Fire-safe hemp insulation that still absorbs sound
Two open-access papers show that a bio-based phytic acid and urea treatment makes bulk hemp fibres far less flammable, and that spraying it works better than immersion with ten times less solution. A follow-up study shows that the treated fibres remain excellent sound absorbers.
Dr. Fouad Laoutid · Flame retardancy of polymers & compositesThe challenge. Hemp fibres are attractive raw materials for building insulation, thanks to carbon storage and good acoustic and hygrothermal performance, but they burn easily. A useful flame-retardant treatment must be bio-based, effective and scalable, whereas most published grafting routes treat only a few grams of fibres after multi-step pre-treatments. This work is part of the PhD of Thomas Schatzmayr Welp Sá, in collaboration between Materia Nova, Université Gustave Eiffel, Cerema and Université de Lorraine.
Fire performance (Industrial Crops & Products, 2025). Batches of 300 g of bulk hemp fibres were treated with an aqueous solution of phytic acid and urea, bio-based sources of phosphorus and nitrogen, either by immersion or by spraying, then cured at 80 or 120 °C.
- Spraying uses a solution-to-fibre ratio of 1 instead of 10, needs no washing step, and gives the highest phosphorus content (up to 1.85 wt.%, versus about 0.6 wt.% by immersion).
- In the mass loss cone at 35 kW/m², the peak heat release rate drops from 109 to 49 kW/m² (−55%) and the total heat release by nearly 90% for sprayed fibres, while time to ignition rises from 3.5 s to about 11 s.
- A protective char keeps the sample intact: burning time falls from about 4.5 min to 50 s and 77% of the mass is preserved. In the single-flame source test (EN ISO 11925-2), treated fibres stop burning as soon as the flame is removed, and smoke opacity (ASTM D2843) is reduced.
Acoustic performance (Applied Acoustics, 2026). Impedance-tube measurements and JCAL modelling at 30, 60 and 90 kg/m³ show that the treatment only slightly changes sound absorption: average differences range from −9% to +8% between 200 and 2000 Hz, and stay below 2.3% at 90 kg/m³. The treatment mainly lowers airflow resistivity. The Tarnow model points to an effective fibre radius about 10% larger after treatment (25% for the most heavily loaded sprayed fibres, from 32 to 40 µm), which may come from a surface coating, fibre agglomeration or swelling.
Sources: Schatzmayr Welp Sá et al., Ind. Crops Prod. 228 (2025) 120903 · Schatzmayr Welp Sá et al., Appl. Acoust. 249 (2026) 111287