A building envelope is the physical separator between the conditioned and unconditioned environment of a building, which includes resistance to air, water, heat, light, and noise transfer.[1]​[2]​.

The term envelope is used in construction to refer to the surrounding surface, “membrane” or “skin” built that separates the interior of the building from the exterior in which it is located, “filtering” the conditions of that exterior. Obviously the conditions inside and outside are different, and the outside conditions are rarely compatible with people's well-being. Hence the need for a technical filter that will take advantage of the beneficial aspects of the external conditions, while avoiding the harmful ones, thus taking care of the well-being of those who inhabit the building. In addition, separating from the outside allows the also basic objective of achieving privacy[3]​.

Thus, the thermal envelope of a building is the part that is in contact with the outside air, the layer through which we notice the cold or heat and which is most affected by inclement weather.[4]​.