How to Properly Vent a Bathroom Exhaust Fan in an Attic

Bathroom exhaust fans perform an important function by removing excess moisture from your home. When venting a bathroom exhaust fan, make sure to vent the air to the outside, rather than into your attic where it can cause mold and mildew to form.

Options for venting a bathroom exhaust fan include (best to worst):

  1. Through the roof or an exterior gable wall.
  2. Behind a gable vent.
  3. Behind a soffit vent.

To improve venting and reduce air resistance in the vent pipe:

  • Keep the run of vent pipe as short and straight as possible.
  • When possible use smooth metal pipe, rather than flexible ductwork.

Watch this video to find out more.

Further Information

Danny Lipford: Jim asks, “Is it OK to have a bath exhaust fan vent into my attic?”

No, it’s never a good idea to have any exhaust fan in your home dump all that hot, moist air in your attic. It can cause you a lot of problems with mold and mildew forming on the underside of your rafters and decking as well as getting into your insulation.

Several different ways you can move that hot air to the outside. One is to route it over to a soffit vent and attach it to the back of the soffit vent, but this can work against your exhaust fan because that’s actually an air intake, so not as good as other methods.

Another method, a little bit better, is if you have a gable-style roof and a gable vent on one end of the house, you can attach this to the back of a gable vent.

But the absolute best way to get that hot, moist air out of your house is to route it straight up through the roof and out a roof cap on it. Even better, instead of using this flexible type pipe, use a smooth metal pipe to route it through the roof.




  3. My bathroom exhaust fan drips water down on the toilet and floor and also the ceiling is wet around it. It vents into the attic. How can I fix this?

  4. My home is like Betty’s. The master bathroom and the 2nd bathroom vent through the roof and through the same opening. The 2nd bathroom vent drips on the floor and is ruining the ceiling. The 2 pipes, one a 4″ (master) and the other a 3″ (2nd) do not go through the ceiling but into a common box that goes through the roof. The house is too high to see if that box has any kind of fitting on the roof but it doesn’t look like it from the attic. None of the pipes are insulated and neither is the box they go into. The master bath pipe is about 30′ long and the 2nd bath pipe is about 10′ long. Thank you for this forum. Pat Thompson

  5. I don’t like the idea of putting holes in the roof so I routed mine through the exterior wall like a dryer vent

  6. Are we sure it’s okay to put it behind the gable vent? Mine was vented directly into the attic, so I ran 4″ duct and butted it up against the gable vent. I did this based on your article and for some reason it has me a bit worried. Can you show some pictures of this being done properly? Thanks

  7. I want to go with option 2, exhausting through my existing gable vent, but I cannot find a fitting that will easily mount behind the vent. I think I’m going to have to improvise with a wall vent. Don’t they make a simple fitting for this purpose?

  8. Can I tie it into my gas water heater vent. That already passes through the roof and I’m thinking it will save another hole in roof. I was thinking of an in-line backflow preventer just to keep any carbon monoxide from back feeding if that would happen.

  9. I have my fan vented out through the roof, but near the inside of the roof in my attic the vent leaks a bit and condensation gets on the wood and a bit down in the insulation around where the vent goes out. I can feel some water around the metal wrapping of the vent where there is an opening. Would anyone recommend wrapping a towel around this area to absorb the condensation and then switch the towel out monthly or so? Thanks!

  10. Feedback on several of the questions:
    1. No, you cannot tie the bathroom exhaust to the hot water vent. That is a recipe for disaster and may very well not be to code. Don’t do it.
    2. Two bathroom exhausts should not be tied to one vent. They should vent separately. They will never vent properly otherwise.
    3. The questions relating to water leaks and/ or condensation problems…. 30′ is way too long of a run. It should be under 10′ if at all possible and have a straight route . Each bend in the duct run reduces the effective distance max. You should call the manufacturer to find out their recommended vent distance. If there isn’t a shorter route, you will need to add an in line booster fan. The long run is likely the cause of the water damage. The high humid air doesn’t vent out of the house before it condensates in the duct and then runs back into the house causing damage. Insulating the duct may also help.
    4. No, you shouldn’t use a towel or other absorbing material. You may have a bad boot around the roof vent that is leaking into the attic. Or you may have a distance issue. See comment 3 above.

  11. I have a bathroom exhaust fan that is vented directly into the attic. It has started to leak water back through the exhaust fan into the bathroom. What is the best thing to do to correct this problem. If you take it through the roof, using a metal pipe, do you use insulation around the pipe?

  12. I have installed an electric extract fan in an inside bathroom however the council inspector has told me I require an air intake due to lack of air flow through hallway if doors are closed My options appear to be
    A Change the existing ceiling fan
    B Provide a separate vent to the vented ridge tiles
    C Provide a separate vent to the attic space

    Please advise


  13. We recently bought a 50 year old house so it’s a “project house”.
    There’s a full bathroom in the basement and the exhaust fan / duct goes out into the unfinished storage room behind it. That’s a no-no and the previous owners either were too lazy or clueless to realize that.

    Since it would be uneconomical and too long to run an exhaust duct route from the basement through 2 stories and roof, so how do you feel about us running the exhaust duct horizontally by 10 feet along the basement ceiling to CMU block wall?

    It appears to be the only solution, but I wanted to see what your thoughts are.

    thank you,

    • Hi, Scott!

      We think if you use the proper-sized pipe as recommended by the manufacturer of the exhaust fan, it should work great.

      Good luck!

  14. I see many questions about leaking bath exhaust vents. I recommend replacing the vent pipe with insulated vent pipe. Next, make sure it is securely fastened on both ends of the pipe. Next, make sure you have one of the new roof exhaust vents on the roof and properly sealed. Most bath vents for homes are 3″. They will have to be transitioned from 3″ to 4″ exhaust pipe. The hard part is no hardware or home improvement store carries an offset 4″ to 3″ reducer and there is usually no clearance between the exhaust vent exit and the ceiling sheet rock. Use a short piece of 3″ pipe to get away from the exhaust fan and an elbow. There you can place the 4″ transition peice and use the insulated flex hose to get to the roof outlet.

  15. I think the contractor didn’t install a piece. We can not only FEEL the hot air from the attic in our bathroom but SMELL that attic-y smell in our bathroom. Isn’t there something in the fan that is supposed to stop backflow of air?

    • Hi, Stephen! No exhaust fan is “draft proof,” but there is a way to minimize the air that enters your bathroom. You might check to see if the fan has a damper flap; this piece is lightweight but can help prevent a draft from entering your bathroom. Good luck!

  16. During heavy rain, water comes out of the exhaust fan in my first floor bathroom. We have a story and a half Cape Cod style house. How do I fix this? Thank you.

  17. I’s staying in an 1895-built large house.

    I’ve just discovered that a bathroom exhaust fan is venting directly into the building’s small attic.

    That bathroom has a large, easily opened window.

    Would it be better for the building if I stopped using the exhaust fan, and just opened the window when necessary?

    Thanks! 🙂

    • Hi, GC!

      A bathroom exhaust fan removes moist air from the bathroom. The fan does this quickly and safely (within 20 minutes of turning it on). You could achieve the same result by opening a window, but the process could take much longer. And an unattended window could invite hazards into your home.
      Good luck!

  18. My 2 story house was built 17 years ago and we have had problems with our exhaust vent over our shower. I finally took it down to replace it and discovered there is no vent pipe. In fact, there is a floor above it and insulation around 3 sides. I just can’t believe they didn’t vent this properly and I don’t see how I can fix it. The one side has about 6 inches tall, about 12 inches wide and about 6 feet long until I see more insulation. What should I do?

    • Hi, JR,
      Danny says, “If you cannot access it from the attic space, the only way to provide the necessary venting is to actually cut a hole in the roof to gain access — certainly an extreme measure, but the only way I would know to be able to accomplish that.
      Good luck with your project!”

  19. So I’ve installed metal duct from my bathroom exhaust fan to the gooseneck vent on my roof. We have a train that comes close to our house and when the fan isn’t running and the train blows it’s whistle the sound reverberates down the pipe into the house. Do you have any solutions for dampening noise that comes through the pipe into our home.

    • Hi, Cyler,
      Danny says, “This is a unique problem but you should be able to deaden the sound by wrapping duct insulation around your exhaust pipe. Good luck, and thanks for your question!”

  20. I am remodeling a basement bathroom. The exhaust fan is vented into the space between floors. My dryer vent runs right over the bathroom. Can I just attach a vent for the exhaust fan to that and have them both vent to the outside via the same hole?

  21. We have a house built in 1925. The bathroom was an add on and they did not build the roof up to code nor is it attached to the attic. Our problem is that we want to but in an air vent to get rid of the mold problems we have due to moisture. We do not want to put in a wall vent because the wall to the outside is in the shower. There is only about 2 inches between the roof and bathroom ceiling. What can we do?

    • Hi, Regent,
      We don’t recommend this. Building codes probit the practice, and there are electrical safety and sanitation concerns. Thanks for your question.

  22. Why do you say that the air is hot that is being vented into the attic? It is coming from inside the house.
    Granted, the bathroom exhaust air could be moist. Every attic I have ever been in was dry as a bone. If anything, they could use some moisture. While the bathroom exhaust air volume is miniscule compared to that of a ventilated attic, if anything wouldn’t a little moisture be helpful in terms of the condition of the wood?
    For decades bathroom exhaust fans were just vented into attics. Is there any documentation of the negative effects of that practice?

    • Hi, Wes,
      Venting into the attic sends excess moisture to that space, which can cause mold and other problems.
      We’ve also seen homeowners have to replace all their roof decking because it became rotted from the excess moisture.
      And many building codes require discharging exhaust to the building’s exterior, so it’s just a good rule of thumb.
      Take care,


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