An insulating glass window is constructed of two 5-mm glass plates separated by an air layer having a thickness of 4 mm. The air layer may be considered stagnant so that pure conduction is involved. The convection coefficients for the inner and outer surfaces are 12 and 50 W/m2 ∙ ºC, respectively. Calculate the overall heat-transfer coefficient for this arrangement, and the R value. Repeat the calculation for a single glass plate 5 mm thick.