/ Field knowledge · IMC 402
Windows are not ventilation until the 4% is on the sheet.
IMC 402: natural ventilation is openable area, not “there are windows.” Four percent of the floor area. Occupants have to be able to operate it.
Educational 2024 IMC 402. Confirm adopted edition and energy-code override. This is not a ventilation design. Managed buildings that keep windows locked are not a 402 strategy. Packet: 402 PDF · ES.
- 402.1: windows, doors, louvers, or other outdoor openings. Operating mechanisms readily accessible to occupants.
- 402.2: openable area not less than four percent of the floor area served. Put the math next to the room — do not make the reviewer derive it from the A-sheets.
- 402.3 adjoining room: unobstructed opening not less than eight percent of the interior-room floor area and not less than 25 ft². Outdoor opening based on the total area served. Sunroom/patio-cover exception: eight percent and not less than 20 ft².
- 402.4 below grade: outdoor horizontal clear space at least 1½ times the opening depth (average adjacent grade to bottom of opening). Dimension the well.
Windows on one wall, no table
Total 4% on the same wall is not a documented airflow path. Use free area of the grille/transom, not the nominal frame size. Keep operable openings away from exhaust outlets the same way you would an OA intake.
Source: DCT 402 · tool: 4% screen · mechanical: 403.
What to send
Floor area by room, which openings actually operate, and whether the owner’s policy is to keep windows closed.
If the comment is “show the 4%”
Send the room areas and the window schedule. MasterBuild will say whether 402 even works — or this building needs 403 mechanical OA.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
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