Green Waste and Wheat Straw in Mycelium-Bound Composites: A Multi-Parameter Study on Dimensional Stability, Strength, Humidity Response, and Thermal Insulation Properties

• Volume shrinkage of composites during drying is dominated by reduction in height. • The positive correlation between growth duration and tensile strength is limited. • Vigorous growth is not a safe indicator for strong composites. • Mycelium-bound wheat straw composites exhibit thermal conductivity of ∼ 40 mW/(m∙K) • Removal of surface mycelium can limit humidity uptake and thus thermal conductivity. Mycelium-bound composites enable circular material use by binding bio-based substrates with fungal mycelium into lightweight materials that can be recycled or composted. Optimizing material properties for specific applications depends on pairing suitable substrates and fungi. Fungal selection is often guided by growth speed without considering whether rapid vigor leads to optimal adhesion. Similarly, the influence of mycelium on key properties such as humidity uptake or thermal conductivity remains insufficiently understood. The current study characterized mycelium-bound composites from Ganoderma sessile and Trametes versicolor on green waste, wheat straw, and a mixture of wheat straw and kapok in terms of dimensional stability, humidity response, tensile strength, and thermal conductivity. A deeper understanding was gained by investigating the effects of composite orientation on shrinkage during the drying process, the influence of growth duration on mechanical performance, and the contribution of the surface mycelium on thermal conductivity. Our data revealed a strong preference for height shrinkage during drying compared to the other dimensions, which can be attributed to fiber orientation. While fast-growing fungi such as G. sessile establish adhesion quickly, they can be surpassed by slower species ( T. versicolor ) in the long run. Moreover, strong surface growth can increase humidity uptake, negatively affecting thermal insulation performance. Finally, while composites with wheat straw exhibited competitive thermal conductivities compared to conventional insulators, values for composites with green waste were around 30 % higher. Overall, our findings provide guidance for optimal material design in terms of shape, insulation performance, and stability.