IMC 602.3.7-602.4 addresses materials and flood risks in plenums. Exposed materials must meet strict flame/spread and smoke limits, and plenums in flood zones must prevent water intrusion or resist hydrostatic forces.
Plain English:
602.3.7 Foam Plastic
Exposed foam finish/trim requires BOTH ASTM E84 or UL 723 flame spread ≤25 and smoke developed ≤50 at the maximum thickness and density intended for use AND NFPA 286 meeting IBC 803.1.1. Approval through tests under IBC 2603.9 replaces only the NFPA 286 test.
A protected-foam exception instead requires ASTM E84 or UL 723 flame spread ≤75 and smoke ≤450 at maximum intended thickness and density AND separation from plenum airflow by an IBC 2603.4 thermal barrier, corrosion-resistant steel ≥0.0160 in. base metal, or masonry/concrete ≥1 in. Confirm the exact product and assembly. 2024 IMC 602.3.7 source.
602.3.8 Plastic Plumbing Piping and Tubing
Must be listed and labeled with flame spread <=25 and smoke <=50 (ASTM E84/UL 723).
Exception: UL 2846-listed water piping with defined smoke/flame performance.
602.3.9 Pipe and Duct Insulation
Insulation, adhesives, and coatings must meet flame spread <=25, smoke <=50 (ASTM E84/UL 723), and cannot flame, glow, smolder, or smoke per-ASTM C411 at service temperature >=250 degrees F.
602.3.10 Other Combustible Materials
Must be listed and labeled with flame spread <=25 and smoke <=50.
602.3.6 Discrete Electrical, Plumbing, Mechanical Products
Exposed products must be listed for plenum use per UL 2043.
Exception: metallic electrical enclosures.
602.4 Flood Hazard
Plenums in flood zones must be above IBC 1612 utility elevation or resist water entry, hydrostatic and buoyancy loads.
On Plans: Why it matters
Exposed foam, piping, insulation, and devices without proper listing or separation are common redlines. Flood-zone plenums are often overlooked in plan review.
Code Path: Where to show it
M-001: plenum-material and flood-hazard notes
M-101: plenum extents and exposed components
M-501: protection details for foam, insulation, piping, and enclosures
E / FP / P / controls sheets: listing basis and exceptions
Arch / civil: flood elevations and resistant construction
Check: Do
Call out the listing/test basis for all exposed materials
Show protection or separation where using exceptions
Coordinate flood-zone elevation with plenum location
Review Risk: Don't
Don't assume "above ceiling" is compliant
Don't place non-listed materials in plenums
Don't ignore flood design requirements
Field Tip:
Review the plenum like a product matrix: foam, plastic piping, insulation, adhesives, and exposed devices. Confirm listing, protection, and flood compliance.
MasterBuild QA Lens
For a flood-hazard plenum, compare its elevation with the IBC 1612 utility elevation. IMC 602.4 permits an engineered below-elevation path only with water-entry/accumulation protection and resistance to hydrostatic and hydrodynamic loads, including buoyancy. Coordinate equipment and electrical requirements separately.
Drawing / Submittal Check
For 602.3.7-602.4 (plenum materials and flood hazard): if the project is in a flood hazard area, M-001 must note the flood hazard zone and confirm that the plenum design accounts for potential flooding. For projects outside flood zones, M-001 should state 'not in flood hazard area' to prevent the reviewer from asking. Confirm the plenum is not serving as a water drainage path.
Common Review Risk
A plenum below the required flood elevation needs a documented IMC 602.4 design; a generic flood-zone note is insufficient. Show the elevation and the chosen water and load-resistance details on the drawings.
When To Escalate
Escalate when the project is in a flood hazard area and the plenum contains mechanical and electrical equipment that has not been flood-proofed. FEMA and the local floodplain administrator have jurisdiction over flood hazard area installations in addition to the building official. Confirm the review path with all authorities before the permit is filed.
Rated Assembly Coordination
Sections 602.3.7-602.4 address plenum material restrictions and separation from adjacent spaces. Where the plenum is adjacent to a rated shaft or corridor, confirm the common wall meets the rated assembly requirement and is not compromised by plenum openings.
Load Assumption Check
Plenum material restrictions in 602.3.7-602.4 may eliminate some insulation or duct liner products, requiring a higher R-value assembly with less thickness. Confirm the plenum-rated insulation system achieves the required R-value and that this is reflected in the energy model or load calculation.