How Reflective Insulation R-values Can Change

Reflective Insulation

The only difference between a radiant barrier and a reflective insulation is that a reflective insulation has an R-value. The R-value is achieved by having the low-emissivity, high-reflectivity surface face into a dead air space. While some products also have a layer of insulation laminated to a low-e surface, that insulation layer is typically just an R-1, so even those products still rely on the dead air space.

The FTC (Federal Trade Commission) allows the R-values of reflective insulation products to be tested and marketed two ways.

  1. The product can be sent to a testing lab, which will construct a mock-up assembly representative of how the product will be installed in the field (or example between wall joists or under a crawl space) and run a test measuring the BTUs of energy lost when the temperature, air flow, direction of heat, and all other factors are accounted for.
  2. Manufacturers to just use numbers in a table provided by the ASHRAE (the American Society of Heating, Refrigeration and Air-conditioning Engineers), which has the exact R-values for low-e surfaces facing into different sized dead air spaces, under different temperatures and different directions of heat flow. This seems simple enough, and most manufacturers are happy to measure the exact emissivity of their product and find the corresponding number in the table to list as their R-value.


However, things get complicated when the emissivity of the product changes over time due to corrosion. As explained in the previous blog post on corrosion, when aluminum is exposed to heat and humidity over time, the emissivity increases (and the reflectivity decreases accordingly). A higher emissivity means a necessarily lower R-value, all other factors being equal.

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Testing radiant barrier and reflective insulation products for emissivity before and after corrosion testing revealed that, while some products corrode less aggressively than others, nearly all aluminum products corrode to some degree. One popular product made with aluminum foil had it’s emissivity increase from 0.03 to 0.08 (97% to 92% reflective) after 15 minutes exposure to high heat and humidity while another increased to nearly 0.4 (60% reflective). The corrosion can be even more aggressive if liquid water (or condensation) is involved.

For example, let’s say a single-sided reflective insulation with an initial emissivity of 0.03 is installed in a 2x4 crawlspace. The heat flow is down, the air temperature is a steady 50 F with a 30 F temperature difference. You would get an insulation value of R-9.60. Now if that product were a coated aluminum metalized film or aluminum foil that saw some humidity, and the emissivity increased to 0.05, you wouldn’t think it would make a big difference in R-value. However, that small change in emissivity brings the total R-value down to R-8.17. Now suppose this product is an aluminum foil that sees some water. According to ASHRAE, when just a little condensation is visible on aluminum foil, the emissivity can reach 0.2 to 0.3, which would decrease that R-value down to R-3.86. Then take the worse case scenario that maybe your low-e surface is made with an unprotected, uncoated metalized film and has an emissivity of about 0.50. We’ve seen the aluminum on an unprotected film completely oxidize in high heat and high humidity environments. In that case, the R-value would drop to R-1.88. Now your product has just lost over 75% of its R-value and you’ll have to re-insulate all over again.

Reflective Insulation 2

Unfortunately, reflective insulation manufacturers don’t mark an “aged R-value” so the consumer doesn’t know to expect lost performance over time. If the reflective insulation gets installed in an application where it’s not visible, the customer won’t know it’s a problem until it’s too late. So as a consumer what can you do about it? Well, for one you can look to only buy products from manufacturers that run extensive corrosion testing and can show the stability of their products over time, or at the very least, only buy products with multi-year warranties so you won’t be out the money if or when it does need to be replaced.


One such product is Sigma’s 3100 Series Thermflux. This product holds up better to corrosion than all other aluminum products tested (including a wide variety of aluminum foil and metalized film laminates) because Sigma tests each and every roll they make to an aggressive, accelerated corrosion test. This product starts off at 0.04 emissivity (96% reflective) due to the protective coating over the aluminum and holds its emissivity (and R-value) under laboratory tests with 100% humidity condensing on the aluminum and heat so high it starts to shrivel the material. (If your house ever gets that hot, you have bigger problems than the R-value of the insulation.)

Another solution to the corrosion problem is to use a product that isn’t made with aluminum at all. Sigma’s new Copperflect reflective insulation is made with 99.99% pure copper and no aluminum whatsoever. Copper, like gold, is considered a “noble metal” because it is so resistant to corrosion. Copper is also lower emissivity than aluminum, so the product can easily maintain 0.03 emissivity, even after an anti-patina coating is applied. Sigma is so confident in the continued performance of Copperflect reflective insulation, they stand behind it with a Lifetime Warranty.


How Corrosion Affects Radiant Barrier

Radiant barrier type insulation relies solely on its low emissivity to insulate, providing no resistance to convection or conduction. As a result, anything that might affect the radiant barrier’s emissivity, like metal corrosion, should be a primary consideration in purchasing a product. Emissivity is expressed as a percent indicating how much infrared radiant heat is allowed to “emit” from it, as opposed to reflecting off it.

Corrosion Foil

Historically radiant barriers have been made with aluminum, from aluminum foil to aluminum vacuum deposited onto a plastic film or substrate. This makes sense with aluminum being inexpensive, easily available, and naturally low emissivity - pure aluminum having an emissivity of 0.03 (or 3%). One issue with aluminum, though, is that it is a highly reactive metal, corroding when exposed to humidity and increasing in emissivity. Radiant barrier manufacturers have a developed a number of solutions to deal with this corrosion problem and mitigate the effects of humidity.

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Aluminum foil radiant barriers consist of thick aluminum (over 100 microns), so even if the aluminum on the top surface corrodes and increases in emissivity, the hydroxide and oxide layers are still relatively thin, and the uncorroded aluminum below is able to maintain its low emissivity. Due to the relative stability of the oxide and hydroxide layers, once the outer surface is corroded, the emissivity won’t increase significantly more. These aluminum foil radiant barriers may only increase emissivity to 0.08 (or 8%) unless exposed directly to liquid water, in which case the process can continue.

Corroded RB

After issues with electrocutions and changes to the fire code in 2009 and 2010, many radiant barrier manufacturers started using vacuum metalized aluminum plastic films and fabrics instead of aluminum foils. These products are made in vacuum deposition processes and consist of aluminum just nanometers in thickness applied to a plastic film, one molecule at a time. Due to the small amount of aluminum actually on the film, once these products see humidity in the air and start to corrode, there isn’t enough aluminum below it to stabilize, and the aluminum corrodes all the way through. As a result, these products can increase in emissivity to above 0.35 (35%).

To address the loss of performance caused by corrosion, many of these manufacturers developed methods of coating the aluminum to protect it. Different manufacturers use different formulations in their coatings, but the end goal is the same - to apply a layer thin enough to not dramatically affect the initial emissivity, but strong enough to stand up to extreme humidity over time.  Generally speaking, radiant barrier products with this type of coating usually start out with an emissivity of 0.04 (4%) to 0.05 (5%) as opposed to 0.03 (3%), which is a small price to pay to ensure the radiant barrier will not increase emissivity after it’s been installed. Depending on the product, the emissivity could increase up to 0.08 (8%). Of course, some coatings are more effective than others, and the formulation of the coating and the level of protection offered is seldom shown on the manufacturer’s spec sheet, let alone the product’s packaging.

Copper sample

Recently, Sigma Technologies found a different solution, which seems obvious in retrospect - use a metal other than aluminum. The lowest emissivity natural metals are aluminum, copper, silver and gold. These noble metals are all lower emissivity than aluminum and significantly more resistant to corrosion. By using a similar vacuum deposition process, Sigma was able to apply nanometers of copper to a plastic substrate, and then apply a protective top-coat over the copper to prevent patina, achieving a final emissivity of about 0.03 (97% reflective). The resulting radiant barrier, Copperflect, only increased in emissivity by 0.002, which is well within the margin of error of the test. This is because, as a “noble metal” copper doesn’t corrode in moisture, even under the most extreme conditions.


Why Low Emissivity Matters in Radiant Barrier Applications

Low Emissivity Radiant Barrier

Simply speaking, emissivity represents the percentage of infrared radiant heat a product absorbs and emits rather than reflects. It is represented as a number between 0 and 1, or as a percentage. As we explain in an earlier blog post, surface emissivity of insulation can play a significant role in the insulation’s efficiency because the lower the emissivity of the outer surface, the lower the percentage of radiant heat the insulation allows to “emit” though. In residential and commercial construction, a radiant barrier uses just a low emissivity surface to provide all of its insulation.

A radiant barrier is a thin material with low emissivity on at least one side, which faces into an open, vented air space, like an attic. In an attic application, as infrared heat radiates through the high emissivity roofing materials into the attic, a radiant barrier reflects and doesn’t emit a large percentage of the radiant heat. ASTM C1313 defines a radiant barrier as a material having an emittance of 0.1 or less, meaning that less than 10% of the heat is able to emit down through the insulation to the living space below.

Because a radiant barrier is installed in an open, vented system, the rejected heat isn’t trapped, but instead is allowed to vent out naturally, meaning that having adequate ventilation in the attic is still just as important. Because the radiant barrier is only addressing radiant heat, and not conductive or convective heat, having traditional blown or batt insulation on the attic floor, below the radiant barrier, is still required. While the contractor or home builder may not necessarily be able to reduce the amount of ventilation or insulation with the addition of a radiant barrier, the radiant barrier can still significantly reduce the energy used in the home, especially when it comes to heating and cooling.

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According to testing from Oak Ridge National Labs and the US Department of Energy, a radiant barrier can save up to 17% of cooling energy in the summer and 10% of space heating energy in the winter. Of course, the summer savings are most significant when the ambient temperature is highest, resulting in a greater influx of radiant heat, which the radiant barrier is able to reflect. In the winter, the heat from the home escapes to the outside, cooler air. The portion of the heat that is in the infrared is unable to emit through the radiant barrier and is reflected back down. Testing at Tennessee Valley Authority analyzed the performance of a radiant barrier at various temperatures and times of the day and found that in the winter, the radiant barrier realized the greatest savings after the sun went down and when the outside temperature dropped the most.

Since low emissivity is the primary determining factor in the efficiency of a radiant barrier, it is important to compare the exact emissivities of radiant barrier products before specifying a product for a new project or before making a purchase for your own home. The emissivity of any given radiant barrier should be available on the  manufacturer’s Fact Sheet or on the product label itself. The Federal Trade Commission (FTC) requires in its “R-value Rule” that insulation products of this type must be tested for emissivity in order to indicate its insulating value, and each manufacturer must keep records of the emissivity testing results for 3 years in order to verify any claims made.



The Importance of Testing Radiant Barrier

The primary performance characteristics, like surface emissivity, corrosion resistance, breathability and even the strength of radiant barriers can change, batch-to-batch and run-to-run during manufacturing. Products need to be tested and retested to make sure they are meeting spec, especially if you’re marketing or advertising a certain quality or performance.

In addition to emissivity of aluminum products changing over a long exposure to oxygen or water, the emissivity is also affected by the protective coating over the top (used to prevent corrosion over time) and the application of the coating, or even the metal itself, in the case of vacuum metalized products, can vary depending on the application. Additionally, the pins used to perforate products for breathability can dull over time, and must be periodically sharpened or replaced to maintain the radiant barrier’s breathability. Even the strength of the products can change depending on the quality and consistency of the internal reinforcement and the heat used in manufacturing. It’s important that manufacturers, resellers, distributors and even contractors be aware of the variation that can occur and have products periodically tested to ensure quality.

Radiant Barrier Testing

Some manufacturers are able to test performance characteristics of their radiant barriers in-house, and spot-check for quality, whereas others must send samples to outside labs. Doing it in-house requires specially trained staff to make sure the tests are being run correctly. While most of these tests are relatively easy to perform with practice, the user error of someone untrained on the equipment can lead to inaccurate results. Additionally, even relatively small testing equipment, like the Portable Emissometer or the Tensile Testing Machine can be very expensive to purchase (thousands of dollars), and relatively inexpensive to have tested by an outside lab (hundreds of dollars), so depending on the frequency of the quality checks, it may be more cost-effective to send samples out to a lab as needed.

In the case of ASTM tests, ASTM provides the contact information of testing labs certified to test to each standard, making it easy to get competing quotes. Sigma Technologies International is able to test every roll of radiant barrier they manufacturer, but they already have engineers on staff and own much of the testing equipment anyway since a significant part of Sigma’s business is making custom insulation, films and fabrics (both metalized and unmetalized) for large companies. Therefore, it isn’t a significant additional expense to have engineers test the radiant barrier prior to its packaging and shipping.

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Another reason manufacturers, distributors, and installers may need to periodically test their radiant barrier is to keep records on file to back up claims made. As the Federal Trade Commission (FTC) lays out in its “R-value Rule”, any claims made on the performance of the insulation (like emissivity or R-value) must be backed up by testing data from no longer than 3 years from the date of the last claim. Specific states or municipalities may require a higher standard. The state of California, for example, requires data from no more than 1 year from the date of the claim. This means, even if you are 100% confident in the quality and consistency of the radiant barrier you sell, you must have recent performance data on record in order to advertise the performance of your radiant barrier.

Of course, any contractor or installer offering a warranty on the radiant barrier, on anything from the performance of the material itself and the savings the customer should expect, to the workmanship and quality of the installation, should have the performance characteristics of the product tested to avoid liability. While manufacturers, resellers and distributors sell the product for cents per square foot, contractors and installers make dollars per square foot, so warranty claims can be much more expensive. A product with a higher emissivity or a lower insulating value than advertised, or a product that is so weak that it is difficult to work with and properly install, may lead to a warranty claim.

While manufacturers in the US may be used to testing to ASTM, UL, and FTC standards, manufacturers overseas may not. Any distributors or resellers buying material from overseas may want to verify the quality of the material with each shipment. While attributes like strength may be easy to give a “pass or fail” without additional testing, some important attributes like breathability/permeability and reflectivity/emissivity are practically impossible to detect with the naked eye.


With emissivity, for example, products that are visually reflective may not be very reflective in the infrared, whereas products that look visually dull may still reflect very well in the infrared. Some manufacturers could put on too thick of a clear coating over the aluminum, resulting in a product that will hold up well to moisture, but may only be 50% emissive and 50% reflective. On the other hand, a manufacturer may put on too thin of a coating or neglect a coating all together, resulting in a product that is 3% emissive and 97% reflective, but corrodes quickly and easily, losing its performance, down to 35% emissive and 65% reflective. A radiant barrier should be no more than 10% emissive and no less than 90% reflective to meet the definition of a radiant barrier according to  ASTM C1313.

Breathability or permeability is also hard to verify with the naked eye. Products with large perforations may actually be less permeable than products with smaller perforations, depending on the frequency and positioning of the perforations. Even the smallest change in the size of the holes due to the dulling of pins or the heat applied to the material (resulting in shrinkage of the plastic reinforcement) can reduce the breathability. Likewise, small variations like a shifting of the material going through the perforator, the height distance from the material to the perforator, the pressure of the pins, and even the speed of the perforator can affect the final breathability and permeability. ASTM E96 describes the test method for measure the perm rating of a product by measuring how much water vapor is able to pass through. ASTM C1313, the Standard Spec for Sheet Radiant Barrier, and ASTM C1224, the Standard Spec for Reflective Insulation set the acceptable standards for radiant barrier and reflective insulation products. A perm rating of greater than 5 is required for any product intended to be breathable, according to ASTM C1313 and ASTM C1224. If a manufacturer produces a product hovering above 5 perms, it would be wise to monitor the material periodically to ensure that it doesn’t fall to 5 perms or less at any point. While slight variations run-to-run or batch-to-batch may be less urgent for those companies with a higher perm product, it still may be wise to keep an eye on it over time.

There are many factors that go into producing, selling and installing radiant barrier products, and all of these performance characteristics can change. Attributes like emissivity, corrosion resistance, breathability, and strength, are essential for any radiant barrier product, and therefore, should be continually tested to make sure the radiant barrier is performing as it should.


Multi-Layer Insulation -- What Makes it Different

Multi-layer insulation spaces heat-reflective insulation material across air space. This minimizes conduction because of the space between layers, and each layer minimized radiation by reflecting back 95% to 98% of infrared radiation. Dozens of alternating layers combined create an incredibly efficient insulator, used everywhere from NASA space suits, to modern cryogenics, to textiles and building materials.

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