Reflective-Insulation and Radiant-Barrier Systems
December 30, 2013
This online-only article is a supplement to the January/February 2014 print edition of Home Energy Magazine.
A reflective-insulation system consists of a low-e surface, like aluminum, which is installed facing an enclosed air space. Enclosed air typically has an R-value of about R-1 per inch, but by reducing the radiant-heat transfer through the air space, the R-value can be increased to over R-4 per inch.
When discussing home insulation, the products that come to mind for most insulation professionals are fiberglass, cellulose, and foam—products designed to prevent convective heat transfer by trapping the air, and to prevent conductive heat transfer by using materials that have low thermal conductivity. Reflective-insulation products, on the other hand, are specifically designed to prevent far-infrared radiant-heat transfer.
Most building materials, including insulation, readily absorb and re-emit radiant heat at about 80–90%, giving them an emittance value of 0.8–0.9. Reflective insulation, on the other hand, typically has a surface that absorbs and re-emits only 5% of the radiant heat, giving it an emittance value of 0.05.
Since all radiant heat that is not reflected from an opaque surface is absorbed by the surface, a material’s reflectance is often expressed instead of its emittance, but for opaque materials, these two properties must add to a value of unity, so that an opaque product with an emittance of 0.05 must have a reflectance of 0.95.
How Does Reflective Insulation Work?
The most common reflective-insulation wall systems include a combination of mass insulation (like a batt) and a reflective material facing a small, enclosed air space. If the distance between surfaces is small enough (on the order of 3/8 inch), a reflective insulation can be more effective than batt insulation, but it may have a lower effectiveness in a larger space, especially when the direction of heat flow is up or horizontal, (where convection can be a major factor in overall heat transfer). When the direction of heat transfer is down, convection is less of a factor, and radiation plays a larger role in the overall transfer of heat.
Exact reflective-insulation system R-values can be found in a table in chapter 26 of the 2013 ASHRAE Handbook of Fundamentals for reference, but manufacturers of reflective-insulation products provide the R-value for each product and system (for each direction of heat flow) on their fact sheet. Consult the manufacturer for the R-value for your specific system.
What Is a Radiant-Barrier System?
There are some applications in a home where it would be beneficial to reduce the heat transfer across open spaces where the space is vented or exposed to open air, as in an attic or crawl space. In these applications, a reflective surface will still reduce the heat transfer across the space by reducing radiant-heat transfer across the open space. These open systems are referred to as radiant-barrier systems.
A radiant-barrier system is most commonly used for attics in warm climates, because it effectively works like a shade tree, protecting the ceiling insulation from heat that would otherwise radiate downward from the hot roof heated by the sun. Because it’s impossible to measure an R-value where there is no defined closed air space, a radiant- barrier system technically doesn’t have an R-value. By effectively eliminating the radiant-heat transfer from the hot roof, the temperature of the attic space may be reduced by 20 0F or more on very sunny hot days. There are energy-related benefits. Ducts located in the attic don’t gain as much heat from the attic air, so the A/C doesn’t have to work as hard; an HVAC air handler located in the attic will have a reduced run time; and the insulation on the attic floor is under a reduced heat load. There are also nonenergy-related benefits. Any storage kept in the attic will be better preserved due to lower attic temperatures; and the whole house will feel more comfortable because less heat will be emitted from the ceiling down to the living space.
Examples of Radiant-Barrier Systems
Figure 1 shows cross-sections of three typical 2 x 4 walls, one with traditional such as fiberglass batt insulation, and two with reflective-insulation systems.
Wall A is filled with traditional batt insulation. As the sun’s rays strike the wall, the heat is absorbed and quickly conducts until it reaches the insulation batt, which is less conductive. The combination of the low-conductive fibers and the small air spaces significantly slows the rate of heat transfer, and this resistance is what is referred to as R-value. The R-value of a typical 3½-inch batt is R-11–R-15. Wall B has a reflective surface between the wall studs instead of a batt, creating a reflective-insulation system. As the sun’s rays hit the wall, the heat conducts to the cavity as it did before, but this time there is an air space in the cavity. Since heat can’t conduct well through air space, two things happen. One, the air adjacent to the hot surface heats up, and that hot air rises and circulates; and two, the heat is emitted by long-wave radiation across the air space. With the cavity properly sealed and with the size of the air spaces reduced, the air movement is limited; and with a reflective surface, 95% of the radiant heat is reflected rather than absorbed by the low-e material.
Although this system works differently than batt insulation, the exact resistance of the reflective-insulation system can be calculated, and an R-value determined, in a similar way. With this same 3½- inch cavity divided into two spaces by a reflective surface, the system has an R-value of R-5. Higher R-value for reflective insulation can be achieved by making the air spaces smaller, to limit the convective air movement.
Wall C is an extreme example of a reflective-insulation system. The same 3½-inch cavity is subdivided into seven air spaces of ½ inch each. With air spaces this small, convective air movement is reduced, and two reflective surfaces share each air space, which further improves the resistance of each air space. Using the calculated R-value tables from the ASHRAE Handbook of Fundamentals, the R-value of this system would be R-18. While six layers of aluminum foil or thin metalized film are still inexpensive, this particular system is impractical because of the labor involved in installing six layers.
Figure 2 illustrates a cross-section of a similar 3½-inch cavity with the heat flow down. Instead of a wall, let’s say this is a sealed roof cavity in a warm climate or a sealed crawl space cavity in a cool climate.
Cavity A still consists of a traditional insulation batt, and because this type of insulation functions primarily to reduce conductive heat transfer, it is still an R-11–R-15 construction. The way the insulation works has not changed with the direction of heat flow.
Cavity B, however, is now operating at a much higher efficiency. As we saw above, this reflective-insulation system achieved a lower R-value because convective air currents were circulating the air in the wall cavity. This time, the direction of heat transfer is down and convection is reduced, so reducing the radiation transfer results in a larger R-value. Since radiation plays a bigger role in heat transfer in this direction, the reflective insulation system is able to resist a higher heat load. By simply changing the direction of heat flow, we have increased the insulation value of this system to R-12.5.
Cavity C now becomes even less practical. Since the main reason for creating multiple small air spaces was to reduce the convective air currents, the system becomes almost irrelevant. If the cavity is divided into seven small air spaces with reflective insulation, the R-value is still just R-18, and the only reason for any improvement at all is that we still have multiple reflective surfaces sharing an air space.
How Do Radiant-Barrier Systems Work Without R-value?
In an attic system in a hot climate, for example, a low-e surface can be installed under the roof. As the sun heats up the roof, the roof tries to re-emit that heat. The radiant barrier prevents the radiant heat from reaching the attic and ceiling insulation below, reducing heat transfer to the living space below. As cooler outside air is brought in through ventilation, the attic air temperature is normally maintained within a few degrees of the outdoor air temperature.
The energy-related benefits are very easy to quantify, despite the fact that a radiant- barrier system doesn’t add an R-value to the insulation. Major universities and government-funded laboratories have conducted numerous studies on the improvement in HVAC efficiency and heat load reductions in attics with radiant barriers.
In the summer of 2008, Appalachian State University used side-by-side test houses in Charlotte, North Carolina—the Bellmont and the Parkwood—to evaluate the performance of typical houses with and without attic radiant barriers. The Parkwood has blown cellulose insulation on the attic floor. The Belmont started with fiberglass on the attic floor. The fiberglass was removed so the attic could be air-sealed around penetrations and weather stripped. Then the original fiberglass was put back in place on the attic floor and the radiant barrier installed. Thirty temperature sensors were placed in each house, and four Omnisense sensors were placed in each HVAC system.
After the radiant barrier was installed in the Bellmont, the peak attic temperature was reduced by 23°F, the peak HVAC run time was reduced by 20%, and the efficiency of the cooling air delivered through the ducts during peak temperatures improved by 57%.
Testing from Oak Ridge National Laboratory, working in conjunction with DOE, has evaluated over the course of about 25 years the benefits of radiant-barrier systems. In one study, researchers looked at three unoccupied houses, one as the control and two with radiant-barrier systems installed in different attic configurations. They concluded that an attic radiant barrier can save up to 17% of cooling energy in the summer and up to 10% of space- heating energy in the winter. These savings are attributable a number of factors, including the exact placement of the radiant barrier, the efficiency of the ductwork and HVAC, the level of existing insulation, and the climate where the radiant barrier is installed.
In fact, Energy Star requires a radiant barrier in the attic for the prescriptive path in the Energy Star Certified Homes program. This applies to all houses in southern climates with more than 10 feet of ductwork in an unconditioned attic. For a radiant barrier to qualify for the program, Energy Star requires a maximum emittance of 0.1 and a minimum reflectance of 0.9. While most radiant-barrier products meet this standard, you may need to consult the product label or the manufacturer’s fact sheet to ensure that the product you purchase qualifies.
Where Do Reflective Insulation and Radiant-Barrier Systems Work Best?
The reason that Energy Star does not require a radiant barrier for attics in cooler climates is that in winter, in those climates, the direction of heat flow would be mostly up and the heat from inside the house would be rising to escape. A radiant- barrier system is subject to the same convective heat transfer as a reflective-insulation system. Radiant-barrier systems do not consist of enclosed or sealed air spaces, and air is free to enter and exit the system. Typically radiant-barrier systems consist of large open spaces, such as an attic or a vented crawl space, rather than the few inches or sometimes fractions of an inch, that make up closed reflective- insulation systems. Because radiant-barrier systems consist of open spaces and have moving air, the effects of convection can be greater in these spaces than they are in the smaller, enclosed cavities typical of reflective-insulation systems.
Because convection plays a larger role in an open, vented space like an attic, the difference in direction of heat flow is even more pronounced. For this reason, an attic radiant barrier will offer greater overall reduction in heat transfer across the attic in warm climates in summer, where heat flow is down, than in cold climates in winter, where heat flow is up. A home in a cold climate can still benefit from a radiant barrier in an attic, but the resulting energy savings will be less significant. In some cases, the savings may be negligible. It may be wiser for a homeowner to install a radiant barrier in a floor or crawl space, where the space is smaller and the heat flow could still be down, to better prevent heat loss.
With reflective-insulation systems, manufacturers must disclose the R-values achieved by using their product in certain defined air spaces in all directions of heat flow, and those values will apply in all climates. Installers just need to apply the R-value that represents their specific application.
Get additional information on radiant barriers from DOE.
Exact reflective insulation system R-values can be found in American Society of Heating, Refrigerating, and Air-Conditioning Engineers. 2009 ASHRAE Handbook: Fundamentals, Chapter 26, Table 1 on page 26.1and Table 3 on page 26.3. Atlanta, Georgia, ASHRAE, 2009.
Studies on the improvement in HVAC efficiency and heat load reductions in attics with radiant barriers:
Davis, B. Radiant Barrier Impact on Selected Building Performance Measurements. Charlotte, North Carolina: Appalachian State University, 2008.
Mixon, W. R., et al. An Overview of the Building Energy Retrofit Research Program. Fourth Symposium on Improving Building Systems in Hot and Humid Climates. Oak Ridge, Tennessee: Oak Ridge National Laboratory, 1987.
Although radiant barriers are most often placed in the attic of a house, there are other places—for example, exterior walls—where radiant-barrier and reflective-insulation products can conserve energy. Radiant-barrier products are available in the form of a breathable house wrap, facing an air space created behind brick or siding, or on the interior side of an exterior wall. Other products for interior use have a lower perm rating but can still transmit vapor, while others are solid vapor retarders. Florida Solar Energy Center found that radiant-barrier wraps applied to an exterior wall were effective as thermal barriers on sun-bearing walls in hot climates. However this application did not perform well in cold climates, because blocking the incoming solar heat was counterproductive. Only the interior-applied reflective systems were equally effective in both climates, with R-values of R-5 to R-6.
If you are considering installing one of these products, consult the manufacturer to get the product specifications. These include perm rating or breathability, resistance to fungus and mildew growth, fire and smoke ratings, and other ASTM test requirements. Then consult your local code official to see what the requirements are for your specific application. In wall and ceiling applications high-perm products may be required for certain climates, and specific configurations and vapor retarders may be required for others. Be sure to follow the installation instructions provided by the manufacturer.
Reflective insulation, however, is used for a wide variety of applications in all directions of heat flow, and is recommended for all climates, since the R-value is not dependent on climate. R-value already takes into account variables like the direction of heat flow, the size of the air space, and the difference in temperature drop between inside and outside a building envelope. When deciding which type of insulation to install on your project, all you have to do is compare the R-values, and you’ll know which one will perform best.
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