Section 403.3/Outdoor Airflow Rate (2018 Edition)
IMC 2018 Section 403.3 covers outdoor air ventilation rates aligned with ASHRAE 62.1-2016.
Mechanical ventilation systems must provide outdoor air at the rates specified in Table 403.3, which uses the two-component approach: a per-person rate (Rp) plus a per-area rate (Ra). The designer must determine the design occupancy from Table 403.3 or the actual expected occupancy (whichever is greater) and calculate the required outdoor air for each zone. The system must be capable of delivering the required outdoor air to each zone during all modes of operation.
Key differences from IMC 2021 and 2024
The 2018 IMC Table 403.3 rates align with ASHRAE 62.1-2016. The 2021 edition updated to ASHRAE 62.1-2019 rates, and the 2024 edition to 62.1-2022. Some space categories have different rates between editions. The section numbering for 403.3 is consistent across all three editions. Jurisdictions on the 2018 IBC cycle use the 2018 IMC rates.
What plan reviewers look for
Plan reviewers check the ventilation schedule for outdoor air rates at each air handling unit. They verify the per-person and per-area rates against Table 403.3 for the correct edition. They check that the design occupancy is documented and matches the occupant load calculation. They verify the outdoor air damper sizing on the equipment schedule.