Biomass-based aerogels offer a promising potential as alternatives to plastic-based foams for thermal insulation applications. However, their inherent flammability has hindered their practical usage. In this work, we addressed this issue by employing a layer-by-layer assembly technique to deposit two oppositely charged biobased materials, namely phytic acid and chitosan, onto a fully biobased aerogel system. These aerogels were fabricated using cellulose filaments and chitosan and cross-linked with citric acid, resulting in a mechanically robust 3D structure. The synergistic effects of phytic acid and chitosan in the layer-by-layer deposited aerogels significantly enhanced their fire resistance and mechanical strength. The developed aerogel with six bilayer depositions (LBL6), showed an outstanding peak heat release rate (pHRR) of 6.0 kW.m
−2, and total heat release (THR) of 0.4 MJ.m
−2, substantially lower than the previously developed cellulose-based aerogels and foams. LBL6 also demonstrated immediate self-extinguishing behaviour, boasting an impressive limiting oxygen index (LOI) value of 63 %, which is the highest reported for a biobased aerogel. Furthermore, the developed aerogels exhibited a superior Young’s modulus of up to 4.5 MPa, surpassing previously developed flame-retardant aerogels. Additionally, they excelled in thermal insulation properties, with a thermal conductivity of less than 38.2 mW·m
−1·K
−1, placing them in the same range as, or even lower than, commercially available thermal insulators. Given the simplicity of the aerogel development process and the well-known advantages of a completely biobased system, our developed aerogels present a sustainable and environmentally friendly alternative to current commercial thermal insulators that are derived from petroleum-based materials.
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