IMC 601.4 is a separation rule. The goal is simple: do not run positive-pressure exhaust, chimneys, or vents through ducts or plenums in a way that can leak contaminants into the air stream.
Plain English: Plain-English Highlights
601.4: The code intent is contamination prevention. If the system leaks, the plenum or duct becomes the distribution path.
601.4 exception 1: Positive-pressure exhaust ducts are allowed in ceiling return plenums only over spaces permitted to have 10 percent recirculation under 403.2.1 item 4, and joints, seams, and connections must comply with 603.9.
601.4 exception 2.1: Chimneys and vents can pass through plenums where the venting system is listed for positive-pressure applications and sealed per the manufacturer's instructions.
601.4 exception 2.2: Chimneys and vents can pass through plenums where fittings and joints between sections are not installed in the above-ceiling space.
601.4 exception 2.3: Chimneys and vents can pass through plenums where they are installed in a conduit or enclosure with sealed joints separating that space from the plenum.
On Plans: Why it matters
This is a common coordination miss. A vent or positive-pressure exhaust run may fit above the ceiling, but that does not make it plenum-compliant. If the path is wrong, the redline is usually straightforward.
Code Path: Where to show it
M-001: IMC 601.4 note and exception basis if used
M-101 / coordination plans: identify plenums and any vent/exhaust routing through them
M-501: details for duct, vent, conduit, or enclosure separation
M-601: system classification notes for positive-pressure exhaust vs vent/chimney routing
A / P / FG sheets: plenum limits, vent listing basis, routing, and enclosure details
Check: Do
Identify return plenums before routing exhaust ducts, vents, or chimneys
Call out when the 10 percent recirculation exception is being used
Show the listing, sealing method, or enclosure detail if using an exception
Coordinate routing early so the path is code-compliant, not just physically convenient
Review Risk: Don't
Don't route a positive-pressure exhaust duct through a plenum without showing the exception basis
Don't assume a vent can pass through a plenum just because it fits above the ceiling
Don't leave sealing, listing, or enclosure requirements implied
Field Tip: Field tip
Start with one question: is this ceiling space a plenum or just a concealed space? Once it is a plenum, trace every positive-pressure exhaust duct, chimney, and vent through that zone and make any exception obvious on the drawings.
Comment "IMC601" if you want a paste-ready IMC 601 review checklist.
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.
Special Exhaust Coordination
For specialty exhaust, start with the contaminant and source. Then confirm capture method, duct material, routing, discharge, separation, controls, and whether another consultant or AHJ review is required.
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.