IMC 604.8 through 604.10 is about what happens when duct insulation and lining meet real equipment conditions. This is where fire dampers, heaters, interrupted liner, and buried access panels can turn a clean duct detail into a coordination problem.
Plain English: Plain-English Highlights
604.8 Lining Installation
Duct liner must be interrupted at the area of operation of a fire damper.
Duct liner must also be interrupted at least 6 inches upstream and 6 inches downstream of electric-resistance and fuel-burning heaters in the duct system.
Where exposed duct liner edges face opposite the direction of airflow, they must be protected with metal nosings or sleeves.
604.9 Thermal Continuity
If duct liner is interrupted, a duct covering with equal thermal performance must be installed.
In other words, removing liner at a fire damper or heater does not eliminate the thermal requirement. The thermal performance still has to be maintained another way.
604.10 Service Openings
Service openings cannot be concealed by duct coverings unless the exact location is properly identified.
If insulation covers an access opening, the field still needs to know exactly where that opening is.
On Plans: Why it matters
These comments usually come from coordination gaps. The mechanical detail may show lined duct, the fire damper detail may show a damper, and the heater schedule may show required clear space, but nobody carries the insulation transition through clearly. The result is interrupted liner with no thermal replacement, raw liner edges, or access panels lost under insulation.
Code Path: Where to show it
M-001: IMC 604.8-604.10 note for liner interruptions, equal thermal replacement, and access identification
M-101: lined duct routing near dampers, heaters, and service-access locations
M-501: damper details, heater-adjacent liner interruption details, metal nosing details, and insulation transitions
M-601: duct insulation / lining schedule with equal thermal performance basis
Check: Do
Interrupt duct liner where fire dampers and duct heaters require it
Show the replacement insulation method where liner is removed
Protect exposed liner edges with metal nosings or sleeves where required
Identify service openings if duct covering passes over them
Review Risk: Don't
Don't run liner through the operating area of a fire damper
Don't leave interrupted liner with no equal thermal replacement
Don't bury service openings under insulation with no identification
Field Tip: Field tip
A good QC check is this: every time liner stops, ask what replaces it thermally and what protects the exposed edge. If the detail does not answer both, the drawing is probably incomplete.
Comment "IMC604" if you want a paste-ready duct insulation 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.