IMC 604.6 and 604.7 are about two review basics: duct insulation cannot compromise rated or fireblocked assemblies, and insulation products need to be identifiable in the field. A generic "insulate duct" note is not enough.
Plain English:
604.6 Penetration of Assemblies
Duct coverings cannot penetrate walls or floors required to have a fire-resistance rating.
Duct coverings also cannot penetrate walls or floors required to be fire blocked.
The duct may pass through where properly detailed, but the covering cannot be carried through in a way that compromises the assembly.
604.7 Identification
External duct insulation, except spray polyurethane foam, and factory-insulated flexible duct must be identified at intervals not greater than 36 inches.
Identification must include manufacturer name, R-value at installed thickness, and flame-spread / smoke-developed indices of the composite materials.
R-value must be based on insulation only, not air films, vapor retarders, or other duct components.
Duct wrap R-value is based on 75 percent of nominal thickness because 25 percent compression is assumed.
Spray polyurethane foam must provide aged R-value per inch in writing to the customer at the time of application.
On Plans: Why it matters
These comments usually show up when insulation details are treated as afterthoughts. Insulation crossing a rated wall, missing product labels, or R-values based on ideal thickness instead of installed thickness can create plan review, inspection, and energy compliance issues.
Code Path: Where to show it
M-001: IMC 604.6 / 604.7 insulation identification note
M-101: insulated duct routing near rated walls, floors, and fireblocked areas
M-501: penetration details showing insulation stop / transition at assemblies
M-601: insulation schedule with R-value, installed thickness, manufacturer, and flame/smoke criteria
A / LS sheets: rated wall, rated floor, and fireblocking coordination
Check: Do
Stop duct covering before rated or fireblocked assemblies where required
Show insulation identification requirements in notes or schedules
Use installed thickness, not just nominal thickness, when documenting R-value
Review Risk: Don't
Don't carry duct covering through rated or fireblocked assemblies
Don't rely on unlabeled insulation in the field
Don't count vapor retarders or air films toward insulation R-value
Field Tip:
For duct wrap, remember the code assumes compression. A nominal insulation thickness does not automatically equal the installed R-value. If the design depends on a specific R-value, the schedule should make the installed-thickness basis clear.
Masterbuild QA Lens
Vapor retarder requirements on cooling ducts (604.3) depend on the duct location and the dew point of the ambient air. Confirm the vapor retarder is specified for all cooling ducts in unconditioned or humid spaces, that the retarder class and installation method are on M-001, and that any insulation applied without a vapor retarder has a documented exception basis. Missing vapor retarder on cooling ducts is an energy code violation.
Drawing / Submittal Check
For 604.6-604.7 (penetrations and identification): confirm the drawings show insulation through penetrations and at supports. Insulation must be continuous at penetrations - a bare metal sleeve through a wall surrounded by duct insulation that stops at the wall face will conduct condensation. Show the insulation type at penetrations on a detail. Insulation identification markings must be called out on the spec or schedule.
Common Review Risk
Insulation gaps at supports and hangers are the most common 604.6-604.7 finding. When the duct insulation is installed before the hangers are set, the hangers compress the insulation, creating a thermal bridge. The correct sequence is to set hangers first, then insulate with insulation split around the hanger saddle. Confirm the installation sequence is in the spec.
When To Escalate
Escalate when insulation gaps at supports and hangers are identified after occupancy - when condensation on exposed metal hangers has caused water damage to the ceiling below. Hanger condensation in an occupied space requires a coordinated repair scope involving the mechanical contractor, the ceiling contractor, and the building owner.
Rated Assembly Coordination
Insulation penetrations at 604.6-604.7 must maintain fire-resistance ratings at every rated assembly crossing. Show insulation termination and sleeve details at each rated wall and floor penetration - insulation cannot fill a gap that requires a listed firestop.
Duct System Coordination
Penetration requirements at 604.6-604.7 govern how insulated ducts pass through walls, floors, and ceilings. Confirm the duct sleeve and insulation continuity detail is shown at every rated and non-rated penetration on the drawings.