The reduction of the world-wide CO2-consumption is a key challenge for the international community as a whole. The building sector and in particular the energy consumption of buildings is one of the major contributors to this consumption. Therefore, not only the national and international legislation is challenged, but also the building industry to provide innovative and sustainable solutions for the reduction of building energy consumption. For new buildings, standards and requirements concerning the thermal performance are already set very high. Therefore the energy-saving potential is used rather efficiently in this sector nowadays. However, a much larger energy saving potential lies in the building stock, and particularly in the group of buildings erected before 1980. For these buildings, the transmission heat losses are very high whereas an external insulation is often not applicable. Consequently, internal insulation is the only option. Internal insulation changes the thermal performance of the outer walls significantly. This can lead to moisture problems which is the main reason why internal insulation is often seen critically. This paper introduces an innovative and sustainable internal insulation system based on a lightweight autoclaved aerated concrete. Starting with a discussion on the advantages and disadvantages of the different internal insulation options, the benefits of diffusion-open systems are derived. The working principles of such insulation systems are explained and the particular advantages of AACbased internal insulation – combining the good thermal properties with the excellent hygric and mechanical properties – are highlighted. Finally, the proof by calculation is compared with experience and evidence from building application.