A laboratory furnace wall is constructed of 0.2 m thick fire clay bricks (k = 1.0 W/m K). The wall is covered on the outer surface with a thick layer of insulating material (k = 0.07 W/(m K)). The furnace inner brick surface is at 1250 K and the outer surface of the insulating material is at 310 K. The maximum allowable heat transfer rate through the wall of the furnace is 900 W/m2, determine how thick must the insulating layer be?