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Community and Q&A

Flex duct “elbow” vs. smooth elbow

kevinjm4 | Posted in Energy Efficiency and Durability on

(Not talking about insulated flex duct) When the term flex duct is used regarding bathroom fan ducting, exactly what is it we are talking about? foil flexible, or aluminum semi-rigid, or something else…

Also, I was wondering if this is a good idea or not. As i understand it, there is more resistance in a smooth elbow vs. a flex duct all things being equal. If there is plenty of room to have a long swooping 2-3′ flex-duct “elbow”, why not implement this instead of a smooth 90?

This question is specifically related to my situation, where my fan’s outlet points at the soffit, and is in the rafter bay. So I need to route the ducting up and over the ceiling joist while turning it 90 degrees aiming it then at the gable.

Am I better off just using flex-duct for the first 3′? (Have flex at the fan outlet –  then swooping elbow up and over joist – then straight to the gable 16′ away using smooth switching to smooth at that point) Lastly, if this is a good idea, is it easy to connect smooth to flex? Do couplers exist for this application, or just tape, crimper?, etc.?

any help appreciated. prefer not to use mastic as this is a temporary bathroom, and tape seems easier to cut through and re-assemble the system elsewhere.


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  1. Expert Member
    BILL WICHERS | | #1

    “Flex duct” is a pretty generic term that can include everything from the plastic dryer vent stuff up to fancy insulated stuff. For a bathroom fan, you probably want some insulation on the duct but you can add insulation to a non-insulated duct with a sleeve or wrap (I prefer the sleeves).

    Corrugated metal flex duct is the most durable type of flexible duct. Any flex duct will have more resistance to airflow compared to a rigid duct of the same cross sectional area due to the uneven nature of the flex duct’s internal surface. I see what you’re saying about a big sweep of flex duct maybe being better for flow compared to a rigid duct 90, but my instinct is that the rigid 90 might actually be better (I’ve never measured this).

    Since your duct run is fairly long, I’d try to make it all from rigid duct. It is possible to connect flex duct to rigid duct (this is a common application), but I’d just avoid the flex duct unless you really need it to make an oddball bend. If you want to reduce backpressure, two 45s will cause less resistance than one 90.


    1. kevinjm4 | | #2


      I appreciate your feedback.

      you said my run is pretty long, and I agree. Both my bathrooms are in the center of the house and have runs of about 16-18’ to the gable.

      After looking at some of the charts I found for determining duct/fan size, the best I could come up with was I am inbetween sizes - whether it’s 4” or 6” and whether it’s 80cfm, or 110cfm.

      What would be recommended for my runs? I’d prefer GBA opinion over manufacturers recommended specs, and over my own research which I couldn’t quite definitively conclude much.

      I should add that I will have a Backdraft damper towards the end, and also in the wall cap. So two dampers and one or two elbows...

      Any help greatly appreciated.

  2. Expert Member
    BILL WICHERS | | #3

    You have to round up with duct size. If you use a too-small duct, you’ll have excessive back pressure which will reduce the actual amount of CFM that the fan will blow. It is possible to oversize the fan, use a too-small duct, and still get the airflow you require, but in that case you’d be essentially using fan power to overcome duct resistance which is inefficient and not recommended.


    1. Expert Member
      Akos | | #4


      With duct work the length doesn't matter as much as fittings and bends (for example an adjustable 90 deg bend is equivalent to 30' of length).

      If you want to use flex for the bend, you can do so if you can keep the radius of the bend to about 5x diameter (ie 20" for a 4" duct). This would not add a lot of extra restriction. Also important to fully stretch the flex and slope it to the exhaust so that condensation won't collect in a sagging section.

      If you don't want flex, the simplest is semi rigid aluminum pipe (dryer exhaust type). HVAC supply houses have these in long lengths (I would avoid joints if possible) and you can pull an insulation sock over it. This would be less losses then a flex duct and maybe 15% more restrictive then rigid.

    2. kevinjm4 | | #5

      It sounds like 6” is almost always preferred. At least preferred in my case. It seems it is simply safer than 4” no matter what - Unless you have a simple 4’ run or something like that... you mentioned too large a fan, but conversely can your ducts be too large - could that have adverse effects?

  3. Expert Member
    Peter Engle | | #6

    Too large ducts won't move air fast enough. For bathroom air, this means it will have more time to cool in the duct, and more condensation of the moisture carried in the air. Adding insulation helps, but it won't eliminate the problem. Attic-mounted ducts can also start out very cold because they are soaking in the cold attic. Short-duration runs of a bathroom fan with high humidity would be the worst case, as the air might never have time to even warm up the duct interiors.

    1. charlie_sullivan | | #7

      In that case, making it slope downward towards the gable so condensation can drip out the termination at the gable is extra important.

  4. charlie_sullivan | | #8

    It is possible to get longer-sweep hard duct elbows. You can also use two regular ones, set up for 45 degrees each, to get a longer sweep 90 degree elbow.

  5. JC72 | | #9

    Wouldn't duct typically used for an ERV/HRV be a good alternative? IIRC Zehnder has flex duct with a smooth interior.

  6. PAUL KUENN | | #10

    Does anyone know if you can use 4" PVC schedule 40? Fire code issue? We had a long run (14') climate zone 5 and used 2 large radius 45 elbows to take it above ceiling joists enough to get good 1/4" pitch per 4' run towards exit. We added butterfly spring backdraft damper at the fan - fit with PVC perfectly. Buried all under attic insulation to keep it near room temp. That was 20 years ago and I've never had an issue. You do see icicles growing at the exit so the moisture is getting out that far and dripping in the right direction.

    Kevin, are you in a cold climate? That is a long run. Last year we emptied 5 gallons of condensate out of aluminum flex tubing that had dips in the 15' run. It was about 20 years old.

    1. GBA Editor
      Martin Holladay | | #11

      PVC pipe -- either thin-wall pipe or Schedule 40 -- is fine for exhaust fan ducts (but not for fresh air ducts). I mentioned the use of PVC for exhaust ducts in my 2014 article, "Bathroom Exhaust Fans."

  7. kevinjm4 | | #12

    I’m still hung up on something... here’s where I’m at so far:

    Manufacturers don’t tell you a whole lot about how to select the appropriate fan size, they just ask how much sq. Ft is your bathroom then select based on that - but what if I told them I have 1” ducts... for some reason that’s irrelevant to some manufacturers.

    My understanding is as follows:
    You want to find sq. ft. of bathroom and if it’s under 100 sq. ft. then that’s the amount of cfm you need.

    But then you have to calculate your actual duct length based on duct length, and fittings, duct type etc..

    Then from that somehow you calculate your static pressure in w.g. And that’s where I’m lost.

    I understand how to find the actual duct length, just don’t know where to plug that into an equation for SP in w.g. If that’s even what needs to be done... And I’ve read elsewhere you want 8ACH, and not sure how that plays in here either.

    Any help appreciated.
    Thanks again.

    1. Expert Member
      MALCOLM TAYLOR | | #14


      All the exhaust fan manufacturers I've used specify the minimum size duct and maximum length of duct run for each model. For example:

  8. Expert Member
    BILL WICHERS | | #13

    The “you need x CFM of fan per Y square feet of room” is the rule of thumb for fan sizing. Code pretty much just gives a minimum (50 CFM for bathrooms if I remember correctly).

    You generally size the duct based on required flow rate and allowable static pressure level using a table or one of the nifty calculator gizmos (see link). Fittings like 90s and 45s are given as equivalent feet of straight duct. Basically if a 90 is worth 15 feet of 4” pipe, and you have two 90s and a 16’ run (the total of all the straight parts) to connect your fan to the vent, you add it all together and get 30’ equivalent (for both 90s together) + 16’ (straight parts) for a total of 46 feet. You then check your chart to make sure you can run the CFM you need at an allowable static pressure in the size duct you want to use for that 46 feet.

    More info is available here:


    1. kevinjm4 | | #15

      Bill thanks again for the info, and I think I’ve settled on having an 80cfm fan with 6” ducts. But a lot of the 80cfm fans come with 4” duct outlets, not 6”. Some come with an adapter and there are a few 6” out there. The particular one I want has a 4” duct outlet.

      My question is how will putting a 4-6” increaser right at the beginning affect the actual duct length/performance/etc.. Does that add 15’ like an elbow would? Or do something else to the system.

      My major concern is that the fan would act as though it was pushing through 4” instead of 6” and work harder than it needs to...

      Thanks agin.

      1. Expert Member
        BILL WICHERS | | #16

        The reducer, acting as an “increaser” here, won’t present any back pressure to the system. A small amount of smaller duct does not restrict flow the way a longer run would.


        1. kevinjm4 | | #17

          Would there ever be a need for two backdraft dampers, one at the wall termination, and one a few feet from the wall - inline. I’ve seen this done a few times but don’t understand the benefit.

          Is one preferred to the other if only one is needed?

          Thank you.

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