Upcycling waste PET bottles into high-performance fiber/flake composite panels for sustainable acoustic and thermal insulation in buildings

Recycled polyethylene terephthalate (rPET) provides an effective pathway for upcycling post-consumer PET bottles into sustainable building insulation materials. This study develops fibrous rPET, flake-only, and hybrid rPET/flake insulation panels bonded with polyvinyl alcohol and systematically evaluates their coupled acoustic and thermal performance for building applications. Fibrous rPET panels exhibit typical porous, viscous–resistive sound absorption behavior, with performance strongly governed by thickness and bulk density and an optimum observed at intermediate density (∼180 kg/m³). Flake-only panels show limited acoustic efficiency, whereas hybrid fiber/flake configurations demonstrate clear synergistic effects. The optimized hybrid panel, consisting of a 30 mm fibrous layer combined with a 10 mm flake layer, achieves a high sound absorption average (SAA) of 0.73, outperforming single-layer configurations of comparable thickness. Thermally, fibrous rPET panels exhibit low effective thermal conductivity ( K e f f ) in the range of 0.031–0.041 W/m·K, comparable to commercial glass wool and rock wool, while hybrid panels remain within 0.039–0.052 W/m·K. Additionally, K e f f remains statistically unchanged under increased relative humidity up to 75%, confirming the moisture-resistant behavior of rPET-based insulation. The fibrous load-bearing layer of the optimized hybrid panel exhibits a flexural strength of 1.3 MPa, adequate for non-load-bearing insulation applications. Overall, the results establish practical design guidelines for thickness, density, and layer configuration of rPET-based insulation panels and demonstrate the potential of PET bottle waste to produce high-performance, multifunctional, and moisture-stable insulation materials for sustainable building envelopes.