The wall of a refrigerator is constructed of fiberglass (k=0.035W/m. k) insulation sandwiched between two layers of 1-mm-thick sheet metal (k=15.1W/m. k). The refrigerated space is maintained at 3°C, and the average heat transfer coefficients at the inner and outer surfaces of the wall are 4 W/m² °C and 9 W/m² °C, respectively. The kitchen temperature averages 25°C. It is observed that condensation occurs on the outer surfaces of the refrigerator when the temperature of the outer surface drops to 10°C. Determine the minimum thickness of fiberglass insulation that needs to be used in the wall in order to avoid condensation on the outer surfaces.