Heat transfer by radiation is pretty much entirely different from the types of heat transfer discussed in the previous two articles: conduction and convection. That’s why I’ve saved it for last in this series. You could make a solid argument that radiant heat transfer is the hardest to understand of the three, but I bestowed that title upon convection because it is so easily confused in some circumstances with conduction.
To briefly summarize the last two articles, heat energy is basically how much jiggling and wiggling is going on within the microscopic particles of a substance. That energy gets spread from one place to another in three primary ways—the three types of heat transfer—always from a hotter item or region to a cooler item or region. Conduction is when the energy is transferred by the collisions of those microscopic particles, one into the next, such as within the handle of a hot poker. That is, conduction transfers heat through a substance—solid, liquid, or gas—or from one substance to an adjacent substance, such as from the handle of the hot poker to your hand. Convection is a bit more complicated, but occurs when heat energy is whisked along on a “conveyor belt” of air or water rather than waiting for all the tiny microscopic collisions to do their work. For example, the air from a forced-air furnace is mechanically blown through your vents, which is much faster than just waiting for the hot air molecules to bump into one another.
Radiant heat transfer is totally different than conduction or convection because it doesn’t rely on a “medium” such as airflow or the handle of a poker to spread. In fact, heat transfer by radiation is most effective in a vacuum such as outer space, which is exactly the way in which you’re most familiar with radiant heat: The sun is constantly sending heat through the void of outer space in the form of radiation. When you stand outside, that heat energy is reaching you at the speed of light, diminished but not slowed by obstacles like earth’s atmosphere and clouds. It’s not that the stuff between the sun and you is irrelevant, but unlike with conduction and convection it’s not how the transfer is happening. The sun can burn your skin even though the air right in front of you is nowhere near hot enough to hurt you.
Radiant Heat Transfer Explained
Instead of relying on a medium, radiant heat transfer occurs through thermal radiation. Thermal radiation is just one of many types of electromagnetic radiation. You’ve heard of all of them—here they are arranged from longest wavelength (lowest frequency) to shortest wavelength (highest frequency):
- Radio
- Microwave
- Infrared
- Visible (light)
- Ultraviolet
- X-rays
- Gamma rays
For reasons I won’t pretend to understand, all of the microscopic jiggling and wiggling that we call “heat” has the side-effect of sending out thermal radiation in all directions away from the object. Every object that’s not at absolute zero—which, for practical purposes, is everything—is sending out thermal radiation. You, your walls, your furniture, your iced coffee. In most cases around room temperature, the effect isn’t huge: your body is emitting thermal radiation, and everything around you is sending thermal radiation right back, so the net effects are low compared to heat transfer by conduction and convection. But it’s happening everywhere, all the time, and there are certain circumstances (discussed below) where radiant heat transfer becomes significant or dominant.
You can’t see thermal radiation in everyday life because it usually occurs in the infrared spectrum. Yes, this is why thermal imaging focuses on infrared—that’s where most of the action is. But when objects get super hot (relative to room temperature), the energy starts to be emitted in the visible part of the spectrum. So once you’re approaching about 1000 °F, you can indeed see thermal radiation. That’s why the hot poker we keep talking about in these articles can turn “red,” or even “white” hot.
Radiant heat transfer is a little counterintuitive. I’m going to show you a simplified equation for figuring out radiant heat transfer between two objects—not because you need to know it, but because it illustrates a couple of crazy aspects of thermal radiation:
Temperature change = (0.174) * Area1 * (T14 – T24)
Ignore the “0.174”, which is just a constant that makes the equation work. Then you can see that the only factors that affect radiant heat transfer between two objects are the area (of the first object) and the temperatures of the two objects. One insane takeaway from this equation is that the distance between the two objects doesn’t matter. You can thank the earth’s atmosphere for reflecting and absorbing a lot of the sun’s radiation (this is why the ozone layer is important!), because our distance from the sun per se is not what keeps us at the right temperature.
The second key thing to focus on in the radiant heat transfer equation is those exponents at the end. Because the temperatures involved get raised to the fourth power, small differences become large and large differences become enormous.* When you have a large difference from room or body temperature—say, a flame in your oven, on your stovetop, or in your fireplace—that 2000° to the fourth power sort of blows up the equation. Circumstances like that are when radiant heat transfer steps out of the background and can start to overpower conduction and convection in terms of how heat energy is getting moved around.
Radiation in the Kitchen
As I just alluded to, radiant heat transfer is playing a role in your kitchen any time you’ve got a flame—or for that matter, a glowing hot electric coil—going in the oven or on your stovetop. Depending on exactly what you’re doing, the effect on your food might be small compared to conduction or convection, but at a minimum you might feel the heat radiating from the burners. In other cases, radiant heat transfer is the dominant way that your food is cooked: examples include broilers and toasters. Picture things that are glowing hot and in line-of-sight, but not making contact with, your food.
Finally, microwave ovens are a good illustration of heat transfer through electromagnetic waves, though technically the mechanism is slightly different from what we’ve been talking about. Microwaves have longer wavelengths (lower frequencies) than the infrared waves that usually carry heat energy. My understanding is that the microwave portion of the spectrum is chosen to provide optimal penetration into many foods (though I welcome corrections here or in the comments). But big-picture, the effect is the same: the waves have the effect of making the molecules in the target food jiggle and wiggle (i.e., heat up) without directly heating up the air in the waves’ path.
Radiation and Material Properties
Material properties have a direct role in how much heat energy is transferred between two objects by thermal radiation. Omitting material properties was the major simplification I made in the equation above. The good news is, there are only three major terms to know:
- Absorptivity is how efficiently an object receives thermal radiation.
- Emissivity is how much thermal radiation an object sends out into the world.
- Reflectivity is how much thermal radiation bounces off of an object rather than being absorbed.**
Absorptivity and reflectivity have an intuitive relationship, where a highly reflective object doesn’t absorb much thermal radiation, and a highly absorptive object isn’t very reflective. Emissivity is a much less intuitive property, but it has a remarkably simple characteristic:
Emissivity is always equal to absorptivity.
That is, objects that are good at soaking up thermal radiation are always equally good at sending it back out. I will admit that I don’t understand why this is true, but it’s a pretty easy fact to remember!
You’re definitely familiar with some of the ways that material properties interact with thermal radiation. Take clothing, for example. You know that wearing white in the sun will keep you cooler than wearing black. That’s because black objects absorb more wavelengths—more energy—than white objects. You’ve also heard of the “heat island” effect, which is what happens when all the buildings and paved surfaces in an urban area absorb—and then correspondingly emit—a ton of thermal radiation, raising ambient temperatures higher than in the countryside.
However, intuitions can be a little bit misleading here because color differences between white and black represent only a tiny range of absorptivity: both a black and white t-shirt are going to have absorptivities above 90%. Objects such as metals that are highly reflective, on the other hand, can have absorptivities approaching 0%. Next time you see somebody walking around with a tin-foil hat on, remind yourself that they may just be excellent at keeping their head cool.
Radiation around the House
The same principles that apply to clothing apply to building materials. The most significant way that radiant heat transfer affects your house is solar radiation—and your roof almost certainly bears the brunt of that bombardment from the sun. As you might expect, white roofs have lower absorptivity than black roofs, but just like in the clothing example, absorptivity can be remarkably lower when reflective materials are used. You may have heard of “silver coating” a roof, for example.
Of course, solar radiation impacts your whole house, warming your walls and even the interior of your home by passing through window glass. Obviously this is to your benefit in the winter and works against you in the summer. And you can imagine how radiant heat transfer ends up combining with the two other methods: conduction transferring the heat energy from the warm exterior of your walls to the interior, and natural convection moving the air warmed by the sun (more accurately, the air warmed by contact with interior surfaces warmed by the sun) around your house.
In that sense, insulation helps guard against the impacts of radiant heat transfer, even though insulation technically only slows conduction. As discussed, the use of light-colored and reflective roof and cladding materials has a direct effect on radiant heat transfer. And the use of low-E coatings (i.e., low-Emissivity coatings) on window glass can help to optimize the amount of heat energy that is reflected, absorbed/emitted, and transmitted while still allowing light to pass through. (Remember, visible light and heat energy are usually different wavelengths.) The specific window coatings you want will vary based on your local climate and what you’re trying to achieve, but they all directly impact radiant heat transfer.
Finally, a quick note of clarification about two “radiant” household technologies that usually do not primarily involve radiant heat transfer: radiators and radiant floor heating. Both types of systems involve heating elements that are located at the lowest part of a room. As such, natural convection (discussed in the previous article) causes the heated air to rise to the ceiling; the constant replenishment of this warm air and the resulting movement primarily heat you by convection. Of course, radiant heat transfer is also part of the story, and I imagine especially so for radiant floor heating. If you review the equation presented earlier in this article, you’ll note that radiant heat transfer increases based on the area of the hotter surface. While it’s not nearly as important to the equation as the temperature difference, you can see how a large floor area could send a lot of infrared waves in your direction.
We did it! That covers the three methods of heat transfer. I’m sure you can see how important it is to identify the different, specific ways in which heat energy is moved around and their implications for your personal comfort. Particularly when you’re planning modifications to your old home, it’s critical to understand which types of heat transfer you’re trying to increase or decrease—as well as which types will be totally unaffected by your efforts. Perhaps it has taken me too many words to arrive at the conclusion that a holistic approach is best: you need to think about the sun, you need to think about air leakage, and you need to think about insulation. But thinking carefully about each of the three modes of heat transfer will keep you honest and save you a lot of frustration in your efforts to save money and energy.
*Note of caution if trying to use these equations: I left out a few important details for simplicity, most significantly material properties (discussed near the end of the article) but also units. Your temperatures need to be measured in degrees Rankine, which is just like Fahrenheit but where 0° means absolute zero like in Kelvin. To do a rough conversion, add 460 to your Fahrenheit degrees. If you want to play around with the full thing, look up the Stefan-Bolzmann law.
**There is also transmissivity, which is how much thermal radiation passes through an object rather than being absorbed or reflected. This property is not relevant for most traditional building materials except glass and is omitted here for simplicity.