One of the most common questions we hear is:
"Does this product create the air gap I need for a radiant barrier?"
It's an understandable question because, after all, many products are full of air. Bubble insulation contains air, spray foam contains tiny air pockets, fiberglass insulation is mostly air, and even mesh products seem "open." But none of these products count as the air gap a radiant barrier requires so you can't sandwich the foil between them. Here's why:
What Is an Air Gap?
A radiant barrier works by reflecting radiant heat (infrared energy). In order for that reflection to happen, the foil surface must face an actual air space of at least ½" or more.
An air gap is exactly what it sounds like:
- Empty space
- Air only
- No solid material touching the reflective surface on at least one side
Contrary to popular belief, the air gap does not have to be vented. It simply needs to be a void occupied by air rather than another material.
Why Doesn't Foam Count?
Closed-cell foam, open-cell foam, rigid foam board - they all contain air. But the foil isn't facing air, it's touching foam.
Even though foam contains millions of tiny air cells, those air pockets are trapped inside a solid material. Heat moves through the foam by conduction, and the reflective foil is no longer able to reflect radiant energy across an open space. The result?
The foil behaves much like any other facing material instead of functioning as a true radiant barrier when it's sandwiched between foam and another layer.
What About Bubble Insulation?
This is probably the biggest misconception. Bubble insulation contains air bubbles, but those bubbles are sealed inside plastic. Again, the foil is facing plastic - not an open air space. If bubble insulation is sandwiched tightly between two materials, there is no radiant barrier effect because there is no air gap adjacent to the foil.
Only if one side of the foil faces an open air space can it function as a radiant barrier.
Does Blown-In Insulation Count?
No. Regardless of whether it's:
- Cellulose
- Fiberglass
- Mineral wool
...once the insulation is touching the foil, the required air gap is gone. The insulation itself contains plenty of air, but it is still a solid mass of fibers and particles contacting the reflective surface. That contact prevents the foil from performing as a radiant barrier and is the reason why you can't lay foil down on the attic floor and then cover it with blown-in cellulose insulation. The foil would have to be laid on top of the blown-in insulation, as we show on this page of our website: Laying Foil Over Existing Attic Insulation.
What About Fiberglass Batts?
The same principle applies. Fiberglass insulation is mostly air, but the foil is touching fiberglass fibers - not an open air space. That's why foil-faced fiberglass insulation only provides radiant barrier performance on the side facing an actual air cavity. The side pressed directly against another material, including batt insulation like ROCKWOOL®, loses that radiant barrier benefit. If you're pairing it with batt insulation, one side of the foil surface must always face an air gap if the other side is in contact with the insulation.
Does Mesh Count?
Another question we hear is whether plastic mesh (like Keene Easy-Fur), spacer mesh, or woven materials qualify as an air gap. The answer is still no if you're using them across the entire surface area of the foil.
While mesh creates small openings, it is still a material contacting portions of the foil surface. The mesh itself is not the air gap.
Now, mesh spacers can sometimes be used to create an air gap by physically separating the foil from another surface. In that case, it's the open air space between the foil and the adjacent material that's doing the work - not the mesh itself. When used like this, the mesh is cut into strips that span the length of the surface to keep the foil from touching the next material, as seen in the photo above.
The Easy Rule to Remember
Ask yourself one simple question: What is directly in front of the foil?
If the answer is:
- Foam ❌
- Bubble plastic ❌
- Fiberglass ❌
- Cellulose ❌
- Mineral wool ❌
- Another solid material ❌
...then there is not an air gap.
If the answer is:
- Empty space filled only with air ✅
...then you have the air gap needed for a radiant barrier to reflect heat.
Why This Matters
Radiant barriers reduce heat transfer by reflecting radiant energy across an air space. Once another material is pressed against the foil, heat primarily transfers through conduction instead, and the radiant barrier effect is eliminated. That's why installation details matter just as much as choosing a quality foil product.
Summary
Many products contain air, but containing air is not the same thing as providing an air gap.
An effective radiant barrier requires the reflective foil to face an unobstructed air space. It doesn't matter whether that space is vented or enclosed - it simply needs to be air and air alone.
When in doubt, remember this rule:
Air inside a material is not an air gap. Only open air next to the foil creates the conditions a radiant barrier needs to work effectively.
