Building envelope consists of all components that separate the interior from the exterior in a building, it includes the roof, walls and floor. The most important principle for energy efficient construction is a continuous insulating envelope all around the building, which is also key to maintain comfortable indoor conditions.
Figure from https://passipedia.org/planning/thermal_protection
Thermal insulation is a construction material with low thermal conductivity (U-value), often less than 0.1 W/mK. Main industrial insulation materials are mineral fibre products, cellular plastic products, and plant/animal derived products.
A thermal bridge free design is also very important for an efficient building envelope. A thermal bridge is any discontinuity of insulation or localized decrease in insulation. An adequate building's insulation needs to consider the airtightness of the envelope, since a significant air infiltration degrades the benefits of the insulation used.
The overall performance of the insulation lays therefore on 3 pillars: insulating power of the materials, airtightness of their implementation and the reduction/absence of thermal bridge. The insulation material is more effective if placed from the outside (acting like a warm coat), because this way it treats thermal bridges more effectively, it preserves the thermal inertia of the building, as well as limits the risk of condensation in walls and the level of thermal bridges.
For the case of existing buildings (many pre-1945 buildings) it is important to beware of humidity. They were built with materials that allow moisture inside due to their porosity, which allows water vapor to migrate through the walls (like if the building "breathes"). For these cases, it is important not to disturb the hygrotehrmal balance of the walls, since the confinement of the wall promotes the degradation of the insulation and interior finishes, leading then to a weakening of the structure.