Abstract: An inorganic composition for use as a fire- and flame-retardant additive to cellulose fiber thermal insulation, consisting essentially of 10 to 25 parts by weight of alumina trihydrate, 20 to 35 parts by weight of ammonium sulfate, 20 to 35 parts by weight of borax, 10 to 20 parts by weight of boric acid, up to 25 parts by weight of nepheline syenite, and up to 2 parts by weight of soda ash. Loadings of 12% to 18% by weight of the additive, based on the weight of the insulation, provide fire- and flame-retardant characteristics, non-corrosiveness, smoke suppression, and moisture absorption values within the limits prescribed by federal specificatons. The composition is also effective in mold inhibition and vermin control, and has relatively low density.