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This study presents a novel and sustainable method for integrating octadecane phase change material (PCM) into traditional building materials like mortar and gypsum using vacuum-impregnated biochar. Optimising the impregnation conditions resulted in a PCM-biochar composite with 62.21 % PCM loading and a latent heat energy of approximately 116.7 J.g−1, as measured by Differential Scanning Calorimetry (DSC). Thermogravimetric Analysis (TGA) confirmed the composite’s stability at high temperatures, while accelerated DSC validated its phase change capability and stability over 300 cycles. Characterisation via Scanning Electron Microscopy (SEM), Small-Angle X-ray Scattering (SAXS), X-ray Diffraction (XRD), and Solid-State Proton Nuclear Magnetic Resonance (1H NMR) verifies PCM retention within biochar pores and reveals interactions between PCM and biochar. Additionally, the non-pozzolanic nature of biochar is confirmed. Workability tests show reduced consistency with increased PCM-biochar content in mortar. At 40 % sand replacement rate with PCM-biochar, the compressive strength initially decreases by 45.50 % after 28 days, but it improves to 43 MPa after 120 days. Gypsum samples retain adequate strength for retrofitting applications (2 MPa), demonstrating the potential of PCM-biochar composites to enhance thermal energy storage in building materials, thereby supporting Net-zero building objectives.