Oooh, Shiny Stuff! — Radiant Barrier Fundamentals
A primer on how radiant barriers work — and what can make them not work
Today just feels like a good day to talk about shiny stuff. Radiant barriers are a green building product with a lot of sex appeal, if that's possible for construction products. People get really crazy about attics, though. (Don't get me started about powered attic ventilators!) Maybe brains have a tendency to overheat when discussing them. The general category of radiant barriers is an area of great hype and misunderstanding, so I'll tell you what I know, explain the basic physics, and give you a couple of links to some great resources for more information.
OK, first of all, a radiant barrier is something that can keep your attic cooler. Unlike powered attic ventilators, they actually go after the source of the problem rather than treating a symptom — but first, let's look at the physics.
Attics get hot because the sun is beating down on them all day. The heat from the sun comes in the form of electromagnetic radiation. When this radiation hits a surface, it can do one of three things: It can be (i) reflected, (ii) transmitted, or (iii) absorbed. How much of each you get depends on the wavelength of the radiation and the properties of the material it hits.
The heat is conducted from the shingles to the sheathing
The part we're concerned with here is the sunlight that's absorbed by your roof. Once the roof sucks up those rays, it gets hot. That heat at the absorbing surface (shingles on most residential roofs) then does what the Second Law of Thermodynamics tells it to do — it looks for cooler places, so it starts conducting down through the roofing materials. When it finds the underside of the roof deck, it then can radiate down into the attic. Typical roof decking materials — plywood and oriented-strand board (OSB) - are pretty good radiators, so everything they “see” in the attic starts getting hot, too.
You can see the heat transfer I just described in the diagram labeled Figure 1 below. Here's the basic process:
- Radiation hits the roof.
- The radiant energy is absorbed by the roofing materials.
- The thermal energy conducts downward.
- Heat radiates from the roof deck to everthing in the attic.
The heat radiates into the attic from the underside of the sheathing
So, the dominant form of heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. in an attic is from radiation, which heats up all the solid materials it finds — framing, ductwork, insulation, all those boxes of Christmas decorations, and dead squirrels. That's one reason powered attic ventilators aren't a good solution. They're going after the air, but the air is hot only because all the solid materials are hot.
A better way to reduce the attic temperature is to go after the source — the radiation constantly blasting the attic from the bottom of the roof deck and rafters. Plywood and OSB are good radiators. In physics language, they have high emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces., which means they're good at emitting radiant energy.
That's where radiant barriers come in. These are materials that have a low emissivity. When the underside of the roof deck has a radiant barrier installed, the heat still travels through the materials, but once it hits the radiant barrier, it's come to a dead end. Little of that heat then radiates to the attic.
The radiant barrier gets hot. If you go up into the attic and touch it, you'll see that the heat's still getting there. But like that stainless-steel playground slide shown at the top of this article, when you hold your hand near it without touching, it doesn't feel like it's hot. The magic of low emissivity!
Radiant barriers need an air gap
In case you're wondering, the answer is yes, this is the same thing that makes low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows so good. Also, if you're wondering how hot the shingles get, the Florida Solar Energy Center has done research on that, which I wrote about a while back.
OK, so if the radiant barrier doesn't feel hot when you hold your hand an inch away but it can burn you when you touch it, what does that mean for installation? You got it! You have to have an air gap next to the radiant barrier. If you spray foam insulation right on the radiant barrier, you've wasted your money on the radiant barrier. Heat will conduct right through the radiant barrier because generally, materials with low emissivity have high conductivity.
There's so much more to say about radiant barriers, but let me wrap up this article and give you a couple of resources where you can read more about how they work, how to install them, how much they might save, and why you probably don't want to install one in Pennsylvania or other cold-climate locations. The first is the Florida Solar Energy Center (FSEC). They have a bit of radiant heat gain in attics down there and know a thing or two about this topic. Here's a good Question and Answer Primer that's got a lot of good info.
Oak Ridge National Laboratory, which does a lot of great building science research, has several good pages about radiant barriers, too.
Another great study is the Houston Home Energy Efficiency Study (pdf). This was a study done by Michael Blasnik and Advanced Energy, and among other things, they looked at the cost-effectiveness of radiant barriers. What they found was that in Houston, Texas, radiant barriers reduced the cooling consumption by about 3%. The paper is well worth the read, in case you don't already have a copy. (Oh, yeah...it's free!)
Finally, here are a couple of good resources that address the question of if or when you should install a radiant barrier. Martin Holladay wrote Radiant Barriers: A Solution in Search of a Problem a couple of years ago here at Green Building Advisor and included a lot of great info on the topic. Also, a new website called Radiant Barrier Truth tackles the issue of radiant barriers in cold climates.
I'll come back to this topic and write more another time, but for now, let's not forget what my friend Mike Barcik says is one of the most appealing features of radiant barriers: People like shiny stuff!
- Energy Vanguard
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