Interior and Exterior Surface Heat Transfer Coefficients

Heat transfer through a window is also affected by the convection and radiation heat transfer coefficients between the glass surfaces and surroundings. The effects of convection and radiation on the inner and outer surfaces of glazings are usually combined into the combined convection and radiation heat transfer coefficients hi and ho, respectively. Under still air conditions, the combined heat transfer coefficient at the inner surface of a vertical window can be determined from

where T_g = glass temperature in K, T_i = indoor air temperature in K, ε_g = emissivity of the inner surface of the glass exposed to the room (taken to be 0.84 for uncoated glass), and s σ = 5.67 x 10^-8 W/m² · K⁴ is the Stefan–Boltzmann constant. Here the temperature of the interior surfaces facing the window is assumed to be equal to the indoor air temperature. This
assumption is reasonable when the window faces mostly interior walls, but it becomes questionable when the window is exposed to heated or cooled
surfaces or to other windows. The commonly used value of h_i for peak load calculation is 

which corresponds to the winter design conditions of T_i = 22ºC and T_g = -7ºC for uncoated glass with ε_g = 0.84. But the same value of hi can also be used for summer design conditions as it corresponds to summer conditions of T_i = 24ºC and T_g = 32ºC. The values of h_i for various temperatures and glass emissivities are given in Table 18. The commonly used values of ho for peak load calculations are the same as those used for outer wall surfaces (34.0 W/m² · ºC for winter and 22.7 W/m² · ºC for summer).