Heat transport through a material takes place by conduction from the warm to the cold side. Think of a steel pan on a stove. The steel handle of the pan can become quite hot, while it is not heated directly. This is the result of the heat conduction. The kinetic energy is transferred between particles or groups of particles at the atomic level: one steel-particle will pass heat on to the next steel-particle.
The same process takes place in a building construction. Of course not as fast as in the steel pan, because generally the thermal conductivity of the building construction materials will be much lower. In solid bodies including building components, thermal conduction takes place when one part of the component is subjected to higher temperature and the other part to lower temperature condition.
Most cases of thermal conduction are usually analysed and treated in their simplified form as one dimensional heat flow cases, i.e. heat flow in directions other than the main direction is neglected. Similarly, if the changes in atmospheric conditions (inside and/ or outside) are assumed to be very slow, neglecting these changes, the process of heat transfer can be assumed to be "Steady State Heat Transfer" in its simplified form.
When heat is transported by a fluid, like air or water, this is called convection. Thus above a radiator, hot air ascends and heats the upper part of the room. The extent of convective heat transfer depends on a number of things, like the position of the surface (horizontal or vertical), but mainly on the speed of the passing air. Outdoors the speed is determined by wind speed and direction, which are very variable. When the air is driven by an outside force such as the wind this called "forced convection".
When there is no wind, convection will occur by temperature or density differences. This is called "free convection". The example of the hot air ascending above a radiator is an example of free convection. Room air is heated by the radiator and ascends because the density of the hot air next to the radiator is lower than the density of the cooler air in the rest of the room. This results in the warmed air rising and being displaced by the cooler air. Downdraught is also an example of free convection.
The heat transfer through forced convection in general is bigger than that of free convection, because of higher air speed. So the heat-transfer of a surface to the outdoor air will generally be bigger than the heat-transfer from the indoor surface to the indoor air.
The phenomenon of thermal radiation is described as the transport of energy through electromagnetic waves. Unlike conduction and convection, radiative heat transfer is not bound with material, it can even occur through vacuum. Every body whose temperature is above absolute zero radiates energy in the form of electromagnetic radiation. The spectral distribution of the energy radiated from any body depends on the temperature of its surface. The higher the temperature of the body, the lower is the wavelength of the radiation that makes the major portion of the total emitted radiation.
Since the temperature of most building components and their surroundings are much less as compared to hat of sun, the spectral distribution of their emitted radiation as more share of larger wavelength. It means, radiation emitted from such bodies is thermal radiation. Typical wavelengths of radiation from building components are in the range of 3 to 800 ?m that makes the basis of radiative heat exchange between the buildings and their surroundings.
In radiation heat transfer the rate of heat flow depends on the temperatures of the emitting and receiving surfaces and on certain qualities of these surfaces: the emittance and absorbance. Radiation received by a surface can partly be absorbed and partly reflected: the proportion of these two components is expressed by the coefficients absorbance (a) and reflectance
(r). The sum of these two coefficients is always one.
a+r = 1 Light colored smooth and shiny surfaces tend to have a higher reflectance. For the perfect reflective theoretical white surface: r= 1, a = 0.
The perfect absorber, the theoretical "black body", would have the coefficients: r = 0, a = 1.
The coefficient emittance (e) expresses how much of the available heat will be emitted (in relation to the black body, for which e= 1).
Its value is a the same as for absorbance: a = e
For the same wavelengths o radiation, but may differ for different wavelengths.