Effect of Dust On Radiant Barriers?

What About The Effect of Dust Accumulation On Top Of The Foil?

There is both some truth and myth to this claim. Radiant Barrier has two qualities: Reflectivity and Emissivity. Reflectivity is that ability to REFLECT Radiant Energy and Emissivity is the ability NOT to RELEASE heat in it's Radiant Form.

Double-Sided Does The Trick. Both Sides Are Working

Yes, over time there can and will be some dust accumulation on the topside of the foil. If there is an air gap, the topside of the foil is using the Reflective quality of a radiant barrier to reject heat. In tests, it takes a significant amount of dust to reduce the reflectivity of the foil.

However, with a double-sided product, there is always a side facing down, right? Dust does NOT fall UP. So, no matter what, the bottom will NEVER have a reduction in effectiveness. This goes back to the EMISSIVITY quality of Radiant Barrier Foil. Or the ability NOT to release heat. The best example I always use is a potato wrapped in foil. Since the foil is in contact with the potato, it is working off the Emissivity quality or NOT allowing the potato to release the heat as fast.

So, EVEN IF you were to have a bunch of dust on the top side, the radiant barrier foil would still be effective. Just think of the dust as the potato. It will want to release all the radiant heat upwards back towards the roof rather than down towards the living space.

This Is How Radiant Barrier Decking like TechShield and PolarPly Work.

Have you seen radiant barrier decking? For new construction, it's a good option. Basically, what they have done is laminate a single layer of foil to the bottom of a sheet of OSB board. It's installed like normal roof decking with the foil facing the attic. Contrary to what is often believed, radiant barrier decking is working off the Emissivity quality and NOT of the Reflectivity quality. The attic is the air gap required.

What About Laying The Foil Out OVER The Insulation, THEN Dust Would Make It Less Effective Right?

Not really, since unless you really push the foil into the insulation, there is very little actual surface contact with the insulation. If you read the specific instructions for the Over-Insulation Method you will see that it specifically recommends trying to keep and inch or two gap between the foil and the Radiant Barrier Foil. Even if the top layer of foil does get a layer of dust, the bottom layer of foil will still stop the radiant heat transfer due to the emissivity quality

This is one reason double-sided radiant barrier foil was developed. Installed properly, dust should have no effect on the effectiveness of radiant barrier foil insulation.