The wall of a refigerator is constructed of fiberglass insulation (K=0.035W/moC) sandwiched between two layers of 1mm thick sheet metal(K=15.1W/moC). the refrigerated space is maintained at 3oC, and the average heat transfer coefficients at the inner and outer surfaces of the wall are 4W/m^2oC and 9W/m^2oC respectively. The kitchen air is at average temperature of 25oC. It is observed that condensation occurs on the outer surfaces of the refrigerator when the temperature of the outer surface drops to 20oC. Determine the minimum thickness of fibreglas insulation that needs to be used in the wall in order to avoid condensation on the outer surfaces.