Provided is an inorganic fiber containing silica and magnesia as the major fiber components which further includes a phosphate additive to the melt of fiber ingredients, or as a coating on the surfaces of the fiber, or both. The inorganic fiber exhibits improved thermal performance properties and is non-durable in physiological fluids. Also provided are methods of preparing the inorganic fiber and of thermally insulating articles using thermal insulation prepared from a plurality of the inorganic fibers.