Hey this is Ed with AtticFoil.com, your complete online source for Do-It-Yourself
radiant barrier foil. One of the most common questions I get all the time is to explain
why exactly an air gap is required for radiant barrier to work. The first thing you fully
have to understand is exactly what is radiant heat. Radiant heat is a heat form that travels
across either an air gap or a vacuum, and I'm going to give you an example of this.
If you go into the kitchen and you stand in front of the oven a few feet way, you feel
the heat coming across the kitchen - that's radiant heat. Now if you go up and you put
your hand ON the oven, you've eliminated the air gap - now you have a solid, basically,
between the oven and your hand. Heat flowing in to your hand is conduction, or conductive
heat flow. Now with the radiant barrier foil, it can only reflect heat that is traveling
across and air gap, and I'll give you an example. If you take a hot skillet and you put your
hand several inches above it, you're gonna FEEL that radiant heat coming off of the skillet,
ok? Now, if you take a piece of foil and you pull it tight across the top of the skillet,
a few inches away, and you put your hand on top, you're going to feel almost NO heat coming
off that skillet. The heat's coming up, it's hitting the foil, it's being reflected back.
This is reflectivity. Radiant barrier foil has a 97% reflectivity, basically it's only
letting about 3% of that heat pass thru. If you put the foil on top of the skillet and
you put your hand several inches above it, it's working off what's called the emissivity
quality. This is the ability to NOT release heat, and it's basically the inverse of reflectivity
- it's only emitting about 3% of the heat. So you could pretty much keep your hand above
that skillet all day long, it's never gonna burn because it's just not emitting that heat.
It's causing that heat to be reflected downward. If you were to take your hand and you put
it down, directly on top of the foil, no you've eliminated that air gap, and now you're back
to conduction. That heat is gonna flow extremely efficiently from the skillet, through the
foil, and in to your hand. This is exactly the same thing as a radiant barrier application
in a home assembly. You MUST have an air gap in order to get either the emissivity quality
or the reflectivity quality that you're looking for. Typically a half inch or more is recommended.
This is why you can never "sandwich" foil between two other products - if you do that
it will actually work against you and increase the heat flow through the assembly by conduction.
This is another reason why you cannot put radiant barrier foil directly under shingles,
between the shingles and the roofing felt. It'll actually work against you, make the
attic hotter and INCREASE heat flow into the assembly. Another example is in walls. People
often thing that they can put the radiant barrier just behind the sheet rock and put
it between the sheetrock and the insulation. This won't work. Insulation is NOT an air
gap, think of nothingness - you've got to literally have a VOID. Insulation is technically
a solid with a lot of air in it. So if you're gonna do a wall assembly you've got to create
an air gap. It doesn't matter on either side it's gonna work either off of reflectivity
or emissivity. Ok, so let's do a quick review! Radiant heat, by definition, is heat that
travels across either an air gap or a vacuum. In order for radiant heat to exist, you MUST
have this air gap. If you don't have this air gap, you CANNOT have radiant heat. If
you don't have radiant heat, you can't install a radiant barrier - it just doesn't work.
If you sandwich two products together and eliminate that air gap, you have conduction,
or conductive heat. So hopefully this explains WHY and air gap is REQUIRED when you install
radiant barrier. For more information on installation pictures, tips, tricks, and to order online,
visit AtticFoil.com.}