When Ventilation Is Required and What Type (401.1-401.3)
Refs: IMC 401.1-401.3; IMC 402, 403, 407; International Energy Conservation Code (IECC); ASHRAE 90.1. Local amendments govern.
Plain English:
401.1: Chapter 4 governs ventilation of occupied spaces. Systems listed in 502 (hazardous exhaust, clothes dryer and cooking exhaust, conveyor dust, subslab soil, smoke control, energy recovery, etc.) must also comply with Chapter 5.
401.2: Every occupied space must be ventilated by natural means per 402 or by mechanical means per 403. Dwellings that meet IECC or ASHRAE 90.1 air-tightness must use mechanical ventilation per 403. Ambulatory care facilities and Group I-2 occupancies must follow 407.
401.3: Provide ventilation during the periods the room or space is occupied.
On Plans: Why It Matters
Early redlines often come from choosing the wrong path: tight dwellings submitted with windows only, clinical spaces sized from 403 instead of 407, or claiming natural ventilation without 402 opening math and controls. Decide the path early and document it.
🧾 Where To Show It (sheet map)
M-001 (General Notes): "Ventilation during occupancy per IMC 401. Natural ventilation when demonstrated per 402; otherwise provide mechanical per 403. Dwellings meeting IECC or ASHRAE 90.1 air-tightness shall include whole-house mechanical ventilation per 403. Ambulatory care and Group I-2 per 407. Local amendments govern."
M-101+ (Plans): Tag each space with Vent Path: NAT-402 or MECH-403. For NAT-402, show operable opening areas and controls. For MECH-403, show outdoor-air route and design cfm.
M-603 or M-002 (Sequences): Occupancy logic proving outdoor air delivery when occupied; define standby and unoccupied modes.
M-601 (Schedules): Space Use Type | Vent Path (NAT-402 or MECH-403) | OA Required | OA Provided | System ID | 407 Flag (Y or N).
Code Path: Ventilation Method Statement on M-001
M-001 must state the ventilation method (mechanical, natural, or combination) and the occupancy basis for the ventilation design.
For mechanical ventilation: reference IMC Table 403.3.1.1 or the ASHRAE 62.1 procedure used, with the adopted edition identified.
For natural ventilation: reference IMC 402 and confirm the operable opening area is shown on the architectural drawings.
Check: Ventilation Compliance Path
Confirm M-001 states the ventilation method and standard - both are first-cycle review items.
For mechanical ventilation, verify the ventilation schedule traces to Table 403.3.1.1 with occupancy type, floor area, design occupant load, OA cfm per person, and OA cfm per area.
For natural ventilation, confirm 4% of floor area in operable openings is shown on the architectural drawings and referenced on M-001.
Review Risk:
Missing ventilation method and standard reference on M-001 is the most common 401 first-cycle comment - the reviewer needs the compliance path stated, not assumed.
Mechanical ventilation designs that reference a standard but do not show a traceable ventilation schedule will be flagged as incomplete.
Natural ventilation projects submitted without operable opening area confirmed on the architectural drawings will be rejected at plan review.
Field Tip:
Dwellings: If the envelope targets air-tightness, specify a dedicated whole-house mechanical ventilator or balanced ERV with duty cycle and filtration on M-603.
Clinical: If ambulatory care or Group I-2, jump to 407 on day one. Do not size from 403.
Natural claims: Put the 402 opening area math on the plan. If you cannot prove it, switch to mechanical now.
Controls: Tie outdoor-air enable to occupancy; state fan and damper positions for occupied, standby, and unoccupied.
Masterbuild QA Lens
The first question on any ventilation review is whether the system type shown (natural vs mechanical) is permitted for that occupancy and enclosure tightness. Ambulatory care and Group I-2 must follow Section 407, not 401. IECC/ASHRAE 90.1-tight dwellings cannot rely on natural ventilation. Confirm occupancy basis and enclosure type before sizing anything.
Drawing / Submittal Check
For 401.1-401.2 (general ventilation requirements and applicability): M-001 must state the ventilation method (mechanical, natural, or combination) and the occupancy basis for the ventilation design. For mechanical ventilation, reference IMC Table 403.3.1.1 or the ASHRAE 62.1 procedure used, with the adopted edition. For natural ventilation, reference IMC 402 and show the operable opening area on the architectural drawings.
Common Review Risk
The most common 401 first-cycle
When To Escalate
Escalate when the ventilation design basis is disputed between the mechanical engineer and the AHJ - when the reviewer requires ASHRAE 62.1 compliance but the design team used the IMC table, and the two produce different outdoor air rates. Ventilation basis disputes must be resolved before the permit is resubmitted, not with each revision.
Rated Assembly Coordination
General ventilation requires outdoor air and exhaust paths that frequently penetrate rated assemblies. Identify damper type (fire, smoke, combination) at each penetration based on the assembly classification before completing the ventilation drawing.
Load Assumption Check
General ventilation load calculations must state the design outdoor air CFM, the weather basis (ASHRAE 1% or 0.4% summer), and the OA fraction for each air-handling unit. These inputs directly size the cooling and heating coils.