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: Plain-English Highlights
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°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: 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.
Comment "IMC603" if you want a paste-ready duct construction review checklist + M-001 note set.
Masterbuild QA Lens
Duct-system sections are coordination sections. The question is not only whether air moves, but whether materials, insulation, plenums, dampers, access, and penetrations are correct for the location.
Drawing / Submittal Check
Trace the air path across plans, risers, details, schedules, specifications, and reflected ceiling constraints. Confirm duct material, insulation, vapor control, fire/smoke dampers, access, and exposed conditions.
Common Review Risk
Small duct notes create large field cost when they miss rated assemblies, plenum limitations, weather exposure, internal liner restrictions, damper access, or condensation control.
When To Escalate
Escalate when ducts cross rated construction, run outdoors, serve healthcare spaces, use internal lining, connect to smoke control, or pass through congested existing-building conditions.
Rated Assembly Coordination
When rated construction is involved, the drawings should identify the assembly, damper type, access location, actuator/control basis, fire alarm interface if applicable, and who coordinates the opening.
Duct System Coordination
For duct and plenum items, check material limits, insulation continuity, vapor control, access, listed products, and whether the surrounding space changes the requirement.