Bio-based insulating materials are increasingly used in construction due to their environmental benefits. However, these materials are particularly susceptible to smoldering fires, a phenomenon of slow, flameless and self-sustaining combustion, that is very difficult to detect. This study includes two experimental approaches to analyze smoldering fires in wood fiber boards with low (50 kg/m³) and high (140 kg/m³) densities. The tests were conducted using a cone calorimeter (ISO 5660-1) and a smoldering fire test device (EN 16733). The cone calorimeter uses small-sized samples with continuous thermal exposure, whereas the smoldering fire test bench involves discontinuous thermal exposure and larger sample dimensions. The objective is to better understand the differences in thermal behavior, gas emissions, and material degradation characteristics, while considering key factors influencing the propagation of smoldering fires, such as density or additives. Despite several studies addressing these key factors, a full understanding of the underlying mechanisms has yet to be achieved. The results show that the low-density wood fiber board degrades more rapidly, reaching high combustion temperatures in a shorter period. In contrast, the high-density fiber board has a greater thermal inertia and prolonged combustion. Regarding gas emissions, concentrations of CO, CO₂, and methane vary depending on fiber density, with low-density samples producing higher yield of CO. These findings aim to enhance the understanding of the smoldering behavior of bio-based materials and to emphasize the importance of chemical aspects such as toxic gas emissions, The study contributes to inform future improvements in fire safety practices and may serve as a basis for revisiting or complementing existing fire safety guidelines. The results contribute to a better understanding of the smoldering combustion behavior of bio-based materials, particularly in relation to material density and its influence on fire dynamics. This knowledge is essential for informing practical fire safety strategies, such as early detection systems and material selection in low-ventilation environments and could help to refine fire performance assessment methods within the built environment.