IMC 604 governs how ducts are insulated, lined, identified, and protected against moisture and thermal loss. It applies to supply, return, and exhaust ducts in conditioned and unconditioned spaces, and it interacts directly with rated assemblies, vapor control, and cooling coil performance.
Plain English: This section has more residential-specific traps than it looks. R-values vary by climate zone and duct location. Internal lining rules apply in healthcare and specific humidity-sensitive environments. And 604.13 has a one-line restriction on coil discharge that generates more redlines per word than almost any other IMC provision.
**604.1-604.5 Basics, Listings, and Minimum R-Values**
Duct insulation must be listed, labeled, and installed per manufacturer's instructions.
Insulation materials must not burn in a way that produces excessive smoke or flame: ASTM E84 or UL 723 max flame spread 25, max smoke developed 50.
Minimum R-values are in Table 604.3 by climate zone and duct location (supply vs. return, conditioned vs. unconditioned space, attic, crawl space, exterior).
For residential projects: duct location (inside conditioned envelope vs. vented attic vs. exterior) determines which R-value column applies. This is one of the most common residential energy compliance misses.
**604.6-604.7 Penetrations and Identification**
Duct insulation must not compromise fire-resistance-rated assemblies or fire-blocked cavities.
At penetrations of rated assemblies: confirm the insulation does not fill the annular space required for intumescent firestop, does not bridge the damper, and does not extend into a condition where it will be damaged by the fire damper actuator.
Identification: duct insulation must be permanently identified with R-value, material type, and flame spread/smoke developed index.
**604.8-604.10 Internal Lining, Thermal Continuity, and Service Openings**
Internal lining must be installed per the manufacturer's specification and must not reduce the duct design cross-section below the required area.
Lining must be secured to prevent sagging or separation. Liner joints must be sealed or stapled per manufacturer requirements.
Thermal continuity: insulation must cover the duct continuously. Gaps at hangers, supports, fittings, and equipment connections are common inspection failures.
Service openings in lined ducts must be reinstalled with matching liner to maintain continuity.
**604.11-604.12 Vapor Retarders and Weatherproof Barriers**
Cooling ducts exposed to unconditioned spaces: insulation must have a vapor retarder on the outside surface to prevent condensation from penetrating the insulation.
The vapor retarder must have all joints and seams sealed.
Outdoor ducts: insulation must be protected by a weatherproof barrier. UV-rated jacket or sheet metal cover. Metal-jacketed insulation is the most durable solution for exposed outdoor duct runs.
**604.13 Internal Insulation at Cooling Coil Discharge**
Internal insulation or lining that is water-permeable is prohibited within the cooling coil discharge zone.
The coil discharge zone is defined by the AHU manufacturer's installation instructions; if not specified, a minimum of 24 inches downstream of the coil face is the standard applied.
This restriction exists because condensate from the coil saturates water-permeable internal lining, creating a zone that grows mold and cannot be dried out. It is one of the leading causes of IAQ callbacks on new construction.
Code Path: IMC Table 604.3 (R-value by climate zone and location) -> 604.6 (rated assembly penetration restrictions) -> 604.11 (vapor retarder) -> 604.12 (weatherproof) -> 604.13 (coil discharge restriction).
Check: Before You Submit
Confirm R-values on the duct insulation schedule match Table 604.3 for the climate zone and each duct location in the project.
Flag all ducts in unconditioned spaces and verify vapor retarder is specified and joints are noted as sealed.
Flag all outdoor duct runs and confirm weatherproof barrier is shown.
Flag all AHUs and confirm internal lining stops (or changes to water-resistant facing) within 24 in of the coil face.
Review Risk:
Duct insulation schedule showing a single R-value for the entire project without distinguishing conditioned vs. unconditioned locations is a common redline trigger.
Vapor retarder shown but seams not noted as sealed: the inspector will not assume sealed seams.
Internal lining specified as standard fiberglass liner for the full AHU discharge duct including the coil discharge zone: this fails 604.13.
Field Tip: For every AHU, trace the discharge duct back to the coil face and mark the 24-inch zone. That zone needs zero water-permeable internal insulation. Use washable, closed-cell, or sheet metal with external insulation in that segment.
Masterbuild QA Lens
If the duct insulation schedule is a single line item with one R-value and one material for the entire project, it is incomplete. R-value varies by location, vapor retarder varies by application, and the coil discharge zone requires special treatment. A complete insulation spec takes 10 minutes to write and prevents three separate redlines.
Drawing / Submittal Check
The duct insulation schedule should show: R-value by location, vapor retarder requirement by application, weatherproof barrier for exterior runs, and a note explicitly restricting water-permeable internal lining in the cooling coil discharge zone.
Common Review Risk
The most common 604.13 failure is not on the drawings at all: the AHU submittal arrives with a duct liner schedule that includes standard fiberglass liner at the discharge, and no note excludes it from the coil discharge zone. By the time the liner is installed, the coil is done and the liner is wet.