AEO Answer · Parking Garages & Structures

How Does Natural Ventilation Work in Open Parking Garages?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

Natural ventilation in open parking garages works through wind and thermal buoyancy across code-required perimeter openings on multiple sides; the design proves the openness percentage and distribution on every level, keeps the cross-ventilation path clear, and adds mechanical exhaust where openness cannot be achieved.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of building MEP systems: ductwork with diffusers, an electrical panel with conduit, and piping runs.

Proving openness level by level

I build an openness schedule: for each parking level, the perimeter wall area, the open area, the percentage, and the distribution across sides — all taken from the architectural elevations. The schedule is checked against the code criteria for the adopted edition, and any level that falls short is flagged for mechanical ventilation design. The schedule also records the assumptions: opening heights, any screening, and the interior clearances. This document goes to the code reviewer with the permit set, because the open determination is the foundation the mechanical design stands on.

Detailing the openings

Openings have to stay open and stay effective. I coordinate the opening heights with the structural engineer — the spandrel depth and the opening head height are structural decisions with ventilation consequences. Guardrails and barriers at the openings are designed for openness, not solid walls. Where screening is needed for aesthetics or debris, I specify open-area screening and recalculate the net free area. And I detail the transition at stairs, elevators, and attached spaces so garage air does not migrate into the building: vestibules, pressurization, or separation per the code. The openings are the ventilation system — they get engineered like one.

Open garage natural ventilation checklist

An open parking garage natural ventilation design is ready when it clears this checklist. Natural ventilation is free, but only if the design protects it. • Openness percentage and distribution proven level by level against the code criteria • Cross-ventilation path kept clear of storage, tenant space, and solid infill • Supplemental mechanical ventilation where analysis shows dead zones • Opening details coordinated with structure, with net free area after screening • CO monitoring in questionable areas and full mechanical design for any enclosed level

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?How Does Demand-Controlled Ventilation Design Save Energy?How Is Natural Ventilation Designed for Large Buildings?How Do You Model Airflow in Complex Structures?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What makes a parking garage count as open?

The code's openness criteria: perimeter openings totaling a required percentage of the exterior wall area on each level, distributed so air moves across the floor — typically with openings on at least two sides. I calculate it level by level from the architectural drawings, because a garage can be open on the upper levels and enclosed at the ground floor where retail or walls close the perimeter. The determination is made per level, per the code edition the project is permitted under. If a level does not meet the criteria, it gets designed as an enclosed garage with full mechanical ventilation.

Can you add mechanical ventilation to an open garage?

Yes, and sometimes you should. Openness on paper does not assure air movement on a still, hot day in a wind-sheltered urban site. I add supplemental exhaust or jet fans where the analysis shows dead zones — deep floor plates, corners far from openings, levels partially blocked by adjacent construction. The supplemental system is designed to the same standards as enclosed-garage ventilation for the areas it serves. The honest engineering answer is that the code minimum and the real condition are two different things, and the design serves the real condition.

What can defeat natural ventilation in an open garage?

Anything that blocks the openings or the cross-floor path. Solid parapets built higher than designed, signage or screening that covers the openings, storage cages or tenant improvements built against the perimeter, and future adjacent construction that walls off a side — I have seen all of them. The design documents the openness assumptions and flags them for the owner: the ventilation works as long as the openings stay open. I also detail bird and debris screening that keeps the openings clear without choking the airflow, because a screen that blocks 40 percent of the opening changes the calculation.

Do open garages need CO detection?

Not typically as a ventilation control, since there is no mechanical ventilation to control — but CO detection still has a role. I specify CO monitoring with alarming in open garages where the configuration creates questionable areas: below-grade ramps feeding the open levels, enclosed pay booths, and attached occupied spaces. The detection does not run fans, but it warns if the natural ventilation is not doing its job. In a garage with a mixed open and enclosed configuration, the enclosed levels get the full detection-controlled mechanical system and the open levels get monitoring.

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