By Gary Brown and Steven H. Miller, CDT
Heat transfer occurs through three mechanisms: thermal convection, thermal conduction, and thermal radiation.
Thermal convection is the bulk movement of excited (i.e. hot) molecules within a fluid—that is, heat is transferred because hot molecules move from one location to another. The tendency of hot air to rise is an engine of natural thermal convection.
Thermal conduction involves the transfer of heat energy from one molecule to the next. Heat is the excitation of molecules, which then vibrate with greater force and bump into adjacent molecules. So, at the molecular level, heat is pressure—excited molecules pushing against their neighbours. If the neighbour is equally excited, it pushes back equally hard, and heat is not transferred. However, when an adjacent molecule is less excited, some energy transfers from the more excited to the less excited molecule, attempting to equalize its energy. This gets repeated with other adjacent, less excited molecules. Heat energy is conducted from one molecule to the next, even though no molecule ever changes its location in space.
Thermal radiation is, in a sense, a side effect of the movement inside excited molecules. Charged particles moving within the molecule produce electromagnetic radiation: radiant energy. At temperatures greater than absolute zero (–273 C [–460 F]) all matter emits some radiant energy. Highly excited levels emit visible spectrum (i.e. light); lower excitation emits infrared. The sun heats the earth entirely by radiant energy: no molecules move from Sun to Earth, and there is 150 million km (93 million mi) of vacuum preventing conduction.
The three mechanisms often work together. For example, air is heated in a furnace by conduction and radiation, carried throughout the house by convection, and then heats cooler objects (e.g. occupants) by conduction and radiation.
Effective insulation must control all three modes of heat transfer:
- radiant heat can be reflected away, typically by white or light-coloured materials;
- conduction can be reduced by providing little or no physical path for energy—air is a conduction-barrier because molecules are spaced far apart, making energy transfers few and far between; and
- convection can be prevented by sealing the space against interior/exterior air movement.