One of the most common questions we get 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 radiant heat is. Radiant heat is a heat form that travels across either an air gap or a vacuum.
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. Using radiant barrier foil, the foil can only reflect heat that is traveling across an air gap, so, take a hot skillet and you put your hand several inches above it, now you can feel that radiant heat coming off of the skillet, right?
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 of the foil, you’re going to feel almost NO heat coming off that skillet. The heat’s coming up, it’s hitting the foil, and 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 through.
If you put the foil directly in/on the skillet and you put your hand several inches above it, now the foil is working off what’s called the emissivity quality. This is the ability to prevent heat from releasing (ie. not emitting heat), and it’s basically the inverse of reflectivity. Foil has an emissivity of 0.03, or 3%. So you could pretty much keep your hand above that skillet all day long and your hand would never burn because the foil is just not emitting much heat.
If you were to take your hand and you put it down, directly on top of the foil, now 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.
These are the exact same principles that apply to installing a radiant barrier in any assembly. You MUST have an air gap in order to get either the emissivity quality or the reflectivity quality that you’re looking for, otherwise the foil will not work as a radiant barrier.
How Much of an Air Gap is Required? Doesn’t Insulation Count as an Air Space?
Typically we suggest you have an air gap between 1/2″ and 3/4″ for the radiant barrier to work. Air gaps larger than this work well too – they promote ventilation on the foil and help keep the air dry and the air temps lower.
Insulation is technically a solid with a lot of air in it therefore, it is NOT an air gap. You literally must have a VOID, nothing in the air gap except air itself. So if you are installing under a roof or in a wall, you must create an air gap. It doesn’t matter which side the air gap is on, the foil will work the same whether it is using reflectivity or emissivity to block the heat transfer.
No Air Gap = No Radiant Heat = It Won’t Work!
In order for radiant heat to exist, you MUST have this air gap. If you don’t have this air gap, you CANNOT scientifically possibly have radiant heat because if you sandwich two products together and eliminate that air gap, you have conduction, or conductive heat. If you don’t have radiant heat, you don’t need to install a radiant barrier – it just doesn’t work. Hopefully this clarifies exactly why an air gap is REQUIRED any time you plan to install any radiant barrier.