A vertical air space in the wall of a home is 0.1 m thick and 3 m high. The air separates a brick exterior from a plaster board wall, with each surface having an emissivity of 0.9. Consider conditions for which the temperatures of the brick and plaster surfaces exposed to the air are -10 and 18°C, respectively. What is the rate of heat loss per unit surface area? What would be the heat loss per unit area if the space were filled with urethane foam?