In an uninsulated house, there is a 90-mm air gap between the plaster wall and the wood siding. When the inside wall is at 18°C and the outer wall at -9°C, what is the heat loss in watts per square meter by radiation and by natural convection? By what factor would the heat loss be reduced by covering the inside wall with aluminum foil? Would it be better to put the aluminum foil halfway between the two walls? (Correlations for natural convection at vertical surfaces give a film coefficient of 3.9 W/m²·°C for each wall.)