This study presents a multiscale investigation of mycelium-based biocomposites produced via solid-state cultivation of Ganoderma lucidum on agro-food sidestreams. Three lignocellulosic residues, wheat bran (in two particle sizes), rice straw, and spent coffee grounds, were selected based on global availability and chemical diversity. The biocomposites were characterized to investigate how substrate composition and mycelial growth influence microstructure and macroscopic performance. Monosaccharide analysis and scanning electron microscopy (SEM) revealed that wheat bran supported enhanced mycelial growth. Fine wheat bran-based composites exhibited compressive strengths up to 449 kPa at 30 % strain and tensile moduli of 15–25 MPa, significantly higher than expanded polystyrene (EPS), a conventional insulator. All biocomposites showed intrinsic surface hydrophobicity (water contact angles of 106–120°). Thermal analyses, including thermogravimetric analysis (TGA) and hot-plate conductivity measurement, confirmed their suitability as porous insulation. Cone calorimetry demonstrated improved fire safety in wheat bran-based composites, with reduced peak heat release rates (112–115 kW/m2). Embodied energy and carbon footprint assessments indicated up to 89 % lower energy demand and 72 % lower CO2 emissions compared with EPS. Through multiscale characterization and direct benchmarking, this study shows how substrate selection and fungal-substrate interactions can be utilized to tailor performance. The findings provide insights into converting low-value biomass into scalable, fire-safer, and environmentally responsible insulation materials.