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:
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 degrees 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:
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.
Masterbuild QA Lens
Plenum material compliance (602.2) covers wiring, piping, insulation, and any other materials placed in the plenum path. Confirm that every material in the plenum is called out with a compliance note (UL 2043, NEC 300.22, or equivalent). This is a coordination check between mechanical, electrical, and architectural - incomplete coordination generates multi-discipline comments on the same review.
Drawing / Submittal Check
For 602.2-602.2.1 (plenum construction): the drawings must call out the plenum construction materials and confirm each is listed or approved for plenum use. All wiring, piping, and equipment in the plenum must be labeled as plenum-rated. A details note that says 'all materials to meet plenum requirements' without specific material call-outs will get a comment - the reviewer needs the materials, not the promise.
Common Review Risk
Plenum-rated materials are required for everything in the above-ceiling return air space - not just electrical. Pipe insulation, duct wrap, and mechanical equipment supports must also be plenum-rated or enclosed. The most common gap is a set that specifies plenum-rated cable but leaves the pipe insulation and supports unaddressed - one non-rated material makes the whole plenum non-compliant.
When To Escalate
Escalate when the project involves an existing above-ceiling plenum with unverified materials from multiple previous tenants, and the renovation scope requires the ceiling space to be opened. A pre-work survey by a qualified inspector is the most cost-effective way to identify non-compliant materials before the ceiling is opened.
Rated Assembly Coordination
Plenum construction rules limit materials in the return-air space, but the plenum boundary itself may be part of a rated floor-ceiling assembly. Confirm that the plenum boundary construction meets both the rated assembly and the plenum material requirements simultaneously.
Duct System Coordination
Plenum construction requirements govern what materials may be exposed in the plenum. Coordinate with the architect on ceiling system type and confirm any piping, cable, or equipment in the plenum space is listed for plenum use.