It’s been heating up around here, and for the past few weeks we’ve been slowly increasing our use of box, window, and ceiling fans. I sat down this morning to write about how to use, and how not to use, these seemingly simple machines.
But then I realized I was getting ahead of myself.
If you want to make smart decisions about keeping you and your family comfortable—in particular, how to do so without wasting money or energy—one of the most helpful things you can do is to get really precise about what you’re trying to accomplish. Am I trying to cool my body? Warm the whole room? Replace the stale air in here with fresh air from outside?
To help you think about exactly what you’re trying to achieve and how to do it, it’s really valuable to understand the three primary mechanisms of heat transfer:
- Conduction
- Convection
- Radiation
If that sounds intimidating, don’t worry. These are technical terms for things that you already understand intuitively—different ways things get hotter or colder in the world around you. The value in using the technical terms is to help you to be specific about what’s making you comfortable or uncomfortable, which will allow you to make smarter decisions around the house—both in terms of energy upgrades and your everyday behavior.
This article and the two that follow will walk you through each of the three types of heat transfer in turn, and talk about a few intuitive examples of each. That way, when you’re faced with a new situation, you can use these examples as analogies: is this more like being in a hot car, or a warm bath? Try to give yourself a mental shortcut so you don’t have to think too hard about technical definitions.
But before we talk about heat transfer, let’s take a quick second to talk about heat itself.*
What is Heat?
Heat is essentially a way of describing how energetic the microscopic particles of a material are. If the electrons, atoms, or molecules of a substance are super jiggly and wiggly—to use the official terminology—then that substance is going to be hotter than if its electrons, atoms, or molecules are just lounging around.
It doesn’t matter if we’re talking about solids, liquids, or gases: hot air, hot water, and a hot stovetop all have super energetic particles. (Though specific material properties do affect things like rate of heat transfer, as discussed below.) And heat transfer will happen by conduction, convection, or radiation any time there is a temperature difference—a discrepancy in the heat energy—within a material or between adjacent substances.
Conductive Heat Transfer Explained
Conduction is probably the most intuitive mode of heat transfer. If you stick a long metal poker in a fire and wait until the handle is too hot to touch, you’re experiencing conduction: heat transfer through a material. There’s an initial temperature difference between the end of the poker that’s in the fire (hot) and the end in your hand (cold), and the heat energy is going to transfer from the hot to the cold region. Heat energy always travels from “hot” to “cold,” essentially spreading itself out and trying to reach equilibrium, i.e., where everything in the vicinity is the same temperature.
(That’s basically what the second law of thermodynamics says, and you’ve probably understood it on some level since you were a kid. Pretty cool, no?)
Let’s go back to our super scientific definition of heat and think about the particles inside the metal poker. Once the heat from the fire gets them all jiggly wiggly inside one end of the poker, those electrons and molecules are going to be moving around like crazy and bumping into adjacent molecules that weren’t directly heated by the fire. Then those new molecules start bouncing around, spreading that energy to more molecules, etc. This cascading reaction, like a crazy NASCAR wreck or a long string of dominoes, sends that heat energy down the poker towards the handle.
Conduction in the Kitchen
Cooking offers good illustrations of the different types of heat transfer. It’s something most of us do regularly, and when you think about it, a huge part of cooking is controlling heat transfer—for example, from your oven or stovetop to your food—in various ways.
Conduction is the primary way food itself gets cooked, in the sense that the outside of your broccoli, steak, pasta, etc. generally heats up first and then the energy is conducted to the interior of the food. (A clear example that you might be familiar with is “carryover cooking,” where the interior of a solid cut of meat will continue to heat up after being removed from heat.) Backing up a step further, conduction is clearly responsible for every stage of cooking on an electric stovetop: first the coil heats up, which transfers the energy to the pan it’s touching, which transfers the energy to the outside of the food, which transfers the energy to the inside of the food.
But the same process is actually taking place inside a conventional oven, only the conduction is occurring within the air inside before it reaches your food. Remember, conduction takes place within liquids and gasses, not just solids. (Conduction is just one of the methods of heat transfer that takes place within a conventional oven, as we’ll discuss in the following articles.)
Conduction and Material Properties
Contrasting hot pokers and electric stovetops with ovens is a good example of the way specific material properties affect heat transfer. Metals generally conduct heat very well for two main reasons. Zooming way in with our microscope, metals generally have a lot of “free electrons” that not only do a lot of the jiggling and wiggling, but can freely jump from one metal ion to another. (This is also why metals are so good at conducting electricity.) Zoom out a little bit, and molecules of pure metals also typically feature a highly regular, tightly packed crystalline “lattice” structure that is also really good at jiggling and wiggling and sending heat energy down the line. Metal alloys, which introduce other elements and disrupt the perfectly regular lattice structure, generally conduct heat less well than pure metals.
The air inside your oven, in contrast to any metal, has a much less densely packed and well-ordered molecular structure. Once the air particles adjacent to the heat source start jiggling and wiggling, it takes longer for that energy to reach their neighbors. That’s why it takes your oven longer to reach temperature than your stovetop.
The details aren’t nearly as important as recognizing the general fact that different materials conduct heat at varying rates due to different intrinsic properties—and those properties can be selected to achieve specific goals. For example, contrast the example of a metal poker with batts of fiberglass insulation. Fiberglass is literally glass that’s been drawn out like spaghetti into superfine threads, and glass itself is a relatively poor conductor of heat.** In addition, the glass strands are joined together with a binder in a way that ensures that there are air pockets, which slows the conduction of heat through the batts even more.
Conduction around the House
Insulation is where the concept of conduction most clearly intersects with practical home energy concerns. You’ve probably heard of “R-value,” even if you don’t really know what it means. R-value essentially measures how resistant a material is to heat conduction; it’s used because higher numbers mean better insulation. (The inverse of R-value, “U-value,” measures the heat conductivity of a material. It would probably be confusing to purchase insulation by U-value, because lower numbers would mean better insulation.)
It’s not just your insulation; everything has an R-value: your wood siding or brickwork, your framing, your plaster or drywall, and even air gaps. And you can combine those R-values together to measure the insulative value—or lack thereof—of your entire wall assembly. Because conduction doesn’t just happen within materials, it happens between materials. After all, it’s conduction that transfers the heat energy from the pan to your food, or from the hot poker to your hand.
In that way, conduction doesn’t just contribute to the general temperature of your house as heat transfers in or out through your walls. It also has specific, localized effects on your comfort. For example, ever notice how a metal object often feels colder than room temperature? What’s actually happening is that metal is such a good conductor of heat that it draws heat away from your skin so rapidly that it feels cool. That’s the same reason 50º water feels so much colder than 50º air: water is much better at conducting heat away from your body than air is.
Air is actually a decent insulator under certain circumstances—for example, if you’ve properly air sealed your home. Air sealing minimizes uncontrolled air leakage, which is a type of convection, the type of heat transfer discussed in the next article. Insulation, including air, works by being bad at conducting heat, but is less effective at stopping convection (air movement) and radiation. By minimizing these other forms of heat transfer, you’re letting your insulation do its job. And even an air cavity—between your plaster and your siding, or between your window and a storm window—becomes part of your old home’s “insulation” when that cavity’s borders are properly sealed.
Next up, we’ll talk more about convection: how it can undermine your comfort and energy efficiency goals, but also how it can be a tool to help you achieve those same goals.
*The internet leads me to believe that technically you can’t really have heat energy without heat transfer, so defining “heat” by itself may be kind of meaningless. I’m not a physicist, mechanical engineer, or even an HVAC expert, so I’ll admit to not being sure. But for practical purposes, I stand by the definition in the article (though please feel free to reach out with corrections, here or in the comments).
**You might be wondering: if glass isn’t a great conductor of heat, why does glassware get so hot? Keep in mind that we’re talking about rates of heat transfer here, not whether something is capable of getting hot or not. A material that conducts heat well, such as copper, will heat up and cool down more quickly than a material like glass or ceramic (or the air in your oven!) that will heat up slowly—and then retain that heat for longer as it slowly cools.