In this study, the effect of boric acid dosage used as a fire retardant in waste paper based cellulosic insulation has been studied and optimized. Since Na2SiO3 (sodium silicate) is also used as binder in most cellulosic insulator recipes, its effect with and without the presence of boric acid on fire retardancy has been investigated. The ratio of boric acid to sodium silicate or water was kept at 1 to 1 by wt. The results of fire resistance were evaluated by two techniques. In most cases, the classical flammability tests were utilized and then compared with the standard LOI (limiting oxygen index) values. The results indicate that boric acid definitely enhances the fire retardant properties of cellulosic fiber even in the presence of water as binder. However, the combustion properties such as weight loss, flame based ignition temperature, char state of ignition temperature and smoldering combustion temperature, collapse period of ignited material were all found to further improve upon using Na2SiO3 as binder. A improvement of 30% in fire retardancy upon adding boric acid was achieved.