Section 401.2/Ventilation Required
IMC 401.2 requires mechanical ventilation for spaces that cannot be naturally ventilated, establishing when HVAC drawings must include outdoor air provisions.
Every occupied space must be ventilated by natural or mechanical means. Mechanical ventilation must comply with the International Mechanical Code or the applicable ASHRAE standard (62.1 or 62.2). Natural ventilation is permitted where operable openings to the outdoors have an aggregate area of not less than 4 percent of the floor area of the ventilated space. Spaces that cannot meet the natural ventilation exception require mechanical ventilation.
Why this section exists
Indoor air quality is essential for occupant health. Without adequate ventilation, indoor pollutants (CO2, VOCs, moisture, odors) accumulate to levels that cause discomfort, illness, and reduced cognitive function. This section establishes the baseline requirement that every occupied space must have a defined ventilation strategy, either natural or mechanical.
What plan reviewers look for
Plan reviewers check that every occupied space on the floor plan has a ventilation strategy identified on the mechanical drawings. For naturally ventilated spaces, they verify the operable window area meets the 4% minimum. For mechanically ventilated spaces, they check for outdoor air intake sizing, duct connections, and compliance with ASHRAE 62.1 or the IMC ventilation rates.
Common violations
Related IMC requirements
Section 403 covers mechanical ventilation system requirements. Section 501 covers exhaust system requirements. ASHRAE 62.1 provides the ventilation rate procedure that is referenced by the IMC for determining outdoor air quantities.