The EU MESSIB project addresses the development, evaluation and demonstration of an affordable multi-source energy storage system (MESS) integrated in buildings, based on new materials, technologies and control systems, for significant reduction of its energy consumption and active management of the building energy demand. This new concept will reduce and manage smartly the electrical energy required from the grid favouring the wider use of renewable energy sources (RES).It will reduce raw material use for thermal performance and improve the indoor environment, the quality and security of energy supply at building, including Cultural Heritage. Furthermore, a significant reduction of the energy unit cost for end-users will be achieved.