In France, and in Europe in general, the building sector is the largest consumer of energy, and accounts for about 43% of the total energy consumption and around 25% of CO2 emissions. The building sector offers significant potential for improved energy efficiency through the use of high-performance insulation and energy-efficient systems. In this study, the thermal behaviour of buildings with an advanced thermal insulation system, particularly, with an aerogel-based rendering/mortar exterior insulation system is examined. In addition to new buildings, the rendering is very suitable for application to retrofit existing ones since it offers high insulation performance and its application is easy, compatible with traditional masonry facades, and using common well-known techniques. Numerical modelling simulations are carried out on three different scales: (a) 1D envelope scale to examine the aerogel rendering’s impact on the thermal and moisture transfer of exterior walls, (b) 2D envelope scale to examine its impact on limiting the heat losses through some types of thermal bridges, and (c) full-scale house to examine its impact on reducing the heating demand. Results show that adding an aerogel-based rendering on the exterior surface of un-insulated or already internally insulated walls reduces significantly or removes entirely the risk of moisture. It also reduces wall heat losses significantly, especially for old un-insulated buildings, and consequently the building’s energy consumption. In addition, this insulating rendering can act as a very suitable solution for some thermal bridges such as the window reveals. Due to its application technique and its high insulating performance, a small thickness can have great impact on reducing such heat losses. So, this type of insulating renderings/plasters can serve as a good solution for places where traditional insulation is difficult to apply or where small insulation thicknesses are needed due to space or construction constraints.