The use of natural fiber insulation materials is gaining increasing importance in sustainable construction, owing to their low environmental footprint and renewable origin. Among these materials, straw-based insulations represent a readily available, CO₂-sequestering alternative with significant potential. This research investigates how pressurized wet heat treatment modifies the thermal and structural properties of barley and wheat straw bulks. The experiments were carried out in both bonded and unbonded states, with particular attention paid to the relationship between thermal conductivity, bulk density, and average stem content (total stem length). The study questions the general practice of characterizing natural fiber insulations solely by bulk density. The results show that the changes induced by heat treatment —such as alterations in fiber porosity, density, and geometry— have a significant effect on thermal conductivity. While untreated straws exhibit optimal insulation performance at a bulk density of 120 kg/m3, this correlation weakens after treatment. This suggests that cumulative stem length (average stem content) becomes a more accurate descriptor. The investigation of the optimal stem length is addressed in another section of the paper; in the present study, we simply state the determined optimal value of 6 cm, which was used in all cases. The research recommends using average stem content instead of bulk density to determine the optimal parameters of treated straw-based materials. This new approach enables better performance control and increases the competitiveness of straw-based panels against industrial insulation materials.