The Science of Insulating Ducts

Thursday, February 23, 2017 2:22 pm MST



Brennan Hall, HVAC Product Line Manager, Johns Manville HVAC

Properly specifying insulation for duct systems is not a one-size-fits-all endeavor. In order to effectively optimize system performance, ensure indoor air quality (IAQ), and meet code requirements, there are a number of critical variables specifiers should consider prior to selecting an insulating material. These include things like coatings, facings, installation methods, duct system components, code requirements, and the specific performance needs of the application itself.

By understanding the unique conditions and environmental influences of the application, engineers can begin to narrow down which materials will optimize system performance. In the first part of this two-part blog series, we’ll look at how insulation performance and material characteristics contribute to designing robust HVAC insulation systems. Below are four critical factors that will shape the design of an optimized HVAC insulation system

1. The type of duct: Naturally, before you specify an insulation, you need to know whether you will be insulating spiral, rectangular, or flexible ducts. While both types of ducts can be lined or wrapped with insulation, there are distinct differences in the insulation installation methodologies for rectangular and spiral ducts, and as such, they require different insulating options.

Generally, rectangular ducts can be one of the following:  lined with duct liner, wrapped with duct wrap, wrapped with board insulation, or fabricated from the insulation itself (for example SuperDuct® RC or Micro-Aire® Duct Board).

In contrast, the circular shape of a spiral duct, poses different design challenges that the system engineer has to address in the insulation selection. This primarily comes down to determining whether the assembly will use a double-wall configuration, a spiral duct liner, or a duct wrap. Each of these assemblies offers different benefits for insulating spiral ducts. While duct wrap is fairly straight forward in terms of installation and benefits, double wall configurations and spiral duct liners are the only two options for spiral ducts that offer sound-attenuating performance.

Double-wall configurations (which can be used in both spiral and rectangular ducts)wrap the insulation around a metal inner core that fits inside the metal outer shell, isolating or semi-isolating the insulation from the airflow. In some circumstances, this configuration can be time-intensive, and it may require re-work if the insulation snags when the inner duct is being inserted into the outer duct. Additionally, it increases the weight of the ducts themselves.

Spiral duct liner, like Spiracoustic Plus®, is a pre-kerfed duct board that can be rolled into a circular shape to fit the duct. Spiracoustic Plus installations can be very rapid, and the end result weighs less than a standard double-wall assembly; however installing the insulation in fittings can become challenging if not done properly. Click here to learn how to install Spiracoustic into four different types of spiral duct fittings.

2.  Thermal and Acoustical Requirements: When specifying the insulation, the system designer should determine whether the insulation system needs to address thermal needs, acoustical needs, or both. This can help a designer narrow down whether using a duct liner, duct wrap, or external duct insulation would be best for the application. The thermal needs, articulated as R-values, are frequently specified by code requirements, like R-6, R-8 or R-12 as required by the 2015 International Energy Conservation Code (IECC). The higher the R-value, the better the insulating properties of the material.

If the system needs to address thermal requirements only, then a duct liner, duct board, or duct wrap can each perform sufficiently.  Acoustical performance, however, is highly influenced by the type of insulation material utilized. Fibrous insulations, like fiber glass or mineral wool, have better Noise Reduction Coefficients (NRC) than non-fibrous insulations, such as elastomeric foam. Additionally, the system designer should consider whether the noise is coming from inside or outside of the duct. If the intent is to attenuate noise originating from inside of the duct (e.g., fan noise or air-flow friction), then either duct liner or duct board will address the acoustical requirements. If internal noiseandexternal noise (e.g., cross-talk) need to be addressed, then using a duct board that is fabricated to function as the duct itself, like Micro-Aire Duct Board or SuperDuct RC, can effectively meet the acoustical requirements.

3.  Coatings: Coated airstream surfaces offer a number of benefits (you can read about them in detail this blog) that include improved durability, superior water repellency over traditional glass mats, and, depending on the coating, antimicrobial features that can help to resist mold and fungi growth. They can also make it easier for the ducts to be cleaned after installation.

Additionally, the coated surface helps prevent stray fibers from potentially entering the airstream. While respirable fiber glass has been repeatedly proven to pose no respiratory hazards to building occupants, this coating offers further durability to the insulation itself. If you have any questions about the safety of fiber glass in air handling applications, you can view Fiber Glass Health and Safety, a webinar on our Exclusive Content page that details the scientific research supporting the safety of fiber glass for air handling applications.

Using an insulation with an antimicrobial coating, like Johns Manville’s Permacote®, can help to prevent mold or fungi growth from forming on the surface of the insulation should dust or debris happen to enter the system. In hot, dry climates, mold avoidance may not be a top priority; however in humid climates, this can be a critical feature to assist in extending the life of an insulation system and pursuing a healthy IAQ.

4.  Energy Efficiency: Once we have established the thermal and acoustical requirements, we’ll need to look at other preferences.   These include things like energy efficiency, IAQ considerations, and LEED certification requirements. As we observe a shift toward greener building design, insulation is playing a crucial role in improving energy-efficiency in buildings. The drive for “energy efficient living” has caused a number of evolutions, both politically and commercially, that are advancing the energy efficiency requirements throughout the U.S. and have made it economically prudent (and sometimes mandatory) to strive for more efficient structures.

In some cases, manufacturing reports, like Environmental Product Declarations (EPD), are required in order to attain certain energy efficient building accreditations (namely LEED certifications). EPDs are highly detailed reports that calculate the environmental impact of the entire lifecycle of a product, incorporating the impact of the raw material acquisition, the energy savings throughout the lifetime of the product, and the impact of the disposal of the material.

As we consider the evolution toward more environmentally conscious living, energy efficiency performance is proving to be only a fraction of the picture when it comes to material selection. We’re seeing a notable shift toward understanding the full impact of the product lifecycle. In this regard, engineers may need to intricately understand how the performance characteristics of the insulation itself, as well as its manufacturing process, translate into potential energy-saving benefits.

Each of the performance variables we’ve highlighted pertains specifically to the selection of the insulation itself, but the duct system as a whole also has a significant influence on HVAC insulation system design. As such, in Part 2 of this blog series, we’ll look at the key components of the duct system that influence insulation system design and how engineers can account for those components to improve their specifications for optimized systems.  

We will also be hosting a live webinar, The Science of Insulating Ducts, on March 28th that will discuss the intricacies of selecting and specifying duct insulations. Click here to register for the webinar.

If you have any questions regarding your insulating materials or would like input on your HVAC insulation specifications, please don’t hesitate to contact our technical support team at 1-800-654-3103.


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