I need to replace my bathroom ventilation fan. I am in 2nd floor condominium. My bathroom dimensions are 60 square feet and ceiling height is 8'2".

As per formula I found for CFM (height × length × width × 0.13) ,I need fan with around 62.4 CFM.

I can only find 50 and 80 CFM fans, can't find anything between. Should I go with 80 CFM fan ? There is around 10' to 12' of duct work before it opens outside on the wall.

I have attached picture of the flexible white duct which is connected to solid metal duct as you can see in the far end of the picture.

Can I replace flexible duct with solid duct to get more smooth air flow ?

Do I need to buy insulated duct ?

My building is around 34 years old. Not sure if there are specific code requirements for my case.

bath vent duct

  • Keep in mind that more CFM means you don't have to listen to it for as long after the shower.
    – dandavis
    Commented Apr 24, 2021 at 21:04

1 Answer 1


You can certainly go with an 80 cfm unit, the only drawback being additional noise (in general the more powerful the unit the louder they are). This can be remedied by using a high-quality unit designed to be quieter.

You certainly can (and should IMO) use regular duct pipe in lieu of flex, wherever you can.

I don't think you need insulated duct for this. I live and work in a temperate region and am unfamiliar with considerations for vents in unconditioned spaces that could drop below freezing, but the fact that the existing vent is not insulated is a hint that it should be fine.

  • Agreed almost completely, Jimmy! 50 or 60 CFM is the absolute minimum. I do disagree about one thing: Higher capacity, well designed fans can actually be quieter than cheapies. + Commented Apr 24, 2021 at 18:13
  • the companies know we want them quiet, so they make the cheaper fans louder on purpose to position upsells.
    – dandavis
    Commented Apr 24, 2021 at 18:38
  • @dandavis, while I believe in designed failure in that cheap components are bound to fail, when it comes to extraction fans it definitely costs more money to build larger units that are quiet (more expensive rotating elements e.g. motor+fan, sound-deadening insulation, etc.) I don't believe they intentionally make cheap fans louder; nobody realizes how loud it is until it's installed and nobody un-installs them immediately after installation due to noise. In this case you get what you pay for. Commented Apr 24, 2021 at 20:37
  • no doubt better motors cost more, but not that much more, especially when you buy a million at once. It's like a lincoln vs ford; a little more cost, a lot higher price. I took the grill off a cheap fan in an old apt i had, and it got WAY quieter, and cleared faster, so I left if off until I moved. I've done some research in fan housing design. Updating their plastic grill mold to avoid sharp edges (turbulence) and expand openings (decreasing cavitation or flow mismatch) wouldn't result in a more expensive grill; but it would make it harder to charge double for a slightly more robust BOM.
    – dandavis
    Commented Apr 24, 2021 at 20:58

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