Radiant barrier: Why does the reflective surface face the attic and not the roof?

I’ve been looking at radiant barriers for my attic and the instructions usually say to have the reflective barrier facing into the airspace. That is, with the barrier installed on the roof, the shiny side should face into the airspace of the attic. Why is that? I would think the shiny side should face the roof so that the heat from the roof bounces back.

Would it make any difference if the barrier was applied directly to the roof with no air gap or to the truss which would have an air gap of a few inches?

The reflective side will work as well in terms of radiation with it pointing either way, since the ability of the reflective side to emit radiation is directly tied to its ability to absorb radiation.

Reasons to have it pointing down are that if it were pointing up, part of the reflective surface will be touching the roof or roof trusses, allowing conductive heat transfer, reducing its effectiveness. that’s probably the main reason, but also, dust will tend to land on the top surface, and if that were the reflective side, that would also reduce its effectiveness.

Applying it directly to the underside of the roof would be the worst way to install it.

I found this website that explains the technical details of how a radiant barrier reflects in both directions as long as there is an air space on one side. - Physics of Radiant Barrier Foil - See the “Refelctivity and Air Spaces” section.

For future reference, I came across this video which described it pretty well:

Radiant Barrier Foil Insulation - Why Is Air Gap or Space Required? - YouTube.

The reason it doesn’t matter which side the foil is on is because aluminum will either reflect 97% of the radiant heat or it will only allow 3% of it to pass through. So if it’s facing the roof, it will reflect 97% of the heat back out. If it’s facing the attic, only 3% of the roof’s radiant heat will pass through the aluminum layer.

I ended up installing a foil in my attic and it made a huge difference. It’s comparable to how your car feels much cooler when you park in the shade vs. the sun.

A polished metal surface not only reflects radiation (tends not to absorb it as heat) but it is also a very poor radiator of heat, so if it does get hot, it tends not to lose much heat. Ever pick up a shiny wrench that has been laying out in the sun? It got so hot because even though it is a poor solar adsorber, it is a worse radiator. Steam “radiators” are often painted silver, to make them worse radiators. This allows them to heat a room more evenly by conduction/convection, rather than baking the area near the radiator with radiated IR. Baseboard radiators have metal shields to aid convection and block radiation.

Kitplanes magazine once did a test to see what color paint kept composite airplanes the coolest when the airplanes were tied in the sun. Titanium oxide based white was the best by far, and silver/aluminum “hillbilly chrome” was the worst. The silver painted surface got even hotter than either flat or glossy black paint.

Anyway, the radiant block is to be placed shiny side down, because you don’t want it radiating heat toward the living space, and the shiny side has significantly lower heat radiation than the other side.

Radiant barrier works via emissivity which is the opposite of reflectance (E = 1 - R). The barrier gets very hot but because the shiny surface emits very little heat (i.e. it has low emittance) and because the shiny surface faces inwards it radiates very little heat into your attic space. It’s a radiant barrier. Not a reflective barrier.

The barrier itself will be unbearably hot to touch. It’s absorbing the heat but has no way of emitting it. The only ways it can disperse heat is conduction and convection (which is why you need the air gap).

You must have an air-gap. The barrier gets hot - that’s the point - but is unable to emit that heat into your attic space. If there is no air gap the barrier will transfer heat via conduction. Materials that are shiny (low-E) are usually also good conductors.


Apparently the product is so bad he’s embarrassed to admit he sells it.