January 10, 2014
I’ve always been a huge fan of the space program (Shocked to hear that, are you?) and I remember as a kid watching the space shuttle launch and repair satellites and was always curious why everything was wrapped in shiny foil. Now, as an engineer and resident energy nerd for my company, I encounter radiant barriers often. That has closed a loop for me because it turns out the mystery foil on the satellites and equipment was indeed a radiant barrier.
Radiant barriers have been around a long time. They have been used extensively in the space program for decades and even on the Lunar Excursion Module (LEM) used to land on the moon. There are many examples of materials developed for the space program making their way into everyday life and radiant barriers are just that. Incredibly, these materials are cheap and very effective in reducing energy use in a building as well. However, they are also often misunderstood and in order to help that confusion, I recently combined the questions I get about them in a FAQ format and would like to share them with you. So, push up those taped glasses and let’s go!
1. What is a radiant barrier?
A radiant barrier is a special type of insulation that resists transmission of radiation, typically in the infrared spectrum.
2. Gee, that’s nice. Now in layperson’s terms, how do they work?
Let’s back up a little. There is a law in thermodynamics that states heat will always travel from a warmer point to a colder point. And when it does, it does do in three possible modes: Conduction, convection, and radiation. Conduction is generally applicable to solids, i.e., a handle of a metal spoon with the other end submerged in hot soup getting warm. Convection is generally applicable to gases and fluids because they can flow, transferring energy from one point to another. Hot air rising up out of a fireplace, heating the flue as it goes is an example of heat transfer by convection. Radiation is heat traveling at light speed in the form of electromagnetic radiation, mostly in the infrared spectrum for objects at Earth surface temperatures. When you put lighter fluid on a fire and it suddenly flares, you will feel a burst of heat on your face instantly, right? That’s radiation.
So if you think about this, you will come to the conclusion that all heat from the sun must get to the Earth through radiation because of the vacuum of space. That is correct and exactly why radiant barriers are so important in the protecting satellites and astronauts from the extreme temperature swings they would be subjected to otherwise. You see, space isn’t really cold because the concept of temperature kind of breaks down in a vacuum. In reality, objects in space can be either very hot or extremely cold depending on their exposure to an energy source like the sun. So when a satellite in orbit goes behind the Earth, its temperature would plummet suddenly without a radiant barrier. That’s also part of why satellites are constantly rotating, to make them warm and cool evenly and prevent premature failure on the instruments on board.
But back to Earth-bound, near-room temperature objects: Most solids are very efficient (about 90%) at converting heat to infrared radiation or vice versa in order to match the temperature of their surroundings. But there are notable exceptions, one of which being polished aluminum, which is much less efficient at converting heat to radiation and vice versa. This means that in a vacuum (i.e., no conduction or radiation can happen) a warm object coated with polished aluminum will cool slower than it would without the coating. Thus, polished aluminum is a key ingredient of a radiant barrier and thus has saved many astronaut lives.
3. I thought aluminum conducts heat readily but now you’re telling me it is a good insulator?
No, I’m saying it’s a good radiant barrier. Remember, those are different things. Radiant barriers don’t have to be very thick to work, so a common approach is to take a conventional insulation liner and coat it with a thin layer of aluminum. That layer doesn’t have any direct effect on the R-value of the insulation. Now, if you were to touch the radiant barrier with another solid, only then would you have solid-to-solid contact and conduction would be a factor. Fortunately, conductors can only transfer what is transmitted to them, so the insulation still limits the heat loss. But what matters is that the radiant barrier makes the insulation work more effectively when it is placed next to air, either against a cavity or lining a room, by impeding radiation release from the insulation into that adjacent space. Think of a baked potato wrapped in aluminum foil. It will stay hotter than an identical potato without the foil even though aluminum is a good conductor because the foil emits far less radiation than the potato skin, keeping the energy contained in the soon-to-be eaten hotter potato.
4. I’ve seen that but I’ve always called it reflective insulation.
Many people do. But that name is a bit misleading, kind of like putting a statement in a FAQ. (Really, who would do that?) A radiant barrier doesn’t reflect radiation per se; it just does a bad job absorbing it. But we can leverage that behavior to increase the effectiveness of the insulation it’s attached to just as we do with a baked potato.
5. How much does a radiant barrier increase the insulation R-value?
R-value is a measure of the resistance to heat flow through traditional insulation and isn’t really applicable to radiant barriers. While it is true that energy is energy whether it is transmitted by radiation or some other mode, the amount of energy impeded by the use of a radiant barrier depends on how it is deployed. The only way to know with much certainty how much heat it is impeding is to test or model every possible configuration and calculate a total heat transfer coefficient, or U-factor for each one. This is obviously not very practical. However, there are some references you can find on the internet that will give “effective R-values” (equal to 1/U-factor) of a radiant barrier deployed in certain common configurations. They work well as long as you read the fine print and don’t use them out of context.
6. Then how is the effectiveness of a radiant barrier measured?
Radiant barriers can have one active or low-e face and an inactive face but you can also get them with two low-e faces as well. The emittance of the low-e facer is the key number. Remember that the lower the emittance, the better the radiant barrier. The lowest emittance readily available is 0.03. But it is a continuous scale and what really matters is difference between the emittance of the radiant barrier and the other solid objects in the room with which the barrier trades radiation. In fact, any material with lower than average thermal emittance (let’s assume that to be 0.9) will function as a radiant barrier to a certain degree.
If you are using a single-sided radiant barrier, you must be careful to install it in the orientation that will give the best results for your particular climate. Generally, this will be with the low-e side facing the predominately cooler environment, be it indoors or outdoors. If you install one with two low-e sides, then you don’t need to worry about it; winter or summer, it will help you save energy
Aluminum is also commonly alloyed with zinc to make a corrosion-resistant coating called Galvalume. This coating has an emittance around 0.15, so it actually can be used as a radiant barrier as well as a durable coating for a metal roof or wall panel. MBCI makes virtually any one of its profiles in Galvalume as well as painted colors and they can help you leverage that aspect in your building.
7. How much money can radiant barriers save?
It depends. Radiant barriers don’t actually result in a significant direct change in room air temperature, because air is mostly transparent to infrared radiation. (I say mostly because naturally occurring greenhouse gasses like carbon dioxide and water vapor do absorb certain frequencies of infrared radiation causing them to warm slightly.) Instead, radiant barriers work by preventing radiation from escaping the interior environment in the winter and keeping it from intruding in the summer. This keeps the solid objects in the room closer to room temperature and they in turn reduce the heating or cooling load indirectly. Take the summer condition as an example. The radiant barrier slows the release of infrared radiation from the exterior heat coming through the insulation. This makes solid objects in the room (like humans) absorb less radiation from those surfaces. At the same time, those same solid objects are releasing their own radiation at the typical 90% efficiency. This results in a net radiation loss to those objects, cooling them even though the air temperature in the room doesn’t change much. The opposite happens in the winter by keeping the radiation released by the solid objects contained in the room. How much energy this saves is going to depend on what is in the room, what its emittance is, etc. The classic residential application of a radiant barrier is on the underside of the roof, adjacent to the attic air space. Because access is easy and radiant barriers are fairly cheap, paybacks in this scenario are usually in the 2-year range or less. That’s a solid investment from an energy-savings standpoint.
Another ideal and easily accessible place to put a double sided radiant barrier is on the inside of a roll-up door. MBCI’s door division, DBCI, can provide radiant barriers for most of their roll-up doors, aiding the energy efficiency of a conditioned warehouse as a prime example.
So, there you have it: Everything you wanted to know about radiant barriers but were afraid to ask because you didn’t want to sound like a nerd. Fortunately, some of us remain blissfully unaware of our nerdism and are happy to answer your questions.