Daily Code Talk #93: IMC 602 Part 2 (602.2–602.2.1 Plenum Construction)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 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. 🏗️ Why it mattersCommon 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. 🗺️ 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✅ 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⛔ 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 tipFor 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.