Whenever differences in temperature exist between surfaces, heat will migrate from the warmer area to the cooler area.
This is true of all surfaces. However, a glazed surface is special in that it is also transparent to solar radiation, which results in free heat gain.
Heat Exchanges Through a Surface
Heat is exchanged through a surface and hence lost in any of 3 different ways:
Is the transfer of heat within a body or between two bodies in direct contact. No material is physically moved during this type of transfer.
The heat flow between the two faces of a sheet of glass depends on the temperature difference between the faces and the thermal conductivity of the material.
The thermal conductivity of glass is : Ï = 1.0 W/(m.K)
is the transfer of heat between the surface of a solid and a liquid or a gas. This type of transfer involves movement via circulation.
is the transfer of heat by radiation between two bodies at different temperatures.
At ambient temperature, this radiation takes place in the infra-red band of the spectrum, at wavelengths above 5 µm. It is proportional to the emissivity of these bodies.
-emissivity is related to the surface characteristic of a body. The lower the emissivity, the weaker the heat transfer.
The normal emissivity εn of glass is 0.89. Certain types of glass can be modified by means of a low-emissivity coating, in which case εn can be as low as 0.02.