The paper presents the results of laboratory research of indicators of composite thermal-insulation boards filled with irretrievable spinning flax and cotton. As the matrix of the composite used UF binder with various additives. UF binder has advantages and disadvantages: is cheaper but less waterproof than other synthetic binders. The work assessed the properties of the binder and the influence of the composition of the binder on the physical and mechanical properties of the thermal-insulating material. The IR spectrum of the binder, the dependence of the degree of curing of the UF binder on the type and proportion of the hardener additive are given. The results were obtained that the degree of curing and water resistance of the UF binder depends on the type and proportion of the hardener additive and the curing temperature. The physical and mechanical properties and the coefficient of thermal conductivity of the boards on the UF binder were determined. It is shown that the type of additive in the UF binder affects the physical and mechanical properties of thermal-insulation composite boards from spinning flax and cotton. The paper summarizes the results of the determination of the physical and mechanical parameters of boards from plant waste, and proposed a rational combination of factors for the production of composites. The type of additive in the binder does not have a significant effect on the thermal conductivity of the material. Composites have a coefficient of thermal conductivity required for thermal-insulation materials.