Paper-based packaging materials have gained attention from academia and industry for their outstanding environmental sustainability advantages. However, they still encounter major challenges, such as low mechanical strength and inadequate functionality, hindering the replacement of unsustainable packaging materials. Inspired by the remarkable strength of trees provided by cellulose fibers and the water and heat protection of trees provided by bark, this study developed a new biomass-based packaging material (SNC-C) that combines strength, thermal insulation, and water resistance. The material was created by simply blending straw nanocellulose (SNC) with oak bark (i.e., cork), which naturally provides water-resistant, thermal insulation, and unique regenerative properties. The dense layered structure formed entirely by SNC generates a tensile strength reaching up to 60.93 MPa. With the cork cavity structure, the heat transfer rate of the obtained material is reduced to 2.90–3.01 °C/(cm·min). The combining of the closed-cell structure and the suberin component of the cork results in a low water vapor transmission rate (WVTR) of the material of 400.30 g/(m²·24 h). This all-biomass material with excellent performance and low environmental footprint offers a promising solution for the development of sustainable multifunctional packaging materials.