Energy-efficient windows. Energy-efficient windows are constructed with two panes of glass separated by an air gap (Fig. P14.38). In a typical window of this type, the air gap makes a huge difference in the rate of heat flow. The heat-flow rate is much smaller for an energy-efficient window than for a single pane of glass because the thermal conductivity of the air gap is much less than that of a glass pane. 

(a) To compare heat flow for the different types of windows, first calculate the heat-flow rate for a single-pane window. Assume the glass is 3.2 mm thick, it has an area of 0.50 m², and the temperatures on the two sides of the glass differ by 308C. 

(b) Now consider an energy-efficient window consisting of two sheets of glass as in part (a), separated by an air gap that is 1.0 cm thick. Assuming the same temperature difference as in part (a), what is the heat-flow rate in this case? For simplicity, assume all the temperature difference is across the air gap and none is across the two panes of glass. 

(c) How thick would a single pane of glass have to be to give the same rate of heat flow as through the energy-efficient window, assuming the same temperature difference?

Figure P14.38