In Press. Most of the existing buildings in Spain lack any thermal insulation in their envelope. They are in urgent need to be refurbished to reduce their energy consumption. The standard solution for the rehabilitation of facades is to incorporate thermal insulation materials to reduce heat transmission. This work proposes an extension of the conventional approach: the installation of Trombe walls on sunny facades incorporating an outer layer of thermochromic mortar, the thermochromic Trombe wall (TTW). This improves the energy performance of the existing buildings by enhancing their solar gains. This paper describes the design and configuration of the new TTW system. While traditional solutions can be evaluated with a simple heat transfer equation, a complex energy model is needed to evaluate TTW. Energy simulation is used to determine parameters that influence thermal performance. Thermochromic mortar transition temperature is designed to achieve maximum efficiency. The results obtained from the parametric study were used to determine the aspects of the construction of the first prototype on a laboratory test cell. The information obtained from the experiment will be used for the calibration of the energy model that will establish the potential for the incorporation of the TTW to the existing buildings. This work is part of the tasks developed for the project BIA2014-56827-R ‘Development of an intelligent coating mortar based on thermochromic ecoefficient cements (MICET)’ funded by Ministerio de Economía y Competitividad (Spanish Ministry of Economy, Industry and Competitiveness). The construction system analysis was produced under funding of the Spanish Ministry of Economy and Competitiveness project BIA 2017-83231-C2-1-R: ‘HABITA-RES: Nueva herramienta integrada de evaluación para áreas urbanas vulnerables. Hacia la autosuficiencia energética y a favor de un modelo de habitabilidad biosaludable’. Peer reviewed