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This work aims to provide an extensive evaluation on the use of polylactic acid (PLA) as a green, biodegradable thermal insulation material. The PLA was processed by melt extrusion followed by compression molding and then subjected to different annealing conditions. Afterwards, the thermal insulation properties and structural capacity of the PLA were characterized. Increasing the annealing time of PLA in the range of 0–24 h led to a considerable increase in the degree of crystallization, which had a direct impact on the thermal conductivity, density, and glass transition temperature. The thermal conductivity of PLA increased from 0.0643 <inline-formula><math display="inline"><semantics><mrow><mi mathvariant="normal">W</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi mathvariant="normal">m</mi><mo>·</mo><mi mathvariant="normal">K</mi></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> for quickly-cooled samples to 0.0904 <inline-formula><math display="inline"><semantics><mrow><mi mathvariant="normal">W</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi mathvariant="normal">m</mi><mo>·</mo><mi mathvariant="normal">K</mi></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> for the samples annealed for 24 h, while the glass transition temperature increased by approximately 11.33% to reach 59.0 °C. Moreover, the annealing process substantially improved the compressive strength and rigidity of the PLA and reduced its ductility. The results revealed that annealing PLA for 1–3 h at 90 °C produces an optimum thermal insulation material. The low thermal conductivity (0.0798–0.0865 <inline-formula><math display="inline"><semantics><mrow><mi mathvariant="normal">W</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi mathvariant="normal">m</mi><mo>·</mo><mi mathvariant="normal">K</mi></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula>), low density (~1233 <inline-formula><math display="inline"><semantics><mrow><mi>kg</mi><mo>/</mo><msup><mi mathvariant="normal">m</mi><mn>3</mn></msup></mrow></semantics></math></inline-formula>), very low water retention (<0.19%) and high compressive strength (97.2–98.7 MPa) in this annealing time range are very promising to introduce PLA as a green insulation material.