When it comes to selecting fiberglass duct insulation, engineers must decide between specifying either duct liner or duct wrap. Each insulation offers different benefits that can make one or the other more applicable in certain settings. Fortunately, for many applications, either one will perform effectively. That said, the two materials are not identical across the board in terms of performance, and engineers need to be aware of their unique differences to specify the best material for the application.
In terms of rectangular ducts, the selection process can be fairly straight forward. If acoustical performance is a requirement for the insulation, then specifiers should consider selecting duct liner. Acoustical performance is measured by a noise reduction coefficient (NRC). The NRC value measures how well an insulation is able to absorb a range of sound frequencies. The higher the NRC value, the better the acoustical performance/sound absorption. In general, NRC ratings relate to the percentage of sound that is absorbed, with 0 being no absorption and 1 being 100% absorption. For most HVAC applications, NRC values will range between 0 and 1. For example, JM’s Linacoustic® duct liners offer NRC values ranging between 0.55 and 0.85 (depending on type and thickness). For a given insulation design, the thicker the insulation, the higher the NRC value.
In contrast, manufacturers typically don’t list an NRC value for fiberglass duct wrap, like Microlite® EQ, because it offers very little sound absorption because it is installed outside the duct. As a result, most commercial buildings will use duct liner to insulate their rectangular ducts to help reduce sounds like cross-talk. Whereas, in applications like warehouses or facilities, where sound control is a non-issue, then duct wrap is a perfectly viable alternative that will offer the same or similar thermal performance and typically at a lower cost than duct liner.
Keep in mind, however, that while one material may be less expensive, the overall cost for the project can vary depending on the cost of labor and installation. Duct liner is installed into the ducts as they are fabricated, and the ducts arrive on site ready to install. In contrast, duct wrap must be applied around the duct after the ducts have already been installed. This has the potential to eat up the cost savings from using a less expensive insulation, as installing and insulating the duct is a two-step process.
While material selection can be fairly straight-forward when discussing rectangular ducts, spiral ducts pose a different set of challenges and unique needs. Not only are duct wrap and duct liner insulating options, but engineers should also consider a double-wall duct insulation.
Both double-wall insulation, like Spiral SG®, and spiral duct liner, like Spiracoustic® Plus, offer acoustical benefits that duct wrap can’t deliver, but each insulation has different components that may make it a better choice for certain applications.
Double-wall spiral duct insulation has a long-standing history in terms of tried and true performance. It is the primary spiral duct insulation installation method in the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) manual. Because of this long history and the readily available installation resources, there are many contractors who are able to skillfully install double-wall insulation, making it easier for engineers and specifiers to count on their system to perform as specified.
The inner metal core required for a double wall system makes it ideal for applications that have high negative pressure, as the insulation is not exposed directly to the airstream surface nor does it have to be pinned to hold its shape (as would be required if a duct liner were applied in a high negative pressure system). Double wall systems can also be easier to install than a liner or wrap insulation in ducts with diameters of 10” or less.
However, double-wall insulation is not a perfect solution for every application. As the insulation must be wrapped around a perforated or solid inner core, the weight of the duct system can become unwieldy on larger diameter ducts. Additionally, since the inner core must be wrapped in the insulation and then inserted into the outer duct, there is a notable margin for error during the insertion process as the insulation could become bunched or tear if it is not inserted properly. When this happens, installers will have to start over and either reinsert the inner core or, if the insulation is torn, reinsulate and then reinsert the inner core. This can be both time-consuming and costly.
In contrast, spiral duct liner, like Spiracoustic Plus, works similarly to a rectangular duct liner, in that no inner core is needed. Spiracoustic Plus, is a pre-kerfed board, that is designed to be easy to insert into a duct. Installers buckle the insulation at the seam, insert it into the duct, and then release the insulation, allowing it to snap into place. You can see a video of how to install Spiracoustic Plus here.
Since spiral duct liner doesn’t require the inner core, the installation can be significantly easier in some applications, and it reduces the overall weight of the duct. In the case of Spiracoustic Plus, the insulation is also coated with Permacote®, an anti-microbial coating that helps protect against microbial growth on the airstream surface. A duct liner like Spiracoustic Plus can be a better alternative to double wall systems when the application utilizes large or very large diameter ducts - as the weight of the overall system is significantly lighter than a double wall system. Additionally, in some circumstances, it can require fewer people to install than a double wall installation.
As with rectangular ducts, duct wrap will typically offer the same thermal performance as a double-wall or spiral duct liner insulation, however, it does not offer any acoustical benefit.
Selecting the appropriate fiberglass duct insulation for your application is critical to ensuring that systems achieve optimal performance, meet code, and function as designed. If you have any questions about how to select a product for your application or have concerns about meeting codes or specifications, please don’t hesitate to contact our technical support team: 1-800-866-3234.
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