/ Field knowledge · IMC 401.1–401.3
Tight dwellings do not get windows-only ventilation.
IMC 401.2: every occupied space is natural (402) or mechanical (403). A tight dwelling that meets IECC or ASHRAE 90.1 air-tightness does not get windows-only. Ambulatory care and Group I-2 jump to 407 on day one — do not size from 403.
Educational 2024 IMC 401.1–401.3. Confirm adopted edition. Distinct from 402, 403 table, and 407 / healthcare.
- 401.1: Chapter 4 governs ventilation of occupied spaces. Systems listed in 502 (hazardous exhaust, dryer and cooking exhaust, conveyor dust, subslab soil, smoke control, energy recovery, etc.) must also comply with Chapter 5.
- 401.2: ventilate by natural means per 402 or 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.
Tag each space NAT-402 or MECH-403. For NAT-402, Daily Code Talk’s check is 4% of floor area in operable openings on the architectural drawings — if you cannot prove it, switch to mechanical now. For MECH-403, show the outdoor-air route and design cfm. Tie OA enable to occupancy; state fan and damper positions for occupied, standby, and unoccupied.
Tight envelope, windows only
Missing ventilation method and standard on M-001 is the common first-cycle 401 comment. Clinical spaces sized from 403 instead of 407 is the expensive one. If the reviewer wants ASHRAE 62.1 and the set used the IMC table, resolve the basis before resubmittal — not with each revision.
Source: DCT 401.1–401.3 · natural: 402 · table: 403 · intake: 401.4.
What to send
Whether this is a tight dwelling, I-2 / ambulatory care, or a natural-ventilation claim — plus M-001.
If the dwelling is tight and the set shows windows only
Send M-001. Masterbuild will say whether 401.2 even has a 402 path — or this is already whole-house mechanical.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. Equation 4-9 and Ez-free rates below are 2024 IMC education from Daily Code Talk — not a sealed outdoor-air design.