A fiber cement material formulation comprising a cementitious binder, a siliceous material, fiber, alumina trihydrate and a bifunctional low density additive wherein the bifunctional low density additive comprises any one or more of diatomaceous earth, recycled autoclave fiber cement dust or cellulose dust. The fiber cement material formulation optionally further comprises a secondary low density additive which may be perlite. In some embodiments, a fiber cement article manufactured from the fiber cement material formulation comprises a density of approximately 1.1 g/cm3 or below.