A backup thermal insulation plate includes a colloidal inorganic oxide-impregnated, pressed and dried high-temperature-resistant inorganic-fiber blanket or board, the plate having a use temperature up to at least about 1000.degree. C. and maintaining mechanical integrity after exposure to the use temperature, the plate having a density greater than or equal to about 500 kg/m.sup.3, and a compression resistance of at least about 50 kgf/cm.sup.2.