The article considers the results of the study of the uneven distribution of the strength properties of a rigid high-temperature fibrous heat-protective material over the volume of the block. The article presents a comparative study of the uneven strength of two materials that differ in the method of introducing the binder. A conclusion is proposed about the mechanism of the occurrence of unevenness of the strength properties of a rigid fibrous thermal insulation material when a soluble binder is introduced into the material by the strait method. The absence of such a mechanism is shown for materials obtained using a solid-phase binder introduced into the molding hydraulic mass.