/ Field knowledge · IMC 602.2
Supply air in the stud bay is not a plenum.
A plenum is not “the space above the ceiling.” IMC 602.2: enclosure materials exposed to airflow comply with IBC 703.3, or flame spread not over 25 and smoke-developed not over 50 (ASTM E84 / 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 with direct evaporative cooling. Stud cavities and joist spaces cannot be used as supply-air plenums.
Educational 2024 International Mechanical Code (IMC) 602.2–602.2.1 from Daily Code Talk. Confirm the adopted edition. Distinct from 602 overview and 602.3 listings / flood. This is not a plenum listing.
- Exposed-to-airflow materials: IBC 703.3 or flame spread ≤25 / smoke ≤50 (ASTM E84 / UL 723).
- Gypsum plenums: air ≤ 125°F and gypsum surface above the airstream dew point. No gypsum-board supply plenums on systems with direct evaporative cooling.
- 602.2.1 stud cavity and joist space: cannot be used as supply-air plenums. Cannot be part of a required fire-resistance-rated assembly. Stud cavities cannot convey air from more than one floor level. Meet IBC floor-penetration protection and isolate from adjacent concealed spaces with approved fireblocking. Exterior-wall stud cavities cannot be used as plenums.
On the set: M-001 material criteria and gypsum-plenum basis. M-501: gypsum and cavity details, fireblocking, penetration protection. Common misses: no rating shown, gypsum without temperature/dew-point, framing cavities treated as free duct without fireblocking or rated-assembly conflicts.
Supply air in the stud bay, gypsum over a 140°F coil
Or an exterior-wall cavity used as a return. Those fail 602.2.1 even when 602.1 “where allowed” was answered correctly.
Source: Daily Code Talk — IMC 602.2 · 602: overview · listings: 602.3–602.4.
What to send
Whether the ceiling or a stud/joist cavity is used as a plenum, supply vs return, the leaving-air temperature, and whether gypsum or evaporative cooling is in that path.
If supply air is in the stud bay
Send the section. Masterbuild will say whether 602.2.1 still has a supply-cavity miss — or 602.2 still has gypsum over 125°F.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. 25/50, 125°F gypsum, no supply in stud/joist cavities, no exterior-wall plenums, and no gypsum supply with direct evaporative cooling are 2024 IMC 602.2 education from Daily Code Talk — not a plenum listing. This page is not legal advice.