IMC 604 starts with a simple idea: duct insulation must perform thermally and safely. It is not enough to show "duct wrap." The drawings need to address IECC compliance, surface temperature, flame/smoke performance, listing, labeling, and foam plastic conditions.
Plain English:
604.1 General
Duct insulation must comply with IMC 604.2 through 604.13 and the International Energy Conservation Code.
604.2 Surface Temperature
Ducts operating above 120 degrees F need enough insulation to limit exposed surface temperature to 120 degrees F.
604.3 Coverings and Linings
Duct coverings, linings, and adhesives must meet flame spread <=25 and smoke-developed <=50 per ASTM E84 or UL 723 using ASTM E2231 mounting.
They must not flame, glow, smolder, or smoke under ASTM C411 at service temperature, with test temperature not below 250 degrees F.
Coverings and linings must be listed and labeled.
604.3 Exceptions
Spray polyurethane foam on exterior ducts in attics and crawl spaces has a separate path: flame spread <=25, smoke <=450, ASTM C411 performance, IBC 2603 compliance, and ignition protection.
Exterior duct coverings not in plenums can use smoke-developed <=450, but still need the required testing, listing, labeling, and ASTM C411 performance.
604.4 Foam Plastic Insulation
Foam plastic used as duct covering or lining must comply with IMC 604.
604.5 Appliance Insulation
Listed and labeled internally insulated appliances are considered compliant with IMC 604.
On Plans: Why it matters
Most insulation comments are not about R-value alone. They come from missing flame/smoke ratings, missing listing basis, wrong foam plastic assumptions, no IECC tie-in, or generic insulation notes that do not tell the reviewer what product path is being used.
Code Path: Where to show it
M-001: IMC 604 note, IECC basis, flame/smoke criteria, and listing requirement
M-101: locations of insulated duct systems
M-501: duct covering, lining, and foam insulation details
M-601: insulation schedule with R-value, material type, listing, flame/smoke rating, and vapor/finish basis
Specs / product notes: ASTM E84, UL 723, ASTM E2231, ASTM C411, IBC 2603 where applicable
Check: Do
Show the duct insulation product path, not just "insulate duct"
Coordinate IMC 604 with IECC duct insulation requirements
Review Risk: Don't
Don't rely on generic duct wrap notes
Don't ignore adhesives when checking coverings and linings
Field Tip:
For every duct insulation note, ask three things: what is the R-value, what is the fire/smoke listing basis, and where is it installed? If the answer changes between plenum, attic, crawl space, exterior, or equipment, the note should change too.
Masterbuild QA Lens
Duct insulation requirements (604.1-604.2) are set by the energy code and the IMC jointly. Confirm the R-value specified on M-001 meets both the IMC minimum and the ASHRAE 90.1 or IECC requirement for the climate zone. The lower of the two standards does not govern - both must be met. State the governing standard on M-001.
Drawing / Submittal Check
For 604.1-604.5 (duct insulation basics): confirm the drawings show insulation on every duct segment in an unconditioned space - including short stub-ups and transitions. A common miss is the duct from the AHU to the first supported horizontal run - if this segment is in an unconditioned mechanical room, it must be insulated. Show insulation breaks at fire dampers and smoke dampers so damper operation is not impaired.
Common Review Risk
The most common 604.1-604.5 finding is supply duct that is insulated in the conditioned space but not in the unconditioned attic or mechanical room above it. The installer insulated the visible portion and stopped at the ceiling. Confirm the insulation schedule clearly states 'all supply and return duct in unconditioned spaces' - not just 'exposed duct'.
When To Escalate
Escalate when the insulation inspection reveals that the installed insulation does not match the specification - when lighter insulation or no insulation was installed in portions of the duct system. Insulation non-conformance in an occupied building requires a decision on whether to open the ceiling, add insulation, and re-close - or to test the energy performance of the existing installation.
Duct System Coordination
Duct insulation requirements (604.1-604.5) set R-value by duct location and system type. The insulation schedule should list each duct zone (conditioned space, unconditioned space, exterior) with the required R-value and product specification.