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Heat can be transferred by three mechanisms: radiation, conduction, and convection. Solar energy arrives from the sun as radiant energy. This radiant energy can travel through the void of space, and warms the surfaces that it shines on. Insolation is the measure of solar energy received on a surface over a period of time, and commonly expressed as kilowatts per square meter per day. An object warmed by radiation can transfer heat energy by direct contact to another object through the process of conduction. If the warmed object is in contact with a fluid or gas, such as water or air, the heat energy is transferred away from the object by convection.
A passive solar home incorporates all heat transfer mechanisms into its design. Insolation from the sun, captured by large equator-facing windows, shines on the floor and walls that form the thermal mass of the building. The heat spreads throughout the entire thermal mass by of the building by conduction, and is later re-radiated back into the structure in the evening as the outside temperature cools. In addition to incorporating insolation and thermal mass into the design, careful planning allows the designer to take advantage of convective air currents to insure the entire building remains warm, or cool, as seasonally appropriate.
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