The average thermal conductivity of the walls (including the windows) and roof of the house depicted in the figure below is 0.480 W/m · °C, and their average thickness is 20.5 cm. The house is heated with natural gas having a heat of combustion (that is, the energy provided per cubic meter of gas burned) of 9 300 kcal/m3. To maintain an inside temperature of 21.9°C if the outside temperature is 0.0°C? Disregard radiation and the energy lost by heat through the ground.