The presence of an airspace enclosed within a building envelope assembly is known to contribute to the overall thermal performance of the assembly. But, the actual R-value of an airspace can vary significantly depending on various conditions of use, such as the air-tightness of the assembly of materials enclosing an airspace. This research report reviews current technical knowledge and regulatory requirements to provide guidance for appropriate R-value characterization and limitations of use for airspaces within building assemblies for code compliance.