Definition 1: The building enclosure (or building envelope) is the physical separator between the interior and the exterior environments of a building. It serves as the outer shell to help maintain the indoor environment (together with the mechanical conditioning systems) and facilitate its climate control. Building envelope design is a specialized area of architectural and engineering practice that draws from all areas of building science and indoor climate control.
Definition 2: The building envelope consists of the pressure and thermal boundary in alignment with one another. Normally, this would include the exterior walls, floor, and roof assembly of a building.