Building envelope

The building envelope is the physical barrier between the conditioned inside of a building and the outdoors: its exterior walls, roof, foundation, windows, and doors.

For square footage, the exterior walls of the envelope set the boundary. ANSI Z765 measures gross living area to the outside finished surface of those walls, so the thickness of the walls is part of the total.

The envelope also explains why some spaces fall out of living area. A screened porch or an unheated three-season room sits outside the conditioned envelope, and space that is not heated and finished for year-round use is not counted, even when it has a roof and walls.

Further reading: Building envelope on Wikipedia.