: 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 Highlights
• 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.
🏗️ 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).
🔧 Quick 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.
Comment “IMC401” for the Intake and Protection Kit.
DM “ADVISORY” for a fast sweep of your ventilation basis before you file.