Related Resources: heat transfer

Surface Heat Transfer Coefficients Equations for Windows

Heat Transfer Engineering
Thermodynamics

Interior and Exterior Surface Heat Transfer Coefficients Equations for Windows

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 h_i and h_o, 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 σ = 5.67 x 108 W/m² · K⁴ is the Stefa 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:

h_i = 8.29 W/m² · °C = 1.46 Btu/h · ft² · °F (winter and summer)

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

Combined convection and radiation heat transfer coefficient h_i at the inner surface of a vertical glass under still air conditions (in W/m² · °C)*

Ti °C	Ti °C	Glass emissivity, εg
20	17	2.6	3.5	7.1
20	15	2.9	3.9	7.3
20	10	3.4	4.2	7.7
20	5	3.7	4.5	7.9
20	0	4.0	4.8	8.1
20	-5	4.2	5.0	8.2
20	-10	4.4	5.1	8.3

Notes:

* Multiply by 0.176 to convert to Btu/h · ft2 · °F.