IMC 602.2 is where plenum design shifts from "where it is allowed" to "what it is made of." For plan review, this is where exposed materials, gypsum-board plenums, and framing-cavity plenums often fail because the drawings show the space, but not the compliance path.
Plain English: Plain-English Highlights
602.2 Construction of Plenums
Plenum enclosure materials exposed to airflow must comply with IBC 703.3 or have a flame spread index not over 25 and smoke-developed index not over 50 per ASTM E84 or UL 723.
Gypsum board can form a plenum only where air temperature does not exceed 125°F and the gypsum surface stays above the airstream dew point.
Gypsum-board supply plenums cannot be used in systems with direct evaporative cooling.
602.2.1 Stud Cavity and Joist Space Plenums
Stud cavities and joist spaces cannot be used as supply-air plenums.
They cannot be part of a required fire-resistance-rated assembly.
Stud cavities cannot convey air from more than one floor level.
These plenums must meet IBC floor-penetration protection and be isolated from adjacent concealed spaces with approved fireblocking.
Exterior wall stud cavities cannot be used as plenums.
On Plans: Why it matters
Common misses are predictable: no material rating shown, gypsum plenums used without checking temperature/dew point, or framing cavities treated like free duct space without addressing fireblocking, penetrations, or rated-assembly conflicts.
Code Path: Where to show it
M-001: IMC 602.2 material criteria and gypsum-plenum basis
M-101 / coordination plans: plenum type and framing-cavity use
M-501: gypsum plenum details, cavity-plenum details, fireblocking, and penetration protection
A / LS sheets: rated assemblies, concealed-space boundaries, and floor-level limits
Notes / schedules: exposed material listing or ASTM E84 / UL 723 basis
Check: Do
Identify which plenum surfaces are exposed to airflow
Show flame-spread / smoke-developed compliance
Show why gypsum plenums meet temperature and dew-point limits
Detail fireblocking and penetration protection
Check that framing-cavity plenums are not used for supply air
Review Risk: Don't
Don't assume above-ceiling materials are automatically plenum-compliant
Don't use stud cavities or joist spaces as supply plenums
Don't use framing cavities in required rated assemblies
Don't use exterior wall stud cavities as plenums
Field Tip: Field tip
For framing-cavity plenums, review them like a fire-and-air path at the same time: right air type, not in a rated assembly, no multi-floor path, and isolated from adjacent concealed spaces. If one of those is unclear, expect a redline.
Comment "IMC602" if you want a paste-ready plenum 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.
Rated Assembly Coordination
When rated construction is involved, the drawings should identify the assembly, damper type, access location, actuator/control basis, fire alarm interface if applicable, and who coordinates the opening.
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.