Principles of Heat Transmission
Heat flows through a barrier of building materials (e. g., walls, ceilings, and floors) by means of conduction and radiation. When it has passed to the other side of this barrier, it continues its movement away from the source by means of conduction, radiation, and convection. The direction of heat flow depends on the relationship of outdoor and indoor temperatures because warmed air will always move toward a colder space. Thus, in the summer months, the warmer outdoor air will move toward the cooler indoor spaces. In the winter months, on the other hand, the reverse is true: The warmer indoor air moves to the cooler outdoors. The purpose of insulation is to reduce the rate of heat flow to an acceptable level. This, in turn, will reduce the amount of energy required to heat or cool the structure.