This paper presents an integrative and comprehensive examination of sustainable natural-fibre nonwovens and pith-based lightweight bio-composites for acoustic and thermal building applications. The research synthesizes morphological, chemical, mechanical, hygrothermal, and acoustical findings from contemporary experimental and theoretical studies to build a coherent framework for material selection, processing, and application. Key material classes considered include pith-derived mortars and composites, cellulose-based nonwovens (including barkcloth), agricultural residues such as sugarcane bagasse, oil palm empty fruit bunch (OPEFB), almond skins, sheep wool, date palm fibres, and chemically treated wood fibres integrated into flexible polyurethane matrices. The exposition develops causally linked explanations for acoustic absorption, sound insulation, thermal conductivity, moisture interaction, and long-term hygrothermal stability, drawing on microstructural phenomena (porosity, pore size distribution, fibre morphology, cell wall chemistry) and macroscopic performance metrics (sound absorption coefficient, transmission loss, thermal resistance). Theoretical mechanisms are elaborated with attention to viscous and thermal boundary-layer effects in porous absorbers, frame-structure coupling in sandwich panels, and the role of fibre-hydrophilicity and bound water in heat and mass transport. Practical design principles and manufacturing recommendations are given, covering nonwoven web formation, binder selection, density control, surface treatments, and hybridization strategies for simultaneous acoustic and thermal performance. Limitations in current evidence are critically appraised, including variability due to botanical origin and harvest year, measurement standardization challenges, durability under cyclic moisture and temperature loading, and scale-up constraints. The paper concludes with prioritized research directions to advance the deployment of bio-based acoustic-thermal materials in buildings: standardized characterization protocols, predictive multiscale models, life-cycle assessment aligned with performance outcomes, reproducible processing routes, and pilot-scale field demonstrations. The integrated perspective offered here aims to help researchers and practitioners move from promising laboratory-scale materials to robust, certified building products that contribute to circular material flows and improved indoor environmental quality.

natural fibres, pith composites, nonwovens, sound absorption

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