The construction of a laboratory furnace involves sandwiching an insulation material of thermal conductivity k=0.069W/mK between thin metal sheets. The oven air is at Ti=400C, and the corresponding convection coefficient is hi=25W/m2K. The inner wall surface absorbs a radiant flux of 110W/m2 from hotter objects within the oven. The room air is at T0=22C, and the overall coefficient for convection and radiation from the outer surface is h0=15W/m2K. Determine the insulation thickness L required to maintain the outer wall surface at a safe-to-touch temperature of Twall=85C