Integrate bio-resources in materials allows to reduce the environmental impact of the building industry. This study deals with fire-retardant treatment, alternative to boric acid and ammonium salts solutions, applied to hemp fibers for thermal insulation application. The aim is to limit the energy, sanitary and environmental impacts of the treatment, while optimizing the technical performances. A laboratory protocol evaluates the flame-retardant effect of the developed treatments. Treated fibers, including commercial treatments, are subjected to characterization tests: direct ignition small flame, thermal analysis (ATD-TG), cone calorimeter and pyrolysis-combustion flow calorimeter analyses. Some of the methods have been adapted to be applied to fibrous materials. The obtained results orientate the formulation of a treatment and highlight the complementarity of the analysis methods. Coupling the results, a predictive model leading to the estimation of the reaction to fire classification according to the Euroclass is proposed. Euroclass C is probably accessible with tailored treatment.