A hot body may lose heat by radiation through emission of electromagnetic waves The maximum power which can be radiated from a body at a given temperature is called the black body radiation corresponding to that temperature. The radiation power, P, from a black body increases as the fourth power of the absolute temperature, T, of the same body and it is given by Stefan’s law as
P = sT (70)
where s = 5.67 X 10—8 W/m2 K4 is the Stefan’s constant. Heat flux in the case of radiation from a black body is presented in Table 9 for different absolute temperatures.
Similar to sun’s radiation heat can be radiated from the surfaces of heated materials. The amount of radiation is first dependent on the temperature of the radiating body and then
Black body radiation
Radiation amount is also dependent on the surface material emission. Most materials used in the building construction have high emissions of approximately 0.9, which means that they radiate 90% of the theoretical maximum for a given temperature. Usually, the total heat loss from combined effects of conduction, convection and radiation is referred to as the U-value. Its unit is the amount of loss per area per centigrade degree. Typical U-values are provided in Table 10.