In Italy and Spain, almost all Europe, in the second half of the 20th century under the influence of the international style (modern style), several million homes have been built. Something that happened in general throughout Europe. Most of these buildings have been built with cavity wall exterior enclosures, with an air gap inside and without thermal insulation. Starting in the 80s, thermal insulation began to be widely used in both countries but in very small thicknesses (2–4 cm plus air chamber inside), which will have to wait for the European EPBD directives, already in the 21st century, and their transposition to the member states, to be increased to average thicknesses of 8–16 cm (depending on the climatic zone where the building is located, the conductivity of the insulating material, as well as the rest of the envelope). This article addresses, on two different case studies located one in Italy and the other in Spain, the privilege of the reduction of energy demand, passive conditioning, the energy behaviour of the envelope, compared to other measures on active systems (conditioning systems). For this, two energy performance evaluations will be used in two different states, one before and one after the energy restoration, which will be non-invasive and acting on the air gap. The results highlight the energetic privilege of the envelope, what we can call the prerogative of building well, above the dimensioning of thermal conditioning systems and the need for research into new insulation materials to intervene in these types of buildings; within these types of envelopes, safeguarding the architectural heritage of the constructions.