> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/support-and-training/other-support/faqs/design-and-specification/materials/rigid-air-barriers.md). # Rigid Air Barriers Rigid air barriers, also known as external sheathing boards, are key components in rainscreen and open façade systems. Positioned behind the outer cladding and ventilation cavity, they form a durable, continuous layer over the insulation and internal lining, offering a high-performance alternative to flexible membranes. While particularly suited to mid- and high-rise buildings facing harsh environmental conditions, they are equally effective in low-rise applications. ## **Key Features** ### **Condensation Management** Rigid air barriers are vapour permeable, performing similarly to Class 4 vapour permeable membranes. This allows internal moisture to escape through the building envelope, reducing the risk of interstitial condensation and improving long-term wall health. ### **Durability** When joints are taped and the system is fully sealed, the barrier forms a secondary weathertight layer. This makes it suitable for exposure during construction, protecting internal elements and maintaining envelope integrity even before the final cladding is installed. ### **Material Composition** Typically made from fibre cement or similar rigid, durable materials, rigid air barriers outperform pliable membranes in terms of: * Wind resistance * Structural rigidity * Fire safety (some are classified as non-combustible) ### **Additional Protection Options** In more demanding conditions, an additional membrane layer can be applied over the rigid air barrier to enhance airtightness and moisture resistance. ### **Thermal Performance Contribution** While rigid air barriers do not provide significant standalone thermal resistance (typical thickness: 13–16 mm), they support thermal performance indirectly by: * Promoting dry wall assemblies * Preventing thermal bridging from wet insulation * Improving real-world R-values through moisture management ### **Applications and Benefits** Rigid air barriers are a staple in modern building envelope design, offering high durability, excellent weather protection, and robust support for energy and condensation management strategies. Their versatility and reliability make them a preferred solution for both commercial and residential projects, particularly where long-term performance and construction resilience are essential. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.betterbuilding.io/support-and-training/other-support/faqs/design-and-specification/materials/rigid-air-barriers.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.