Building Code Glossary
Lower Flammable Limit (LFL): Building Code Definition
How the IBC 2009, IFC 2009, IMC 2009 define lower flammable limit (lfl), 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
Lower flammable limit (lfl) is a defined term in the International Building Code (IBC) 2009, in Special Detailed Requirements Based on Use and Occupancy. The code's own words are: “The minimum concentration of vapor in air at which propagation of flame will occur in the presence of an ignition source. The LFL is sometimes referred to as "LEL" or "lower explosive limit."” The IFC 2009, IMC 2009 define the same term with different wording, so the controlling text depends on which code governs the question in front of you — the comparison below sets the versions side by side. The term is used in 10 chapters of this corpus, most often in Special Detailed Requirements Based on Use and Occupancy (IBC 2009), Exhaust Systems (IMC 2009), Motor Fuel-dispensing Facilities and Repair Garages (IFC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted.
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?
Lower Flammable Limit (LFL) — IBC 2009, Special Detailed Requirements Based on Use and Occupancy:
The minimum concentration of vapor in air at which propagation of flame will occur in the presence of an ignition source. The LFL is sometimes referred to as "LEL" or "lower explosive limit."
What must be verified before using this guidance?
Three of the I-Codes in this corpus define Lower Flammable Limit (LFL), and the wording is not identical — read the version that governs your question.
| Code | Where it is defined | Definition |
|---|---|---|
| IBC 2009 | Special Detailed Requirements Based on Use and Occupancy | The minimum concentration of vapor in air at which propagation of flame will occur in the presence of an ignition source. The LFL is sometimes referred to as "LEL" or "lower explosive limit." |
| IFC 2009 | Hazardous Materials-general Provisions | The minimum concentration of vapor in air at which propagation of flame will occur in the presence of an ignition source. The LFL is sometimes referred to as LEL or lower explosive limit. |
| IMC 2009 | Definitions | The minimum concentration of refrigerant that is capable of propagating a flame through a homogeneous mixture of refrigerant and air. |
Where lower flammable limit (lfl) is used in the code
The term appears in 10 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Special Detailed Requirements Based on Use and Occupancy | IBC 2009 | 7 |
| Exhaust Systems | IMC 2009 | 7 |
| Motor Fuel-dispensing Facilities and Repair Garages | IFC 2009 | 6 |
| Flammable and Combustible Liquids | IFC 2009 | 5 |
| Semiconductor Fabrication Facilities | IFC 2009 | 3 |
| Hazardous Materials-general Provisions | IFC 2009 | 2 |
| Definitions | IBC 2009 | 1 |
| Definitions | IFC 2009 | 1 |
| Flammable Finishes | IFC 2009 | 1 |
| Definitions | IMC 2009 | 1 |
The terms lower flammable limit (lfl) is built out of
The definition above leans on 3 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what lower flammable limit (lfl) covers:
- Ignition Source — IRC 2009: A flame, spark or hot surface capable of igniting flammable vapors or fumes.
- Refrigerant — IRC 2009: A substance used to produce refrigeration by its expansion or evaporation.
- Explosive — IBC 2009: A chemical compound, mixture or device, the primary or common purpose of which is to function by explosion.
Lower Flammable Limit (LFL): 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 IBC | Special Detailed Requirements Based on Use and Occupancy | |
| Read the definition in the adopted IFC | Hazardous Materials-general Provisions | |
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Special Detailed Requirements Based on Use and Occupancy (IBC 2009) | |
| Re-check the requirement that uses it | Exhaust Systems (IMC 2009) | |
| Re-check the requirement that uses it | Motor Fuel-dispensing Facilities and Repair Garages (IFC 2009) | |
| Re-check the requirement that uses it | Flammable and Combustible Liquids (IFC 2009) | |
| Confirm the dependent definition | Ignition Source | |
| Confirm the dependent definition | Refrigerant | |
| Confirm the dependent definition | Explosive | |
| 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
- Ignition Source: Building Code Definition
- Refrigerant: Building Code Definition
- Explosive: 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 lower flammable limit (lfl)?
The International Building Code (IBC) 2009 defines it in Special Detailed Requirements Based on Use and Occupancy as: “The minimum concentration of vapor in air at which propagation of flame will occur in the presence of an ignition source. The LFL is sometimes referred to as "LEL" or "lower explosive limit."” 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.
Do the I-Codes define lower flammable limit (lfl) the same way?
No. IBC 2009, IFC 2009, IMC 2009 all define the term, but the wording is not identical, and a difference in wording can change which requirements are triggered. The comparison table above sets the versions side by side so you can read the one that governs your question.
Which parts of the code depend on lower flammable limit (lfl)?
In this corpus the term is used across 10 chapters, led by Special Detailed Requirements Based on Use and Occupancy in the IBC 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.
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