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Community and Q&A

Flex Duct Material for Ventilation System Sound Control

Trevor_Lambert | Posted in Mechanicals on

It was recommended to me by a couple of independent sources to include a short run of flex duct between my HRV and the supply and return manifolds, for noise control purposes. The idea is to decouple the unit from the ducting as a means of reducing vibration transmission. I’ve looked at several options:
1. Vinyl dryer duct. Very flimsy, has a pretty nasty smell to it, probably not a good choice from an air quality standpoint.
2. Foil dryer duct. Also very flimsy.
3. Semi-rigid aluminum dryer duct. Seems too rigid to actually achieve the goal of decoupling.
4. Clear PVC hose typically used for dust collection systems. I have this in my wood shop, and it seems fairly inert, but obviously still maybe a concern being PVC. Very durable and smooth walled.
5. Polyurethane dust collection hose. Similar to the PVC stuff, but maybe polyurethane is better in terms of potential off gassing?

Opinions? I’m currently debating between the foil duct and the polyurethane hose.

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  1. Expert Member
    Michael Maines | | #1

    Trevor, it's a good idea to add sound control, and the best solution is to use a sound dampening device from your HRV company, but not all companies supply them. I would just use insulated HVAC ducting with a poly liner, such as this one sold as sound control ducting:

  2. Trevor_Lambert | | #2

    Thanks, I will check that out. The Zehnder sales guy is actually one of the ones who suggested just using a short length of flex duct, rather than the rather expensive muffler they sell. He said it usually isn't necessary.

  3. Trevor_Lambert | | #3

    Having a hard time finding Thermaflex locally. Will this do more or less the same job?

  4. Expert Member
  5. Trevor_Lambert | | #5

    It seems pretty much a certainty that if Home Depot in the US has something, it will NOT be available at Home Depot in Canada. That has been my experience. It's kind of weird, because Thermaflex does have a Canadian distributor. Maybe I'll give them a call and see if they can direct me. The Thermaflex is the only one with "acoustic rating", however that may be mostly snake oil.

  6. Expert Member
    Michael Maines | | #6

    Trevor, that's interesting about Canadian vs. US distribution.

    I would not worry about the extra sound rating; any flexible duct will dampen vibrations. Here's a good, short overview of why you might want sound control: Honestly I either spec a Zehnder unit and their Comfotube, or I leave the duct design to others, so I can't claim to be an expert on this. Maybe someone with more first-hand experience installing HRVs will weigh in.

  7. Trevor_Lambert | | #7

    Digging this up again, because I did a temporary install of the recommended insulated duct and my wife complained about how it looked. I can't blame her, it is pretty hideous. Our utility room is also a throughway; it it was in a basement it wouldn't matter. Anyway, I'm back to wondering if options 4 and 5 raise any offgassing / air quality concerns. My other option is to just remove the insulating jacket from the current flex duct. This is on the interior supply/return ducts, so it doesn't need insulating but I figured it would help dampen sound.

  8. GBA Editor
    Martin Holladay | | #8

    User-6792900 -- or maybe your name is Trevor? Not sure.

    I wouldn't use duct-collection hoses if I were you. Use insulated flex duct.

    Perhaps the installation was sloppy enough to irritate your wife because of workmanship issues. Maybe all you need is a neat installation: Square cuts; the use of clamps and high-quality tape; and the use of duct hangers to prevent sagging.

  9. Trevor_Lambert | | #9

    Yes, I am Trevor. I'm using a new computer, but I'm logged in to my account so I don't know why it's not showing my name. The issue she had was the material itself; the irregular bulkiness and the white on black colour scheme. I will try just removing the insulating sleeve and see how that looks.

  10. Expert Member
    Akos | | #10


    I had similar concerns about plastic ducts, ventilation and noise. For my setup I ended up using insulated flex duct section on the return lines ( main trunk is steel). Each section is about 4' and there is a jog in them to prevent direct line of sight path (bad for flow, just oversize the duct). You can scream into one of the vents and barely hear anything on the other one.

    On the supply lines I ended up going with semi rigid aluminum. For the sections that needed insulation between the ERV and outside, I just pulled the jacket off a flex duct and over the semi-rigid aluminum. I have an inline filter on the outlet of the ERV which also helps.

    I use the bedrooms and bathrooms as the return source, supply goes into the living space. The aluminum piping on the supply side is louder, my noise concerns where mostly on the return side which are whisper quite.

    There is still a bit of air flow noise when the unit is running, significantly less than the sound from a ductless wall mount unit.

    Good Luck.

  11. Yupster | | #11

    I would recommend doing it in a nice round sheet metal duct with a flexible duct connector somewhere in-between. It will look neat and have some sexy curves at your corners. An HVAC guy should be able to supply you some.

  12. Gopher_Baroque | | #12

    Note the original post was concerned with vibration control (to control sound radiated by duct walls or other equipment shaken by the HRV) but responses got off on a tangent of sound propagated through the duct system. For vibration control, the lightest and most limp material would be best to minimize transmission and to provide stiff support at the "far" end of the flex hose to minimize reception into the ducting. Duct mufflers, including the Thermoflex hose, will be most effective at the other end of the duct (just ahead of any outlets) by attenuating all sources (HRV and any flow turbulence generated at bends, transitions, etc).

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