an external wall of a house has an area of 15 m2 and is of single-brick construction having a thickness of 10 cm. the occupant keeps the internal temperature at 22c. how much heat is lost through the wall per hour, if the outside temperature is a stable 0c? what percentage reduction in heat loss through the external wall would be achieved by reducing the internal temperature to 18c? the thermal conductivity of the brick is 0.8 w/m.k.