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Don’t Squeeze a Radiant Barrier
You are probably all familiar with radiant barriers. Sometimes, it is thin foil draped under the roof deck. Other times, it's used on the inside of stud walls or over furring strips before the drywall goes up, and there is even plywood with a radiant barrier attached to one side that is used for roof decking.
The point of this article is to remind you that you eliminate the benefit of a radiant barrier when you sandwich it between materials (in other words, when there is no “air gap”). However, I also want to help you understand why this is.
How Radiant Heat Transfers
Heat energy is the “force” that makes atoms move and molecules jiggle. It's in everything over absolute zero (-460°F). Heat is transferred or moved in one of three ways, but heat itself isn't these things; these are methods by which heat is moved, similar to how we move by walking, flying in a plane, or riding a surfboard.
- Conduction – Heat moving when one molecule bumps into another and imparts some of its force. It's like standing in a line and shoving someone; they move because you impart force directly on them.
- Convection – Heat moving when the molecules in a fluid are free to move around. It's like flying on a plane; you move freely through the air and bring your energy with you.
- Radiation – Heat moving through the air or a vacuum via electromagnetic waves. It's like surfing because your energy is riding a wave, DUDE! (That stupid metaphor was the whole reason for the other two lame ones.)
So, from a practical building science standpoint, we control conductive heat transfer with insulation, convective heat transfer by air sealing to the unconditioned spaces, and radiation with a low emissivity barrier with the shiny side facing an air gap; that last one is if you need a radiant barrier at all.
Radiant heat can only transfer when you have two surfaces pointed at one another that have different temperatures. The rate at which heat will transfer between them is a function of the temperature difference, the distance between them, and the emissivity of each surface. A surface with an emissivity of 1 is a so-called “perfect black body” and is a theoretical perfect emitter and absorber of radiant heat.
A surface with an emissivity of 0 is a perfect reflector of radiant heat energy, and it neither absorbs nor emits radiant heat. In practice, we do not see 1 or zero but a fraction of 1 with a dull black surface being close to one and a shiny, reflective radiant barrier generally being around 0.10, meaning only 10% of the radiant energy is absorbed or emitted.
So, why can't we sandwich a radiant barrier?
Imagine getting a pan on a nice and hot stove and then hovering your hand over it. You would feel the radiant heat emitting from the pan. Now, place a sheet of aluminum foil over the pan and hover your hand again. Very little radiant will be absorbed and emitted by the foil, and your hand will be much cooler.
Push your hand down on the foil and squeeze it into the pan—
NO, DON'T DO IT! ARE YOU CRAZY?
Spoiler alert. It will burn you.
While aluminum foil has a low emissivity, it is very thermally conductive, and heat travels through it easily via conduction (molecule to molecule). This means that the only way it helps you block heat is when one shiny, low emissivity side faces an air gap (or vacuum or other fluid that allows the electromagnetic waves to pass through easily). That is why you see white radiant roofs on shopping centers that face the sky or plywood for roof decking with a radiant layer that faces down into the attic.
If you press anything solid up against both sides of a radiant barrier, you make it a conductive layer, and it does NO GOOD.
Some of you may (incorrectly) assume that a radiant barrier must be pointed at a light source (like the sun) to do any good. Remember, you don't need visible light to have radiant heat transfer, just a temperature difference. So, a radiant layer on the underside of roof decking will help block radiant heat from leaving that roof decking and entering the ceiling, trusses, and whatever else is in that attic, even if it is pitch-black up there because the radiant barrier is bad at absorbing AND emitting radiant heat. Even though the radiant barrier on the underside of the roof deck would be hot to “touch” (conductive), it does much less emitting than wood, so more of the heat stays put.