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: Plain-English Highlights
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°F need enough insulation to limit exposed surface temperature to 120°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°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: 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.
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.
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.