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:
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:
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.
Masterbuild QA Lens
Duct liner requirements and restrictions (604.4-604.5) prohibit certain liner materials at cooling coil discharge locations. Confirm that the duct schedule distinguishes between lined and unlined sections, that liner is not specified within the first downstream section after a cooling coil, and that the liner material is compliant with 604.4 (no loose or deteriorating liner). Liner violations are common on HVAC renovation projects where existing liner is not inspected.
Drawing / Submittal Check
For 604.8-604.10 (lining installation, thermal continuity, service openings): if the design uses duct lining (internal insulation), confirm the liner is on the schedule with the listed R-value and the listed product. Duct liners must not be used in systems where the airstream can degrade the liner (grease exhaust, high-velocity exhaust). Show service openings in lined duct on the plan - openings must be accessible and sealed after access.
Common Review Risk
Duct liner in grease exhaust or high-humidity exhaust is a fire hazard and a code violation. The most common path to this error is a spec that calls for liner in all supply and return ducts - the installer applies liner to the exhaust ducts as well. Confirm the spec excludes liner from all exhaust systems, not just grease exhaust. Liner removal from installed exhaust duct is disruptive and expensive.
When To Escalate
Escalate when mold is discovered in duct liner after occupancy. Liner mold is a health and liability issue that requires a professional mold assessment, a remediation plan, and a root cause analysis before the liner is replaced. The root cause - wet liner from cooling coil discharge, inadequate vapor retarder, or liner in a prohibited location - must be corrected or the mold will recur.
Rated Assembly Coordination
Internal duct liner (604.8-604.10) used inside ducts that pass through rated assemblies does not substitute for the required penetration protection. Confirm that liner specifications and rated assembly penetration details are both addressed independently on the drawings.
Duct System Coordination
Internal duct lining (604.8-604.10) has restrictions on where liner may be used and surface velocity limits that affect duct sizing. If liner is specified, confirm the duct is sized for the liner's friction contribution and that liner edge is sealed at all connections.