A vertical 4-ft-high and 6-ft-wide double-pane window con- sists of two sheets of glass separated by a 1 in: air gap at atmospheric pressure. If the glass surface temperatures across the air gap are measured to be 65 F and 40 F, de- termine the rate of heat transfer through the window by (a) natural convection and (b) radiation. Also, determine the R-value of insulation of this window such that multiply- ing the inverse of the R-value by the surface area and the temperature di erence gives the total rate of heat transfer through the window. The e ective emissivity for use in ra- diation calculations between two large parallel glass plates can be taken to be 0.82.