> 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/surface-emissivity.md). # Surface Emissivity Surface emissivity is a measure of a material’s ability to emit thermal radiation. It is defined as the ratio of radiation emitted by a surface to that emitted by a perfect blackbody at the same temperature. Emissivity values range from 0 (a perfect reflector) to 1 (a perfect emitter). Most building materials fall within the range of 0.8 to 0.95, meaning they emit most of the thermal radiation they absorb. This property is critical for accurate thermal modelling. In platforms like EnergyPlus, emissivity can be set for various building components—walls, roofs, floors, and glazing. These values are used to calculate radiative heat transfer between surfaces and their surroundings. By specifying surface emissivity in Speckel, users can simulate longwave radiation exchange more accurately. This plays a key role in predicting surface temperatures, thermal comfort, and overall energy performance of the building envelope. --- # 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/surface-emissivity.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.