Heat Transfer

Heat is both a chemical and physical property of matter and a fundamental component of our indoor habitat. We all have an intuitive knowledge of heat from our living experience, but the nature of this energy at the elemental level can be surprising. In our daily lives, the heat we feel and use resides in radiation and molecular motion. The jiggling and bumping of molecules gives us the property of temperature.

The heat of sun is the engine of the vast planetary forces that give us wind, rain, seasons, and climate. We can feel sunlight radiation heat our skin. The radiation process delivers large amounts of heat to our houses that can lower our fuel consumption or raise our cooling costs. Radiation also transfers heat within the building and is responsible, in a minor way, for building heat loss.

Heat moves through solid materials by radiation and conduction and through fluids also by convection. In highly conductive materials, heat will move as vibrations directly along the continuous metallic or crystalline bonds. In more resistant stuff, such as organic matter, the molecular bonds do not vibrate as freely and they are interrupted. In cellular insulation, which includes foam and organic fibers, heat cannot take the shortest path from one edge of the insulation to the other. The cells block the path and heat flow is contained to a much more difficult, longer, and restricted route.

When the position of heated particles is less restrained, as with any fluid, we get a more powerful heat transfer process-convection. Using the bathroom shower as an example, we can get a substantial flow of heat from the cellar up to the second floor and back down by turning on the hot water. Within seconds the heat in the water is transferred through the water pipe, onto the bather, and down the drain. If we tried to measure the same flow of heat conducted along a similar length of wooden 2x4, it would take days or weeks. Although air has far less capacity to hold heat than water in a given volume and is not placed under such pressure in normal domestic use, air is much more fluid and the pathways are often much larger than pipes. Typically in our buildings, the most heat transfer is by air convection.

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