The most recent activity in our class has been exploring the science behind thermal energy and heat transfer. Our next few posts will cover our examination of an old farmhouse as apprentice energy auditors and our first forays into the field of engineering and design.
Let’s begin with some basic terminology and principles fundamental to an understanding of the processes by which heat is transferred. First and foremost, “heat” is a common term for “thermal energy.” Thermal energy is transferred, generally speaking, through one of three methods: conduction, convection, or radiation. In conduction, heat travels through a solid material, passing along its energy at the molecular level. We say certain solids (mainly metals) are “conductors” if they have an above average ability to transfer heat in this way; conversely, “insulators” (such as glass, porcelain, and wood) do a poor job of transmitting or spreading heat. Convection is the process by which thermal energy transfers through a liquid or gas. This type of energy transfer is all around us: when we use a fan, boil water, or heat the house with a woodstove, we are taking advantage of one way in which heat travels. A common misconception about heat transfer is that, as people often say, “heat rises.” This is not the case. Heat spreads in all directions, but heated material expands, becoming less dense, so heated material rises. To return to one of the examples above, when we place a pot filled with water on a stove burner, conduction heats the water through the bottom of the pot. (Again, metals are conductors.) Then convection currents form as the water begins to gain energy. The heated water rises through the surrounding liquid, bringing some of its heat with it. At the surface, even more heat is transferred, until the now-cool water returns to the bottom of the pot and the process begins again.
Radiation is the process by which heat travels through the vacuum of space. For example, we receive energy from the sun in the form of solar radiation. An important thing to note is that, while it is possible to stop radiation by insulating with a solid, liquid, or gas or to stop convection and conduction with a vacuum, there is no known method that will completely halt heat transfer. Heat always travels from hot areas to cold (or from areas of energy to those of less energy), and it can only be slowed. That is the main goal of an energy auditor when making modifications to a house to improve its energy efficiency.
The process used for an energy audit begins with knowledge of insulation. In professional circles, the ability of an insulator to resist heat is measured with a unit known as “R value.” A higher R value indicates better resistance to heat. Old houses are prone to cracks and drafts from years of inhabitation and renovation. Most heat loss is incurred from convection at the top and bottom of the house: the attic lets heat escape, while cold air seeps in through the basement.
