The sustainable renovation of the existing building stock is increasingly recognized as a critical priority, requiring interventions that go beyond the sole combination of seismic and energy upgrades. Such interventions must be conceived using a sustainability-informed Life Cycle Thinking (LCT) approach, which emphasizes minimizing environmental, social and economic impacts throughout the entire building life cycle. LCT promotes the adoption of new design objectives such as adaptability, flexibility, damage minimization, and circularity, among others.

Among recent innovations, structural exoskeletons have emerged as promising solutions for integrated seismic and energy retrofitting. These systems offer the additional benefit of avoiding occupant displacement during renovation, thus overcoming one of the major barriers to large-scale building renovation. Early applications featured steel-based exoskeletons, later evolved into timber solutions to leverage the environmental benefits of bio-based renewable materials. However, timber exoskeletons face some limitations, especially when applied to buildings with irregular façade layouts, such as those with ground-floor garages or inconsistent opening distributions.

To address these constraints, this paper proposes a novel hybrid steel/wood shell exoskeleton system designed for the renovation of reinforced concrete (RC) or unreinforced masonry buildings typically built at the end of the last century. The proposed solution utilizes dry-assembled, modular, and prefabricated components, aligned with LCT criteria such as reusability, demountability, and material efficiency. The synergy of steel and CLT (cross-laminated timber) elements is strategically exploited: CLT panels resist in-plane shear forces, while steel profiles handle tensile and compressive forces at the corners, ensuring overturning moment resistance.

This study discusses some limitations of existing exoskeleton systems, especially timber-based, and introduces the proposed hybrid solution as a viable, sustainable, and more adaptable alternative. Its effectiveness is demonstrated through its application to a representative case study, highlighting its potential to broaden the feasibility of structural exoskeleton retrofits, especially for buildings with complex façade geometries.