A furnace wall consisting of 0.20m of fire clay brick, 0.15 m of kaolin, and a 0.05 m outer layer of masonry brick is exposed to furnace gas at 1250K with air at 298K adjacent to the outside wall. The inside and outside convective heat transfer coefficients are 55 and 10 W/m² k, respectively. Suppose that the outside temperature of the masonry brick cannot exceed 350K, by how much must the thickness of kaolin be maintain the same heat flux requirements.