet steel, and have no openings into the garage.
Dampers are not required there unless IBC Chapter 7 requires them.
603.8 Underground Ducts
Ducts must be approved for underground installation.
Metallic ducts without approved coating must be encased in at least 2 inches of concrete.
603.8.1-603.8.3 Underground Requirements
Underground ducts need 1/8" per ft slope to drainage with access.
They must be sealed, secured, and tested before burial or encasement, including IECC leak testing.
Plastic underground ducts must meet the PVC/HDPE material rules and 150 degrees F max design temperature.
603.9 Joints, Seams and Connections
Joints, seams, and connections must follow SMACNA 006 and NAIMA where applicable.
Ductwork must be fastened and sealed with approved closure systems.
UL 181A applies to fibrous glass duct closure products. UL 181B applies to metallic ducts, flex ducts, and flex connectors.
603.9 exception
For ducts under 2" w.c., extra closure systems are not required for continuously welded joints or locking-type joints.
That exception does not apply to snaplock or button-lock joints outside conditioned spaces.
603.10 Supports
Ducts must be supported per SMACNA 006.
Flexible and factory-made ducts must follow manufacturer support instructions.
On Plans: Why it matters
Common misses are garage penetrations shown like typical ducts, underground ducts with no slope or testing basis, wrong sealant callouts, or supports left to field interpretation.
Code Path: Where to show it
M-001: penetration, underground duct, sealing, and support notes
M-101: Duct Routing
M-501: penetration details, underground sections, joint details, and hanger details
A / LS sheets: rated assemblies and dwelling-garage separation coordination
Check: Do
Detail garage penetrations separately from typical penetrations
Show underground duct slope and access point
Call out the correct UL 181 closure system basis
Review Risk: Don't
Don't use a typical duct detail at a dwelling-garage separation
Don't bury duct without showing sealing, slope, and testing
Don't mix UL 181A and UL 181B products incorrectly
Field Tip:
Ask four questions on each run: does it penetrate something, go below grade, need a closure system, or need special support? If yes, the detail should be explicit on the set.
Masterbuild QA Lens
Duct sealing and leakage requirements (603.9) apply to all supply and return ducts. Confirm the sealing class is stated on M-001, that all joints, seams, and fittings are addressed in the materials schedule with the sealant type, and that the test protocol for duct leakage verification is in the permit set. A duct system without a stated sealing class will generate a
Drawing / Submittal Check
For 603.7-603.10 (duct liners, vibration isolation, hangers): the schedule must show the hanger type and spacing for each duct size. Duct liner type and thickness must be called out - confirm the liner is listed for the application (supply, return, or exhaust). Vibration isolation at equipment connections must be shown on the connection detail. Missing hanger spacing on large horizontal ducts is a common comment.
Common Review Risk
Duct hanger spacing non-conformance is a common finding on inspection, particularly for large ducts that require additional reinforcement. A large duct that is under-hung will deflect, causing joint separation and air leakage. Confirm the hanger spacing schedule matches the installed hanger locations before the rough-in inspection.
When To Escalate
Escalate when duct hanger non-conformance affects a large duct that is also a rated assembly boundary - when adding the required hangers requires penetrating or disturbing a fire-rated ceiling or floor assembly. Additional duct hangers through rated assemblies require the fire rating to be restored, which may require a third-party fire stop inspector.
Rated Assembly Coordination
Duct sealing and closure requirements at 603.7-603.10 apply to all ducts, including those passing through or adjacent to rated assemblies. Confirm that duct sealing at rated wall penetrations does not substitute for a required damper or firestop assembly.
Duct System Coordination
Sections 603.7-603.10 cover duct closure, connections, and sealing requirements. The drawings should call out sealant class and indicate where mastic, tape, or mechanical fasteners are required at joints and seams.