Abstract: Mineral fibers, and products (e.g., building insulation products) formed of mineral fibers are treated, and thus rendered fire-resistant, by placing a phosphate-containing compound onto or in close proximity to the mineral fibers. The phosphate-containing compound is at least one which forms a high-temperature protective coating or layer on the surfaces of the mineral fibers. A preferred product of the invention will include two mineral wool layers with a septum carrying the phosphate-containing compound interposed therebetween. Exemplary phosphate-containing compounds include monoammonium phosphate, diammonium phosphate, dicalcium phosphate, monocalcium phosphate, phosphoric acid, aluminum phosphate, tetrasodium pyrophosphate, tetrapotassium pyrophosphate, sodium hexametaphosphate, potassium tripolyphosphate, sodium tripolyphosphate, ammonium polyphosphate, monosodium dihydrogen phosphate, and mixtures thereof.