The increasing demand for environmentally friendly insulation materials has led to the exploration of mycelium-based composites as viable alternatives to insulation materials derived from petrochemicals. This study quantifies thermal conductivity (k) for mycelium composites grown on sunflower husks (unsalted roasted vs. salted roasted) colonized by Pleurotus ostreatus. Across 0–50 °C, k ranged from 0.045–0.058 W/m·K, with the unsalted-husk composite consistently lower than its salted counterpart. The materials are lightweight and produced from renewable feedstocks, aligning with circular-economy objectives. Results suggest competitive insulation performance relative to common commercial materials, while highlighting processing variables (salt content, temperature) that modulate k. Limitations and scale-up considerations are outlined for future work.