: Core Rules (403.1-403.2.2)
Plain English: Plain English Highlights
403.1 Ventilation system: Use supply + return/exhaust. Group R-2/R-3/R-4 may use exhaust, supply, or combo. Supply ≈ return + exhaust (overall balance), but the code does not forbid slight positive/negative building pressure. Ductwork/airside per Chapter 6.
403.2 Outdoor air required: Minimum OA is set by 403.3 (table + equations). You can reduce OA only with a stamped engineered design showing equal or better contaminant control than 403.3.
403.2.1 Recirculation: The 403.3 minimum OA cannot be recirculated. Extra air above the minimum can be. Hard stops:
No recirc between different dwellings or dissimilar occupancies.
Pools/decks: no recirc unless dehumidified to ≤60% RH (Manual SPS), and don't send that air to other spaces (>10% blend rule).
Spaces flagged by Table 403.3.1.1 Note b: all supply becomes exhaust (no recirc out).
Spaces flagged by Note g: no recirc to other spaces if >10% of supply is from those spaces.
403.2.2 Transfer air: Using air from occupiable spaces as makeup for kitchens, baths, toilets, elevators, smoking lounges is allowed (unless table prohibits). Meet the exhaust cfm in 403.3.1.1 and make sure the required OA is delivered either directly to those rooms or to the "source" rooms you're transferring from (or both).
On Plans: Why It Matters
Most redlines here come from (1) trying to "count" recirculated air toward minimum OA, (2) overlooking Note b/g prohibitions, and (3) transferring air without proving the OA basis is still met.
🧾 Where To Show It (sheet map)
M-001 (General Notes): "Ventilation per IMC 403. Supply/return or exhaust as applicable. Minimum OA per 403.3. Recirculation limits per 403.2.1 (Notes b/g). Transfer air per 403.2.2."
M-101+ (Plans): Tag spaces with Note b/g restrictions; show transfer paths and door undercuts/reliefs.
M-601 (Schedules): AHU/RTU OA setpoints; DCV logic; pool unit dehumidification and RH setpoint; exhaust/transfer cfm by space.
Field Tip: Quick Checks
Don't credit return/transfer air toward the 403.3 minimum OA.
For any space with Note b/g in the table, verify the recirc rules explicitly.
If you use transfer air, confirm the OA is introduced where required and the exhaust setpoint is actually met.
Comment "IMC403" for the 403 Ventilation Kit (OA/recirc matrix, Note b/g quick guide, transfer-air worksheet).
DM "ADVISORY" for a fast ventilation/IAQ sweep on your current set.
Masterbuild QA Lens
Ventilation posts should translate code language into a permit-ready airflow story: occupancy, outdoor air quantity, intake location, exhaust relationship, controls, and any exception being used.
Drawing / Submittal Check
Verify the drawings show occupancy basis, ventilation method, outdoor air path, intake and exhaust separation, transfer air assumptions, controls, and schedule values that match the calculation.
Common Review Risk
The common review risk is an airflow value without a traceable basis. Another risk is showing outdoor air on a schedule while the plan, intake location, or control sequence tells a different story.
When To Escalate
Escalate for healthcare spaces, high-occupant-load areas, unusual contaminants, existing air handlers with unknown outside air capacity, or any project where pressure relationships matter.
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.