3. (a) It is proposed to build a store having an outer wall of concrete (100mm thick) and an inner wall of wood (10 mm thick), with the space in between (100 mm thick) filled with polyurethane foam. If the inner wall temperature is 5 C and the outer wall is at ambient air temperature of 20 C, calculate the rate of heat penetration per unit area. Thermal conductivity (k) of concrete, polyurethane foam and wood are 0.8, 0.025 and 0.17 W m K- respectively. (b) Calculate the temperature at the interfaces of the three layers, T2 and T3. T 20C x 100mm 100mm X3 10mm T T3 T 5C