I'm planning on installing a Bathroom exhaust fan (Just a note, the port is 4") and am curious as to what kind of duct should be used.

I've heard that the duct should be insulated to prevent water backflow from condensation during the colder months as my attic is not environmentally controlled. The New York winters can also be rather cold sometimes, so this is especially a concern. I heard that code requires a rigid tube similar to aluminium.

What kind of duct is typically used for these installations? (Home Depot recommendations would be most appreciated because that's where I plan on purchasing the other components)

  • I have never found flex ducting for bath vents not to be allowed, I am on the other side of the US though.
    – Ed Beal
    Dec 16, 2018 at 19:34

3 Answers 3


I used insulated flex ducting like below for my bath vents. Try to keep it flat or sloping toward the outside to avoid condensation pockets.


example product

Edit 1: I'm not familiar with a code that requires rigid ducting for bathroom fans. (Mass local here) Unlike dryer ductwork, it's not going to be subjected to high heat or a potential for significant lint accumulation. That said, there is nothing stopping you from using regular rigid metal ducting. (rigid plastic ducting eg: pipe, might even be acceptable)

There is also available pipewrap insulation, I'm looking to use some on my dryer ductwork, if I ever get the energy to go back into my attic.


8 in. Dia R-6 Ductwork Insulation Sleeve

  • Yes, I figured as much as well, but is that any violation of code? Dec 16, 2018 at 19:15

Most places don't require rigid ductwork. it also appears that 2014 NY code doesn't either

603.6.1 Air Ducts

Air ducts shall be permitted to be rigid or flexible and shall be constructed of materials that are reinforced and sealed to satisfy the requirements for the use of the air duct system, such as the supply air system, the return or exhaust air system, and the variable volume/pressure air system.


Class 0 or Class 1 rigid or flexible air ducts tested in accordance with UL 181 and installed in conformance with the conditions of the listing.

As DaveM noted, insulated lines are your friend, especially in NY in the wintertime. But be sure to exhaust your line through the roof, if able. Exhausting through the soffits is an invitation for the warm, moist air to re-enter the roof through the attic vents and cause mold (this video illustrates the problem)

  • The shortest run is through the gable (Literally about 2 feet). I knew about the soffits and why they're not a sustainable venting location already from reading online, but thanks! Dec 17, 2018 at 9:17
  • The 2014 code quoted above appears to be for HVAC, not bathroom exhaust ducts. Jan 14, 2020 at 4:01

The Code requires a minimum of 50 cfm for toilet rooms WITHOUT baths or showers and a minimum of 80 cfm WITH a bath or shower. (See Section M1507)

Likewise, the Code requires a minimum of 4” flex or smooth duct for exhausting up to 50 cfm and a minimum of 4” metal smooth or 5” flex duct for exhausting 80 cfm and 4” smooth nor 5” flex is not allowed for 100 cfm. (See Table M1506.2)

There are other allowances for exhausting distances, up to 3 elbows, etc. too.

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