Retrofitting aging buildings with CLT-based mass timber envelopes: integrating junction thermal bridges into energy performance analysis

Retrofitting aging reinforced concrete (RC) buildings with regionally optimized ply-lam cross-laminated timber (PCLT) panels represents a viable low-carbon strategy for public facilities in cold and mixed-humid climates. However, conventional energy analyses often neglect junction-level thermal bridges, which can distort predicted retrofit performance. This study establishes an integrated five-step framework combining ISO 10211-compliant three-dimensional junction modeling (Physibel TRISCO) with a calibrated whole-building dynamic simulation (DesignBuilder) to quantify the impact of junction-level bridges on energy efficiency. Results indicate that neglecting these effects can overestimate heating-related primary energy savings by 15–20 %, while optimized detailing at critical junctions—such as wall–external floor and wall–roof interfaces—reduces equivalent transmittance by up to 50 % and raises surface temperature factors to fRsi ≥0. 82, fulfilling Passivhaus hygiene criteria. At the building scale, the comprehensive Retrofit-TB + scenario achieved a 25 % reduction in annual heating demand compared with the validated baseline, and an additional 5 % reduction relative to simplified retrofits without detailed junctions. The proposed workflow connects local heat-loss mechanisms to overall building performance, offering a reproducible bridge between component-level detailing and policy-driven evaluation. Although the present analysis focuses on opaque envelope junctions, the framework is adaptable to other building types and climates through appropriate boundary conditions and material datasets. The findings provide practical guidance for performance-based retrofit design and support the advancement of national building energy codes and carbon-neutral renovation strategies. • Junction-level thermal bridges reduce heating energy savings by up to 20 %. • Detailed 3D TRISCO modeling quantifies thermal losses at critical envelope junctions. • Retrofitting with ply–lam CLT improves insulation and meets Passivhaus standards. • Thermal bridge detailing increases surface temperatures and condensation safety. • Results support robust retrofitting design for aging public buildings in cold climates.