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:
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:
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.
Masterbuild QA Lens
The duct isolation requirement per 601.3 means every duct penetrating a rated assembly must have the assembly type identified on the floor plan, the damper or firestop shown at the penetration, and the UL design number for the protection method in the materials schedule. Missing any of the three will generate an AHJ hold at the rated-assembly review.
Drawing / Submittal Check
For 601.4 (contamination prevention): the drawings must note the requirement to cap and seal all duct openings during construction. Add a construction sequence note on the mechanical drawings confirming that SMACNA IAQ Guideline practices for occupied buildings under construction will be followed. This is a first-cycle
Common Review Risk
Construction contamination of duct systems is a significant IAQ issue on renovation projects. Open duct ends during active construction accumulate dust, debris, and construction materials that are later distributed through the building. Add a construction sequence note requiring duct ends to be capped after installation. This note prevents a common occupant complaint after move-in.
When To Escalate
Escalate for renovation projects where the existing above-ceiling space has never been opened and the materials present are unknown. A pre-renovation investigation of the ceiling space - for asbestos, lead, mold, or non-plenum materials - is less expensive than a discovery during construction that stops work.
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
Section 601.4 contamination prevention requirements set separation distances between outdoor air intakes and exhaust or pollutant sources. Show intake and exhaust locations on the site or roof plan with required clearance dimensions.