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This degree project evaluated the energy and climate impact of thermal bridge mitigation measures in a Swedish Million Programme residential building by linking junction-level performance to whole-building outcomes. The purpose was to identify which junction families contributed most to heat loss and to quantify how selected retrofit strategies affected annual space heating demand as well as operational CO₂e emissions, while also interpreting the findings in relation to practical feasibility, a simplified economic screening, and selected Sustainable Development Goals. The work was conducted as a simulation-based case study of an eight-story building in Malmö using VIP-Energy (v5.0.4). Thirteen representative baseline junction configurations covering balcony-façade, wall-floor slab, and wall-window conditions were modelled in 2D to obtain linear thermal transmittance values (ψ). The ψ-values were combined with recorded junction lengths to calculate building-scale thermal bridge heat loss (H_tb & ΔU_tb) and were integrated into whole-building annual simulations across single and combined retrofit scenarios, including a final BEST package. Operational CO₂e emissions were calculated from delivered space heating energy using a district heating emission factor of 0.1434 kgCO₂e/kWh, and a simplified economic screening was performed using annual energy savings and payback targets of 15 and 30 years. Baseline ψ-values ranged from 0.200 to 0.785 W/(m·K), and the baseline thermal bridge contribution was H_tb = 828 W/K (ΔU_tb = 0.283 W/(m²·K)), dominated by wall slab junctions (49.9%) followed by wall-window (32.9%) and balcony-façade junctions (17.2%). The baseline space heating demand was 198 kWh/(m²·year), and operational emissions were 28.4 kgCO₂e/(m²·year). The BEST packager reduced H_tb by 83.9% and reduced space heating demand to 178 kWh/(m²·year), corresponding to a 10.1% reduction and the same proportional reduction of operational CO₂e to 25.5 kgCO₂e/(m²·year). External insulation continuity at slab edges and improved window detailing and positioning delivered most of the operational benefit, while balcony retrofit measures had negligible whole-building impact in this case. Overall, the study concluded that slab-edge and opening-related junctions were the main priorities in the studied case, and that the most effective measures were best implemented as coordinated façade and window renovation packages. The findings should be read as case-based evidence with indicative relevance for comparable buildings, and were linked most directly to SDG 7 and SDG 13, with qualitative relevance to SDG 11 and SDG 12.