Building insulation products produced from renewable biomass is becoming increasingly common in buildings due to environmental lifecycle requirements. Biomass insulation products are combustible and can contribute to fires through flaming and smouldering combustion. Incidents have been reported where insufficient spacing between combustible insulation and heat-producing electrical appliances has led to smouldering and subsequent development of flaming fires. Insulation materials often contain fire retardants, though their performance with regard to smouldering fire is not well understood. [1, 2] This study investigates the temperature exposure needed to initiate self-sustaining smouldering fires in loose fill wood fibre building insulation, focusing on the effect of fire retardant content and fibre size. The study is a part of the EMRIS (Emerging Risks from Smoldering Fires) project. The test set-up is shown in Fig 1a [3]. The tested material was 100 grams, 34 kg/m3 spruce wood fibre loose-fill insulation with 4 and 9 % added ammonium polyphosphate fire retardant. Tests with short, fine fibres (Fig 1b) were compared to testst with long, thin fibres. The sample was heated from below until a given temperature was obtained 20 mm above the heater. Temperature and mass loss measurements as well as visual observations of the residue after test (Fig 1c) were used to characterize the onset of self-sustained smouldering. An iterative process was used, with 5 to 8 tests per product. It was found that a high level (9 %) of fire retardant gave an onset of smoldering at lower temperatures (225 °C) compared to a low level (4 %) of fire retardant (290 °C). The lower onset temperature indicates that the insulation with the highest fire retardant content is more prone to smouldering, which is contradictory to the expected performance of the fire retardant. For the same fire retardant content, the onset of self-sustained smouldering combustion was obtained at lower temperatures in insulation materials with smaller fiber sizes than in insulation with larger fiber size (225 vs 280 °C). This study is indicative, the absolute temperatures relate to the given test set-up. Further studies should include a range of fire retardant types and content, to obtain knowledge on their effect on smouldering fires.