Core Rules (403.1-403.2.2)
Plain English:
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.
Code Path: Ventilation Schedule
Ventilation schedule must show for each zone: occupancy type, floor area, design occupant load, OA cfm per person, OA cfm per area, and total OA cfm - traceable to Table 403.3.1.1.
M-001 must state: 'Recirculated air does not count toward the IMC 403.3 minimum OA rate' and note any Note b/g spaces with recirculation restrictions.
For systems using transfer air per 403.2.2, show the transfer path and confirm OA is introduced where required and exhaust equals transfer cfm.
Check: OA Schedule and Recirc Restrictions
Confirm the ventilation schedule includes the occupancy-type column - reviewers verify the rate is from the correct row in Table 403.3.1.1.
For spaces flagged by Table 403.3.1.1 Note b, confirm the supply is 100% exhaust (no recirculation out).
For Note g spaces, confirm recirculation from those spaces does not exceed 10% of total supply to other zones.
Review Risk:
Occupant load assumptions are the most common source of ventilation calculation disputes - document the design occupant load and confirm it is consistent with the IBC egress occupant load.
Crediting return or transfer air toward the 403.3 minimum OA rate is the most common 403.2.1 comment - recirculated air cannot count toward OA.
Note b/g spaces without the recirculation restriction addressed on M-001 or in the ventilation schedule generate a first-cycle comment.
Masterbuild QA Lens
Table 403.3.1.1 values are per-person or per-area - not both simultaneously for most occupancies. Confirm which row governs, whether the occupant load is from the IBC or a measured value, and that the calculation sheet or schedule note shows the math explicitly. A reviewer who cannot trace the OA value from table to schedule will write a
Drawing / Submittal Check
For 403.3.1.1 compliance: the ventilation schedule must show for each zone: occupancy type, floor area, design occupant load, OA cfm per person, OA cfm per area, and total OA cfm. The schedule must be traceable to Table 403.3.1.1. Missing the occupancy-type column is the most common reason a ventilation schedule generates a comment.
Common Review Risk
Occupant load assumptions are the most common source of ventilation calculation disputes. If the IBC occupant load (for egress) is used for ventilation sizing without adjustment for actual occupancy, the OA quantity may be oversized. If the design occupant load is lower than the IBC load, document the basis for the reduction - the reviewer will ask.
When To Escalate
Escalate when the IMC Table 403.3.1.1 ventilation rate for the proposed occupancy has not been confirmed with the AHJ - when the occupancy type is unusual enough that the standard table may not clearly apply. Unusual occupancies (data centers with occasional human occupancy, storage facilities with periodic visits) require a written AHJ determination of the applicable ventilation rate.
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
Outdoor air quantities drive supply duct sizing from the AHU to the zone. Lock in occupancy category and ventilation rates before finalizing main trunk and branch duct sizes.