Radiant Barriers: 7 Minutes of BS—Building Science w/a Beat
Thin foil can block massive amounts of heat transfer, but it needs a massive temperature difference to be effective. So, insulation sort of makes radiant barriers irrelevant.
In this episode, we are joined by Jonathan Smegal, M.A. SC a Senior Project Manager with RDH Building Science Laboratories to talk about how radiative heat transfer works and whether an emergency blanket can stop it.
What it is:
A Radiant barrier is a material that blocks radiant heat transfer.
—Jonathan Smegal of RDH Building Science Laboratories
How it works:
"Radiation heat transfer occurs between objects of different temperatures, and it generally moves from hot to cold. So if you are standing next to a cold wall, you will radiate heat to it and you will feel cold.
If you are standing next to a hot wall—or a fire or some sort of heat source—you will start to feel the radiative heat off that material.
Heat is transferred from one object to another through space by way of electromagnetic radiation. Electrons buzzing around inside objects are what ’creates’ the electromagnetic radiation, and the temperature difference between the two objects determines the intensity of the net radiative heat transfer.
If you were to use a radiant barrier between you and either the cold wall or the heat source, then you would not feel as cold or as hot standing next to those materials."
Like one of those foil emergency blankets that your grandfather has in the trunk of his car—”Just in case!” These foil blankets can keep your heat waves from being radiated away when you need to hang onto them.
The so-called blanket interrupts the electromagnetic waves—it acts like a mirror to electromagnetic waves.
"One of the side affects of using a radiant barrier to block heat flow is that it often actually heats up or cools down the surface of the radiant barrier past the surrounding air temperature."
Why it matters:
This does not matter with Gramp’s foil blanket, but when you cool down parts of the roof sheathing or wall, something moist can happen.
"Moisture from the air can condense on the cold surface, and collect into a puddle." And that is one of the biggest risks to using radiant barriers in enclosures such as roofs and walls. Because if the surface of that radiant barrier cools below the dew point, you can get condensation on that surface.
But wait, there’s more!
"And radiant barriers are often vapor barriers so you have to be very careful about where you put it inside the enclosure—because it will stop the flow of vapor and may also result in moisture accumulation."
So a poorly placed radiant barrier can work against you. They can also work very well in some applications. Because radiant barriers block heat flow through space, they need some sort of space adjacent to them to work at all.
"If they are put in contact with another material, they do not have any radiative transfer, it’s all conduction—which is why the foil bubble wraps that were marketed as being put underneath a slab, do not actually work as radiant barriers. Because they are in contact with the ground on one side and the concrete on the other.
How to do it right:
The little air gaps inside the bubbles are too small to do any kind of actual work. One place where large air gaps do exist is in attics, and that is a good example of a place to put a radiant barrier where is can make a lot of sense.
"But again, you need to think about the implications of where you are putting it and what it may cause to happen.
A common place where people have put radiant barriers in the past is on top of the attic insulation in a ventilated attic—which is actually a bad location because it stops the movement of moisture from the inside into the attic space, does not let it ventilate away, and results in moisture accumulation.
A better place for a radiant barrier in a roof assembly is on the sloped sheathing.
"One of the most common uses of radiant barriers in residential construction is in the southern United States in the hot, humid climates, like Texas, where they do not have basements and they put a lot of the mechanical equipment in the attic. They often use radiant barriers on the underside of the roof sheathing to help limit the amount of heat that the attic sees.
To lower the amount of work that the insulation has to do, and lower the amount of work that the air conditioner has to do.
"Another place where it actually makes sense is if you have a metal roof, and it’s on strapping, and then you put foil-faced polyiso or even a radiant barrier on top of that, and you can actually reduce the heat that even gets to the sheathing.
And it actually works on the walls too, so if you have a cladding, and you had a ventilated air space behind that cladding, if you had a radiative barrier on the next layer, on the other side of that space, you could reduce the heat flow.
Again, though you have to be careful in some climates you do not necessarily want a vapor barrier radiative surface on the outside of the wall assembly.
Before the show, Jonathan said he has not actually measured how effective a radiant barrier is on the outside of an assembly like this…
"But I have heard anecdotally, that it has reduced the heat flow—especially in roofs— a fair bit.
The reason why they seem to work so well in roofs in Texas is because there is a huge temperature difference between the surface of the roof that has been heated by the sun and the inside of the attic.
"Radiant barriers work best under a large temperature differences with minimal insulation in the wall cavity.
They kind of compete with insulation, if they find themselves in proximity.
"As we move toward higher R-value wall assemblies, radiant barriers do not make as big a difference as they did when we had smaller amounts of insulation and larger delta Ts, or larger temperature differences."
Basically, the better you insulate, the less sense a radiant barrier makes.
"One of the other things to remember is that a radiant barrier only works when it is clean and shiny. If it gets dirty or dusty during construction or during operation, it stops working as a radiant barrier and works more like a normal surface."
So here’s your residential construction radiant barrier checklist:
❏ Air gap
❏ Huge temperature difference
❏ Clean and shiny
"One of the most practical uses of radiant barriers is not in residential construction, it’s in the construction of ice rinks. Often times in ice rinks you will see the ceiling above the rink it will be a foil facing, shiny reflective surface, and that does two things.
Having a radiant barrier above the ice surface stops the heat transfer between the roof and the ice surface. So the ice surface stays more consistent and uniform in temperature—which is great—and it actually limits the heat loss through the roof assembly because there is not as much of a delta T or temperature difference across the roof assembly when you reflect all the cold back to the ice rink.
Now you can WOW 'em at the hot cocoa stand of your local rink.
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