A radiant barrier FAQ: Everything you wanted to know but were afraid to ask
Here are excerpts from an insightful (and humorous) post on radiant barriers penned by Bob Zabcik, Director of Research and Development for NCI Group Inc., on the MBCI blog.
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.
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.
What is a radiant barrier?
A radiant barrier is a special type of insulation that resists transmission of radiation, typically in the infrared spectrum.
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.
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.
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.
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.
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