Provided herewith is a fiberglass binder comprising an aqueous solution of a polycarboxy polymer, a polyol and at least 30 weight percent of a filler material, preferably clay. The clay filler material is generally present in the aqueous solution as a suspension of particles having an average size of 3 .mu.m or less. The clay filler extended fiberglass binder of the present invention is more cost effective without significantly altering the appearance or function of the binder.