Plain English:
402.1: Natural ventilation is through windows, doors, louvers, or other outdoor openings. Operating mechanisms must be readily accessible to occupants.
402.2: Provide openable area not less than four percent of the floor area served.
402.3: Interior rooms ventilated through an adjoining room need an unobstructed opening not less than eight percent of the interior room floor area and not less than twenty-five square feet. Base the outdoor opening on the total area served.
Exception: Sunroom/patio cover allowed if the interior-to-sunroom opening is not less than eight percent of interior room floor area and not less than twenty square feet.
402.4: For openings below grade, provide outdoor horizontal clear space at least one and one-half times the opening depth (depth measured from average adjacent grade to bottom of opening).
On Plans: Why It Matters
Common redlines: missing four percent math, undersized/obstructed transfer openings, inaccessible operators, and below-grade clear space not shown.
🧾 Where To Show It (sheet map)
M-001: "Natural ventilation per IMC 402. Four percent openable area; adjoining-room transfer opening eight percent and not less than twenty-five square feet. Below-grade clear space one and one-half times opening depth."
M-101+: Tag rooms NAT-402; show floor area, required/provided openable area with window IDs; dimension transfer opening free area; draw below-grade clear-space diagram.
M-601: Room ID | Floor Area (sf) | Vent Path (NAT-402) | Required/Provided Openable Area (sf) | Transfer Opening (Y/N, sf) | Below-Grade (Y/N) | Required/Provided Clear Space (ft).
M-501/502: Details for operable control reach, transfer opening (free area), and below-grade areaway section.
Code Path: Natural Ventilation Opening Documentation
Show operable opening area for each naturally ventilated space on the architectural drawings and reference the calculation on M-001 or a ventilation schedule.
Confirm the opening area meets 4% of the floor area and show the opening dimension on the architectural plan.
For cross-ventilation strategies, show openings on both sides of the space and confirm their locations support airflow through the space.
Check: Opening Area Verification
Confirm M-001 references IMC 402 and shows the 4% opening calculation by space - the reviewer must be able to verify compliance without deriving it from architectural drawings.
For each naturally ventilated space, confirm the opening location is coordinated with the architectural drawings - mechanical drawings showing natural ventilation without architectural confirmation get flagged.
Confirm energy code compliance is addressed - natural ventilation systems may require an override mode to prevent ventilation during extreme weather.
Review Risk:
Natural ventilation designs submitted without the operable opening area calculation and locations on the plan will not pass first review.
Cross-ventilation strategies without openings shown on both sides of the space fail the 402 airflow path requirement.
Opening areas that meet the 4% minimum in total but are placed on the same wall (no cross-ventilation path) will not meet the airflow distribution intent.
Field Tip:
Put the four percent calc next to each NAT-402 room.
List free area for grilles/transoms (not just nominal size).
Note "readily controllable by occupants" and verify reach ranges.
Below grade: depth x 1.5 = required clear space; dimension it.
Masterbuild QA Lens
Natural ventilation compliance lives on the architectural floor plan more than the mechanical drawings. Confirm the four-percent calculation is shown room-by-room on M-101 or on the architectural plans with cross-reference, that the adjoining-room transfer opening dimension and free area are shown, and that below-grade window wells are dimensioned with the 1.5 times clear-space math visible.
Drawing / Submittal Check
For IMC 402 (natural ventilation, 402.1-402.4): the drawings must show the operable opening area for each naturally ventilated space, confirm the opening area meets the code minimum (4% of floor area), and show the opening location relative to the space geometry. For cross-ventilation strategies, show the opening on both sides of the space. Natural ventilation designs submitted without the opening area calculation and the opening locations on the plan will not pass first review.
Common Review Risk
The most common natural ventilation plan review gap is a missing opening area calculation. The drawings show windows and doors, but the reviewer cannot determine if the operable area meets the 4% of floor area minimum without a calculation. A brief summary table on M-001 - space name, floor area, required 4% opening area, and provided operable area - closes this
When To Escalate
Escalate when the natural ventilation design requires the building occupants to operate the windows, but the building owner or property manager has a policy of keeping windows closed. Natural ventilation that depends on occupant behavior is not a reliable ventilation strategy in a managed building. Escalate to a mechanical ventilation alternative or confirm the window-operation policy in writing before relying on natural ventilation.
Ventilation Coordination
For natural ventilation (IMC 402): confirm the operable opening separation from exhaust outlets on the same building - the required distance between outdoor air openings and exhaust outlets applies to operable windows as well as mechanical intakes. A naturally ventilated space that draws air through an operable window near an exhaust outlet is not providing clean outdoor air.