Abstract: This study presents the development and characterisation of sustainable, low-density foams for building insulation, fabricated from banana and flax fibres through a low-intensity manufacturing route. By leveraging mechanical fibrillation, borax-mediated cross-linking, and carboxymethyl cellulose as a thickener, stable three-dimensional fibrous networks were achieved with exceptionally low densities, ranging from 12 to 25 kg/m³ . These engineered foams exhibited superior thermal performance, with coefficients of thermal conductivity in the range 0.020-0.029 W/mK, significantly surpassing the requirements for commercial cellulose insulation. The mechanical response revealed an initial stiffness of 62.6 kPa and 25.6 kPa for banana fibre and flax fibre foams, respectively, and confirmed that the banana fibre variant meets the BS ISO 21844 Category 1 standard for structural and thermal insulation applications. Both materials displayed high sound absorption capabilities, achieving weighted sound absorption coefficients of up to 0.68, and enhanced fire retardancy compared to traditional polyurethane alternatives. End-of-life assessments revealed distinct pathways for each material: flax fibre foams demonstrated substantial 28-day soil biodegradability (68.25% mass loss), whereas banana fibre foams showed greater potential for circularity, retaining functional integrity through three recycling cycles. These results highlight the potential of these fibre-based foams as resource-efficient, multifunctional insulation solutions.

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