If a radiant barrier is on the floor, then dust can land on it and over time won’t that cover up the foil and make it useless? Why would you suggest anyone install the foil over the insulation if this is true?
Myth: The accumulation of dust on radiant barrier compromises it’s reflectivity and therefore its ability to work as a reflector of radiant heat.
To begin, it’s truthful to say that most homes will not accumulate enough dust in the attic area to diminish aluminum’s reflective quality. Even at a glance, a thin layer of dust on your AtticFoil™ radiant barrier has been shown to cause no difference in the ability of the material to reflect radiant heat. A simple way to think of this is to consider a mirror in your home that might get dusty; the more dust that accumulates, the more reflectivity the mirror will lose.
Dust and Single-Sided Foil
Before double-sided foil was created, radiant barrier existed as a single-sided product. Because of this, installation was precise in which side of the foil needed to face up or down. About 30 years ago, when single-sided foil still dominated the market, most installations consisted of laying the foil on the attic floor, over the existing insulation. The foil was installed with the foil side facing up, leaving it prone to dust collection. In cases where dust accumulated rapidly and frequently, it was noted that this had an impact on the reflectivity of the radiant barrier. Large amounts of dust can reduce, or eliminate, aluminum’s ability to act as a reflector of radiant heat. Therefore, it was determined that when using a single-sided product, careful measures had to be taken in order to minimize dust accrual. Some of these measures included stapling the foil up on the rafters and using multiple layers of foil on the floor, but eventually the superior idea of creating a double-sided material was born.
Dust and Double-Sided Foil
The creation of a double-sided radiant barrier proved to be an economical way to increase year-round benefits, while eliminating the threat of dust endangering the reflectivity of the foil’s surface. The main advantage of double-sided heavyweight radiant barrier foil like AtticFoil™ is that it allows for protection against radiant heat by one of two ways: reflectivity or emissivity. Both properties are present in the foil, so if/when one is compromised, the foil can still work off the other property, without a decrease in overall performance.
Reflectivity and Emissivity
Reflectivity is defined as a measure of the ability of a surface to reflect radiation (or energy/heat). Aluminum reflectivity is 97%; therefore, aluminum has the ability to reflect 97% of the radiant heat waves coming in contact with it. This is why AtticFoil™ radiant barrier works so well against radiant heat – when the sun radiates heat to the earth, it is either absorbed or reflected. Typically your roof will absorb the heat (because the majority of roofs are not reflective) and it will travel through the roofing materials until it reaches your attic. At that point there is nothing left to come in contact with so the heat converts from traveling conductively (through materials that are touching in the roof) to its radiant form (heat traveling across an air space) where it can now be reflected (via the radiant barrier) back towards the direction it came from.
Emissivity is defined as the ability of a surface to emit (release) radiant energy. While aluminum has a high reflectivity (97%), it has a low emissivity (only 3%). This means that pure aluminum only allows 3% of radiant heat to pass through it by means of trapping 97% of the radiant heat behind the foil.
Therefore, depending on which side of the double-sided product the air gap is one, the foil will be either using reflectivity to reflect heat or emissivity to block it. Either way, your results are going to be 97% rejection of heat transfer.
Dealing with Dust
So what do reflectivity and emissivity have to do with dust and how it affects foil? Where previously foil had one side, optimal conditions were required in order to make sure the topside would not be compromised by dust accrual. Now, with double-sided foil, even if the dust does get to a point where it jeopardizes the reflectivity, the foil on the bottom side (the side facing the insulation on the floor) will work off of it’s emissivity quality, only allowing 3% of the heat to pass through.
If it’s wintertime and you’re heating your home, this means you’re not losing heat through the roof and so you home stays warmer without using as much energy (to replace what is being constantly lost). If it’s summertime, this means only 3% of the radiant heat entering your attic will be emitted into the insulation on your floor. So rather than absorbing 100% of the heat and having to run your air conditioner to keep your living space cool, you now only have to deal with about 3% of that heat. That means a cooler, more comfortable home and a more reasonable energy bill. Overall double-sided foil is the best choice for all applications of a radiant barrier because it offers you the advantage of both of aluminum’s properties, making dust a non-issue.
