Radiation is the transfer of heat energy by electromagnetic waves and therefore does not require a medium to travel through. Radiative heat transfer occurs when the emitted radiation strikes another body and is absorbed. All bodies above 0 Kelvin emit radiation, the wavelength of which will depend upon the actual temperature of the object. Wein’s Law states that:

Where:

The rate of heat transfer due to radiation is given by:

Where:

The emissivity has a value between 0 and 1 and is a measure of how efficiently a surface emits and absorbs radiation. It is the ratio of the radiation emitted by a surface to the amount of radiation emitted by a perfect emitter at the same temperature.

The resistance of radiation heat transfer is given by:

Where:

Therefore:

The same rules apply as with convection - to lower the heat transfer due to radiative heat transfer you can reduce the area of the body, or decrease the radiative heat transfer coefficient.

Radiation accounts for a large proportion of the heat transfer in cavities, and as such represents significant heat loss in cladding products.  As the above equations show, the emissivities of the surfaces are important. Some relevant emissivites are listed below

The most common way of using the emissivity to lower heat transfer is with the use of low-e coated glazing units.  For example an air filled 6-16-6 double glazed unit has a centre pane U-value of 2.66 W/m²K with uncoated glass, which is reduced to 1.39 W/m²K by using a soft low-e coating.

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