What does the term "thermal envelope" refer to?

The term "thermal envelope" refers specifically to the area between conditioned and unconditioned space within a building. This includes walls, roofs, floors, windows, and doors that serve to separate the indoor environment, which is heated or cooled, from the outdoor environment.

The thermal envelope is crucial for energy efficiency, as it plays a significant role in minimizing heat loss in winter and heat gain in summer. It encompasses all features that affect thermal performance, ensuring the interior space maintains a consistent temperature with minimal energy input.

While options discussing the outermost layer of a building or specific insulation types may touch upon aspects of a building's construction, they do not capture the comprehensive concept of the thermal envelope as a transitional layer that maintains the desired indoor climate. Similarly, while the ceiling structure contributes to the thermal envelope, it is just one component of a more extensive system that involves multiple building elements working together.