Building Code Glossary
Outdoor Air: Building Code Definition
How the IMC 2009 define outdoor air, where the wording differs, and which chapters depend on the term.
On this page
Last updated September 7, 2026 · Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.
Direct answer
Outdoor air is a defined term in the International Mechanical Code (IMC) 2009, in Definitions. The code's own words are: “Air taken from the outdoors, and therefore not previously circulated through the system.” No other I-Code in this corpus defines it, so the IMC 2009 definition is the one to work from. The term is used in 15 chapters of this corpus, most often in Ventilation (IMC 2009), Commercial Energy Efficiency (IECC 2009), Exhaust Systems (IMC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. Record the jurisdiction, the adopted edition, the application date and any local amendment that rewrites the definition before you use it on a live project, because a term redefined by amendment changes every requirement that depends on it.
Content and cited inputs reviewed by Scott Barrington, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.
What does this term mean?
Outdoor Air — IMC 2009, Definitions:
Air taken from the outdoors, and therefore not previously circulated through the system.
What must be verified before using this guidance?
Outdoor Air is defined once in this corpus, in the Definitions chapter of the International Mechanical Code (IMC) 2009:
Air taken from the outdoors, and therefore not previously circulated through the system.
Where outdoor air is used in the code
The term appears in 15 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Ventilation | IMC 2009 | 86 |
| Commercial Energy Efficiency | IECC 2009 | 35 |
| Exhaust Systems | IMC 2009 | 5 |
| Fuel Gas | IRC 2009 | 5 |
| Duct Systems | IMC 2009 | 4 |
| Building Planning | IRC 2009 | 4 |
| Means of Egress | IBC 2009 | 3 |
| Duct Systems | IRC 2009 | 3 |
| Special Detailed Requirements Based on Use and Occupancy | IBC 2009 | 2 |
| Definitions | IECC 2009 | 2 |
| Means of Egress | IFC 2009 | 2 |
| Specific Appliances, Fireplaces and Solid Fuel-burning Equipment | IMC 2009 | 2 |
3 further chapters mention the term.
The term in use
How the code actually writes about outdoor air, quoted from the 2009 text hosted here:
Outdoor air intake openings located in exterior walls shall meet the provisions for exterior wall opening protectives in accordance with the International Building Code.
Both outdoor air supply and exhaust ducts shall be equipped with motorized dampers that will automatically shut when the systems or spaces served are not in use.
If only a mechanical exhaust system is installed for a room or if a greater quantity of air is removed by a mechanical exhaust system than is supplied by a mechanical ventilating supply system for a room, adequate makeup air consisting of supply air, transfer air or outdoor air shall be provided to satisfy the deficiency.
The terms outdoor air is built out of
The definition above leans on 1 other defined term. Each carries its own code definition, and a local amendment to any of them changes what outdoor air covers:
- System — IFC 2009: An assembly of equipment consisting of a tank, container or containers, appurtenances, pumps, compressors and connecting piping.
Outdoor Air: what to open, and in what order
Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.
| Check | Where | Confirmed |
|---|---|---|
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Ventilation (IMC 2009) | |
| Re-check the requirement that uses it | Commercial Energy Efficiency (IECC 2009) | |
| Re-check the requirement that uses it | Exhaust Systems (IMC 2009) | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| Confirm the dependent definition | System | |
| Local amendment to any of the above | Jurisdiction's amendment list |
A research aid, not a code determination. The authority having jurisdiction decides which edition and which amendments apply.
Where can you explore related guidance?
- Explore all glossary guidance
- System: Building Code Definition
- Building: Building Code Definition
- Construction Documents: Building Code Definition
- Registered Design Professional: Building Code Definition
Sources and method
Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The ICC code library publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.
Source links
Frequently asked questions
What is the code definition of outdoor air?
The International Mechanical Code (IMC) 2009 defines it in Definitions as: “Air taken from the outdoors, and therefore not previously circulated through the system.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally.
Which parts of the code depend on outdoor air?
In this corpus the term is used across 15 chapters, led by Ventilation in the IMC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own.
What are the ASHRAE requirements for outdoor air ventilation?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IMC 2009 states, in Definitions: “Air taken from the outdoors, and therefore not previously circulated through the system.” It is defined in IMC 2009. It is not the same as Ventilation, which the IBC 2009 defines separately as “The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.” Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.
What are the requirements for ventilation in Ohio?
Ventilation is a separate defined term. The IBC 2009 defines it as: “The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.” That is not the same as outdoor air, which the IMC 2009 defines as “Air taken from the outdoors, and therefore not previously circulated through the system.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
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