A section with height $2$m and width $3$m of a wall of an industrial furnace burning natural gas is not insulated, and the temperature at the outer surface of this section is measured to be $90$C$.$ The temperature of the furnace room is $20$C and the combined convection and radiation heat transfer coefficient at the outer surface of the furnace is $20$ W$/$m$2/$K$.$ It is proposed to insulate this section of the furnace wall on the outside with glass wool insulation $($with thermal conductivity of $0\mathrm{.}036$ W$/$m$/$K$)$ in order to reduce the heat loss by $90\%$

Assuming the outer surface temperature of the metal section still remains at about $90$C and that the combined heat transfer coefficient at the outer surface of the furnace remains at $20$ W$/$m$2/$K$,$ determine the thickness of the insulation that needs to be used to reduce the heat loss by $90\%$