Mycelium composites offer a sustainable alternative to lightweight petroleum-based plastic products and are attracting great attention in various fields. However, creating a mycelium composite with both superior thermal insulation and mechanical strength remains a significant challenge. Here, different sizes of poplar or birch sawdust arranged in a loose and disordered manner are employed to construct mycelium composites with multiscale hierarchical porous structure, including both meso- and microscale pores. The morphological, biological, and physicochemical properties of the filamentous mycelium and the resulting composites are discussed in terms of how they are influenced by the substrate features and the way they interact with the fungus. Mycelium composites with high porosity (∼93%) demonstrate energy absorption of 32 kJ m⁻³, cushioning coefficient of 5, thermal conductivity of 0.044 W m⁻¹ K⁻¹, and contact angle of 123°, which is comparable to or even better than expanded polystyrene. Lightweight, thermally insulating, hydrophobic mycelium composites with excellent mechanical properties show broad prospects in construction and packaging.