A cold storage wall (3m x6m) is constructed with 15cm thick concrete wall (thermal conductivity =1.37W/(m.oC). An extra insulation layer must be added to maintain a heat transfer rate through the wall at below 500W. The surface temperature outside of the wall is 38oC, and the inside wall temperature is 5oc. Given the thermal conductivity of the insulation material is 0.04W/(m.oC), compute the required thickness of the insulation layer.