Plywood and thermal-insulation boards with phenol-formaldehyde binder (PF), like other composites for construction purposes, must have long-term strength with changing temperature and humidity. The insufficient degree of curing of the phenol-formaldehyde binder reduces the hydrolytic stability of the material. The aim of the study is to increase the long-term strength of materials when exposed to changes in temperature and humidity by improving the structure formation of composites with the introduction of modifying additives in the binder. In the work, the method of cyclic testing of materials “soaking – freezing – thawing – drying” was used. After each cycle, the strength of the samples was determined. Nine modifying additives to the phenol-formaldehyde binder were used – hydrogen peroxide, metal salts, sulfosalicylic acid, dimethylglyoxime. The proportion of additives varied from 0.5 to 1.5 %. Graphic dependences of changes in the strength of PF plywood and thermal-insulation boards from plant waste after cyclic tests are given. Materials on a modified binder have long-term resistance to variable temperature and humidity effects. Plywood on a phenol-formaldehyde binder with the addition of 0.5 % sulfosalicylic acid has, after 15 test cycles, a 2.8 times higher residual strength compared to plywood on an unmodified binder. Thermal-insulation composites from plant waste on a modified binder after cyclic tests have a strength of 9 % higher than plates on an unmodified binder.