<p>To prevent the spread of fire in buildings, achieve excellent thermal performance, and develop eco-friendly building materials with low environmental loads, specimens were fabricated using Hwangtoh, biochar, vermiculite, and perlite. The flame retardancy of the materials, as an indicator of the resistance of the specimens against fire, was evaluated using a cone calorimeter. The heat release rate, total heat release, and mass loss during heating were analyzed. To evaluate the thermal performance for building-energy reduction, the thermal conductivity, specific heat, and heat capacity were analyzed. To evaluate the potential for harm to the human body in the event of a fire, harmful gases generated during heating were measured, indicating that ignition did not occur in any of the specimens. The mass reduction rate corresponded to that of non-combustible materials. Concentration analysis indicated that the emissions of harmful gases were below the standard, and there was no harm to the human body. Mixing biochar with the lightweight materials led to a decrease in the density and thermal conductivity. Considering the decrease in density, the effect on the heat capacity is acceptable. Therefore, the material can be used as an eco-friendly building finishing material with high thermal performance and fire resistance.</p>