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Uninsulated flex duct: Does it lose its sound-dampening qualities?

whitenack | Posted in Green Products and Materials on

I’m working on my HVAC material list. All of my branch lines are short and straight. I see where people suggest flex duct for these applications (if installed correctly) for it’s sound dampening benefits. However, I am running the ducts inside soffits, and I want the ducts as small as possible so the soffits don’t get too big. Since I am inside the thremal envelope, I don’t need insulation. Insulated flex duct adds 2-4″ to the overall duct size. I see there is un-insulated flex duct available, but I wonder if the insulation is what gives the sound benefits and whether that defeats the purpose of using flex.

The next question is whether I need to worry about sound dampening if I am using ducted mini splits?

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  1. GBA Editor
    Martin Holladay | | #1

    The brief answer to your question is that uninsulated ducts are not as good at sound dampening as insulated ducts.

    A more thorough answer would point out that you may be using the wrong type of duct for a ducted minisplit system. Almost all types of flex duct have corrugations that interfere with smooth air flow, so smooth duct (for example, galvanized duct) is preferred when air flow and static pressure matter. Ducted minisplits are low-static-pressure systems, which mean that you need to pay attention to static pressure when designing the duct system. That usually means that you need oversized ducts (compared to conventional forced-air systems usually installed in North America).

  2. whitenack | | #2

    Thanks Martin. Yes, I am concerned about putting unnecessary friction on system that may not have much to spare. Thanks for the extra warning.

    When people use flex to cut down on the sound, what sound are they trying to dampen? The sound of the air blowing through the ducts or the sound of the air handler?

  3. GBA Editor
    Martin Holladay | | #3

    I know that HRV installers like to include at least 4 short sections of insulated flex duct to connect the HRV to the rigid supply and exhaust ducts; this lessens the chance that vibrations and sounds will be conducted from the appliance to the rigid duct systems.

    Most forced-air heating and cooling systems that have ducts located within the home's thermal envelope don't use insulated ducts, and most homeowners accept the level of noise associated with these systems. It's possible, however, that insulated ducts might cut down slightly on noise transmission.

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