# Vapor Retarder Class: Building Code Definition > How the IBC 2009, IRC 2009 define vapor retarder class, where the wording differs, and which chapters depend on the term. - Canonical: https://harth.build/resources/glossary/vapor-retarder-class/ - Last updated: 2026-09-07 - Data source: 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. ## Key facts | Fact | Value | | --- | --- | | Defined in | IBC 2009, IRC 2009 (2009 editions transcribed in this corpus) | | Primary source | IBC 2009 Definitions (model text, not a jurisdiction's adopted text) | | Chapters using the term | 4 (counted across the codes hosted here) | | Wording identical across codes | No (compare before citing one code's wording under another) | ## Direct answer Vapor retarder class is a defined term in the International Building Code (IBC) 2009, in Definitions. The code's own words are: “A measure ofa material or assembly's ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of astm E 96 as follows:” The IRC 2009 defines 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 4 chapters of this corpus, most often in Wall Construction (IRC 2009), Definitions (IBC 2009), Exterior Walls (IBC 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](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.* ## What does this term mean? **Vapor Retarder Class** — IBC 2009, Definitions: > A measure ofa material or assembly's ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of astm E 96 as follows: ![Labeled code-term definition diagram: Vapor Retarder Class: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/vapor-retarder-class/diagram-1344.webp) *How the IBC 2009, IRC 2009 define vapor retarder class, where the wording differs, and which chapters depend on the term.* ## What must be verified before using this guidance? Two of the I-Codes in this corpus define **Vapor Retarder Class**, and the wording is not identical — read the version that governs your question. | Code | Where it is defined | Definition | | --- | --- | --- | | IBC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/definitions.md) | A measure ofa material or assembly's ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of astm E 96 as follows: | | IRC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | A measure of the ability of a material or assembly to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method with Procedure A of ASTM E 96 as follows: | ## Where vapor retarder class is used in the code The term appears in 4 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next. | Chapter | Code | Mentions | | --- | --- | --- | | [Wall Construction](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/wall-construction.md) | IRC 2009 | 4 | | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/definitions.md) | IBC 2009 | 2 | | [Exterior Walls](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/exterior-walls.md) | IBC 2009 | 2 | | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 2 | ### The term in use How the code actually writes about vapor retarder class, quoted from the 2009 text hosted here: > The vapor retarder class shall be based on the manufacturer's certified testing or a tested assembly. — [IRC 2009 §R601.3.2](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/wall-construction.md) ## The terms vapor retarder class 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 vapor retarder class covers: - **[Shall](https://harth.build/resources/glossary/shall.md)** — IRC 2009: The term, when used in the code, is construed as mandatory. ## Vapor Retarder Class: what to open, and in what order Every row is a document this term depends on. The [ICC code library](https://codes.iccsafe.org/codes) and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) cover the middle rows. | Check | Where | Confirmed | | --- | --- | --- | | Read the definition in the adopted IBC | Definitions | | | Read the definition in the adopted IRC | Definitions | | | Re-check the requirement that uses it | Wall Construction (IRC 2009) | | | Re-check the requirement that uses it | Definitions (IBC 2009) | | | Re-check the requirement that uses it | Exterior Walls (IBC 2009) | | | Re-check the requirement that uses it | Definitions (IRC 2009) | | | Confirm the dependent definition | Shall | | | 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](https://harth.build/resources/glossary.md) - [Building: Building Code Definition](https://harth.build/resources/glossary/building.md) - [Construction Documents: Building Code Definition](https://harth.build/resources/glossary/construction-documents.md) - [Registered Design Professional: Building Code Definition](https://harth.build/resources/glossary/registered-design-professional.md) - [Dwelling Unit: Building Code Definition](https://harth.build/resources/glossary/dwelling-unit.md) ## 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](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. ### Source links - [codes.iccsafe.org](https://codes.iccsafe.org/codes) - [iccsafe.org](https://www.iccsafe.org/advocacy/code-adoption-maps/) ## Frequently asked questions ### What is the code definition of vapor retarder class? The International Building Code (IBC) 2009 defines it in Definitions as: “A measure ofa material or assembly's ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of astm E 96 as follows:” 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 vapor retarder class? In this corpus the term is used across 4 chapters, led by Wall Construction in the IRC 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. --- - Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Fvapor-retarder-class%2F - View this page in a browser: https://harth.build/resources/glossary/vapor-retarder-class/ - All resources for agents: https://harth.build/llms.txt