There are three modes of heat transfer: CONDUCTION, CONVECTION, and RADIATION (INFRA-RED). Of the three, radiation is the primary mode; conduction and convection are secondary and come into play only as matter interrupts or interferes with radiant heat transfer. As the matter absorbs radiant energy, it is heated, develops a difference in temperature, and results in molecular motion (conduction in solids) or mass motion (convection in liquids and gas).

All substances, including air spaces, building materials, such as wood, glass and plaster, and insulation, obey the same laws of nature, and TRANSFER heat. Solid materials differ only in the rate of heat transfer which is mainly affected by differences in: density, weight, shape, permeability and molecular structure. Materials which transfer heat slowly can be said to RESIST heat flow.

Direction of heat transfer is an important consideration. Heat is radiated and conducted in all directions, but convected primarily upward. The figures below show modes of heat loss by houses. In all cases, radiation is the dominant mode.    

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