Following the concept of sustainability in the development of new building systems and envelopes based on renewable and natural materials, which is recently obligatory for the EU member countries through the new Renovation Wave Strategy, the authors present research on the topic of energy renovation of buildings using renewable materials in modular systems and its possibilities in two developing countries. This research presents energy savings through a case study of deep renovation of buildings using timber modular systems for envelopes on the characteristic example of residential buildings. Research focuses on the Prosperous Years of construction period between 1961 and 1980 due to a considerable number of modular buildings designed during the peak of the construction progress and promotion of prefabricated buildings. The total potential of energy savings for such building types was considered according to national typologies of residential buildings in Bosnia and Herzegovina (B&H) and Serbia. Through a comparative analysis of these countries, the research presents the parameters of buildings built in Prosperous Years, which indicate the energy condition of buildings before and after the renovation of the envelope using standard measures from national typologies and the application of wooden modular envelope systems, which have the potential for in-depth renovation of buildings.