Development of new materials puts a great emphasis on saving production costs, energy, decreasing the amount and number of chemicals used during the manufacturing process. Bio-based materials can be ecologically produced and recycled after their lifespan, which saves the environment. The recent interest in bio-based materials led to the objective of this work. In this article, the structure of spruce wood (Picea abies) biodegraded by the white-rot fungus Trametes versicolor and brown-rot fungus Gloeophyllum trabeum was studied. Structure of the wood was observed macroscopically and microscopically. Classic and unusual stain combinations were used in this work. Ethanol was intentionally omitted in the process of making permanent mounts. It was done to preserve the coherence of the decayed microsections and to keep small fragments from being rinsed away. Results of the observations suggest that spruce wood decayed by these fungi could be used as an insulation material. Wood decaying fungi decrease the density of wood and increase its porosity. A low density and high porosity are important properties for insulation materials. According to the results, spruce wood decayed by Trametes versicolor would be more suitable to be used as an insulating material.