Bathroom Exhaust Fan

by / Wednesday, 17 June 2015 / Published in Healthy Building Inspections & Testing
Bathroom Exhaust Fan

How effective is your Bathroom Exhaust Fan?

For obvious reasons, bathrooms can be major sources of moisture in homes. And where there is moisture, there is the possibility of growing mold in the bathroom. A bathroom exhaust fan can be essential in removing excess moisture and keeping bathrooms ventilated, but how do we know if our exhaust fans are working properly? Just because we can hear them doesn’t necessarily mean that they are effective.

Ventilation is key to reduce Mold in the Bathroom

A simple way to test your fan’s suction is to take a single square of toilet paper and place it along the fan vent while the fan is running. If the fan holds up the toilet paper, then the fan is working properly. Keeping the bathroom door open while running the fan will allow for air exchange from other parts of the home. It is also important to make sure that the exhaust fan is vented outdoors.

We recommend using bathroom exhaust fans for 15 minutes after using the shower to help move humid air out of the living space. 15 minutes may be longer or shorter than required to keep your bathroom dry, depending on the bathroom size, building materials, windows, etc. It may also be a good idea to install a timer switch in bathrooms which will automatically shut off the exhaust fan after a pre-set amount of time.

Dry, well ventilated bathrooms are important in preventing mold growth in bathrooms.

Controlling moisture with the help of an adequate exhaust fan is key. The following links provide more information on bathroom exhaust fans, and on general mold and moisture issues:

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42 Responses to “Bathroom Exhaust Fan”

  1. Christian says :

    I just installed a 14” exhaust fan wall type no windows in the bathroom , i joticed that when my bathroom door is open the vent behind the fan opens to suck air out of the bathroom but when i close the bathroom door the vents beb8nd the fan closes ? Why is this ty

  2. Joe Carrow says :

    Great post, But I have a question !!

    I use an exhaust fan in my bathroom. Each time that rains seriously Water seeps from the rooftop out of the supporter into our bathroom. Is the excess water in the admirer cause form? I just want to know this question ans. Anyway thanks in advance.

  3. Maya says :

    What if I sealed the vent and placed a dehumidifier in the bathroom? Condo bathroom exhaust system goes to the roof and they are not being helpful.

  4. Alex Stadtner says :

    It may be your fan is powered by a humidistat or timer. Or it could be a sign of an electrical wiring error. The latter is unlikely, but worth having checked out.

  5. Wendy Tomanek says :

    That fan in the guest bathroom keeps coming on several times of day by itself. Is this dangerous? The bathroom shower is seldom used.

  6. Alex Stadtner says :

    The purpose the fan is to remove damp and stinky air from the bathroom. If the fan won’t hold a piece of paper up it’s probably not working as intended. Perhaps the actual fan motor isn’t working? Perhaps there’s a damper that’s permanently stuck shut? Maybe the duct got bent or it was always oversized? Perhaps a bird took up residence in the exhaust flu? For whatever reason it sounds to me like your exhaust fan isn’t working. Depending on where you live this may be a health code violation. Try phoning your local health inspector. Best of luck.

  7. Mary says :

    Hi, i lived in the apartment For 3years now and realizes the exhaust fan never work in my bathroom so I tested it and see if the exhaust fan works in the bathroom but the tissue paper won’t hold in the fan, been seeing some mildew and molds concerns me because I have a son and I know it’s not healthy that way. I had maintenance guy checked and said the fan is working but nothing is coming out of to release the moisture or fog. Can u pls tell me if this something to concern about and my health. What should I do to have someone take a serious action with this because seems like the manager in this complex already knew it’s not working at all. Thank you in advance. I appreciate for your response.

  8. Alex Stadtner says :

    Loads of condensation and pealing paint are bad signs. There are two components of condensation : low temperature surfaces, high indoor humidity. The low temperature surfaces may be addressed by adding heat to the room before bathing, or by adding more insulation on exterior walls, etc. Humidity can be addressed through exhaust air – or dilution, I suppose, if you blew a bunch of non-humid air into the space. Does your exhaust fan pass the toilet-paper test? If so – you may need to add another exhaust fan or more heat to the space. Or – heaven forbid – consider a colder, shorter shower. =+(

  9. Peter Cornwell says :

    Hi. I have a 4″ extractor fan directly above the shower and I still get loads of condensation, peeling paint & fogged mirrors etc. I’m sure it’s either not working or not man enough. As well as leaving the window on the latch (though this is downstairs bathroom so bit concerned to do this) I’ve been advised to consider a fan that blows the moisture out rather than sucks – any advice on these or where I can find out about this?

  10. Alex Stadtner says :

    There is no guarantee that new fans will solve your high humidity and condensation problems in the bathrooms, but fans have improved in the last 25 years. We don’t recommend particular brands, but talk to your local contractors and supply centers to see what they recommend. Many of the newer models have variable speed fans, such as the Panasonic Whisper Series fans, and you can set them to run constantly at a lower speed and ramp up during bathing. Many models allow you to program them to run 15-20 minutes after a shower, too, and this can be really important. Consider installing a humidistat or fan that automatically runs on high when humidity hits 55-60%.

  11. Tracy Rowland says :

    I’m planning to replace 3 exhaust fans in my bathrooms soon. Despite running them, I do get black mold above the shower and the nearby walls . My paint is also chipping and I have faint water streaks down the walls that make the walls look discolored. I’m sure my fans are the original to my home that was built in 1991. Will I notice a big difference after I have them replaced? I really like having heaters with the fans and that is what I currently have. What features should I look for in shopping for my fan w/ heater and is there a particular brand that you prefer?

  12. Alex Stadtner says :

    Yes. Water in the fan or ceiling assembly can cause mold. Get a good roofer to inspect around the fan box. It’s probably a top-down leak or faulty exhaust design on the roof. Good luck!

  13. Happy Room says :

    Hello. I have an exaust fan in my bathroom, and every time it rains heavily Water drips from the roof out of the fan into my bathroom. Could the excess water in the fan cause mold? Thank you so much for your time.

  14. Alex Stadtner says :

    It depends on the level of humidity in the bathroom and the temperature of the mirror. Is there an AC vent blowing on the mirror? Does condensation appear on the windows, ceiling, or other areas within the bathroom? It’s good that the fan is working and moisture seems to dissipate if you leave the fan on after showering. A little condensation on the mirror isn’t an indication of a big problem. But if you’ve got drips falling from the ceiling or other indications of excessive moisture… it could be a symptom of either too much moisture indoors, and/or too little insulation here or there, and/or insufficient humid air exhaust.

  15. Deirdre says :

    Is a bathroom ceiling vent supposed to keep the bathroom absolutely fog-free during a shower? My vent which also has a separate heater blower passes the TP test, but the mirror is always fogged up right after I shower. I’ve taken the thing apart and cleaned out all the dust and lint, and it definitely has better power, but there’s still fog and condensation after a shower. If I leave the vent on, it clears out the condensation after a few minutes, but should there be any condensation at all if it’s on during the shower?

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    Good information.

  17. Alex Stadtner says :

    You could determine if the central system is working by trying the toilet paper trick… and examining the rooftop exhaust to ensure much more air is being exhausted at that central exhaust. But if you’re unsatisfied, and if you can, and all other things being equal, go ahead and add a new exhaust fan. Consulting an HVAC design-build contractor or mechanical engineer would answer your question with more authority, but generally adding a single exhaust fan in a multi-family building will not dramatically influence the central HVAC. There are too many unknown variables to give you a solid answer. I suggest you approach a local design-build firm or mechanical engineer. Good luck.

  18. Michael Rubin says :


    I have not been able to find the answer to my following situation/question/concern:
    I just bought a condo in an older 8 story building from 1967. There is an old fashioned looking vent return in the bathroom wall. It does hold up a piece of toilet paper, but I have learned from neighbors that they feel the central system is not effective at removing moisture from their bathrooms. They also stated that the system breaks down often.
    One of my neighbors, who is an architect, put one in his bathroom. He seemed very knowledgeable about the system, so my take away was that putting in a personal exhaust fan was safe for me and my neighbors.
    But then I read an article that seemed to indicate that balanced pull from centralized system is crucial. That got me thinking that if I installed a personal exhaust fan (to the main centralized stack) that I would be doing harm to the building and affect my neighbor’s quality of ventilation. I would then definitely not want to do it.
    Thoughts? Can I safely install a personal exhaust fan in my bathroom, that is hooked up to the central system, that will be effective for me and not affect my neighbors or the building?
    Thank you so much for your time. Michael

  19. Michael Rubin says :

    Hello, thank you for sharing your knowledge with others. I have not been able to find the answer to the following situation that I am having. I just bough in a 8 story condominium built in 1967 with a central exhaust system. I am in the process of total remodeling. I have lived in other old apartment buildings that had similar central exhaust systems and they were very inefficient. Now that I “own” my condo I would like to install an exhaust fan in my bathroom. My contractor says it is possible. He says he would close the current central return and cut a hole into the central stack and then hook the exhaust fan in my bathroom to that. A neighbor of mine did it in his. He said the central system is often down and not effective at removing moisture.

    I asked the building and they said, “no.” They said that the central system runs 24/7 and is 100% functionality. However, again I know this not to be true from neighbors. I am thinking of going ahead and doing it. However, I won’t do it if putting a personal exhaust fan in my bathroom would negatively impact the system and my neighbors air pull.

    Bottom line question: Would I negatively affect the central system and my neighbors if I install an exhaust fan in my bathroom that is linked to the central stack? Thank you, thank you!

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  22. Cameron says :

    As soon as I read about the toilet paper test I ran to my bathroom fan and tried it. Every part of the vent casing I covered held the TP fine. Yes! How exciting! Fan works great, but I already knew that. My bathroom fan works well enough that it can air out the entire basement apartment with the door open but I just wondered why I couldn’t feel much air flow with my hand..

  23. James Leber says :

    Thanks for tips. I considered installing a timer switch for my bathroom exhaust fan but put it off all the time. I think it’s time to start.

  24. Alex Stadtner says :

    Yes. Three things could be happening that result in your fan contributing to a water leak and moisture problem in your bathroom.

    1) Could be a leak around the seal of the exhaust of your fan. Could be a flashing or caulking issue that needs repairing. It rains and drips down around the fan.

    2) Fan may be literally sucking rain or damp air (100% RH) in. Not sure of the directionality of the fan, or it’s a heat or energy recovery ventilator (HRV / ERV). But it could suck rain in during a storm.

    3) Exhaust air from the bathroom may be reaching dew point in the exhaust duct, condensing, and then dripping back down into your bathroom.

    Good luck with this one. Good to stop the drip as soon as you can and address any water damage or mold growth according to industry standards.

  25. Nikita says :

    Hello. I have an exaust fan in my bathroom, and every time it rains heavily Water drips from the roof out of the fan into my bathroom. Could the excess water in the fan cause mold?

  26. Alex Stadtner says :

    Humm? An exhaust fan mystery!

    There may be several factors. Is the fan sized large enough to “feel” a difference? Placing a piece of toilet paper over the duct or fan is an easy test. Does it move the toilet paper or make it stick to the exhaust grate? If so, you’ve got some pressure drive. Is the duct leaking? Might check the continuity and condition of the entire duct run. Crimped anywhere? Tight or loose? Flex duct is supposed to be pulled rather tight to minimize turbulence and increase air flow through the duct. Lastly – there may be other pressure issues within the building that offset your fan. Other dryers, range-hood and bathroom exhaust fans, stack effect from temperature differentials high verse low within the building?

    Those are a few thoughts.

    Good luck – and let us know if you figure it out.

  27. Sajeda says :

    I live in a condo and we have a system where our exhaust fan is in our laundry room and it is suppose to exhaust and remove the lint from laundry room and exhaust the two washrooms as well. It is an old building and since it did not work well for us we installed a fan in our washroom which worked well but it was blowing lint out from and outlet in the laundry room but it served the washroom well. but as a result we were getting lint and mold in our walls outside the laundry room. we called in a duct specialist and he first insured the system was not clogged. he removed the fan and restored the old system back but now he gets reading in the box but when he closes the hole with a cover there is no suction. Now he is suggesting to put back the fan and add a damper that will stop the air from blowing back into the laundry room. you think this will work. please advise. thank you for your time and effort.

  28. Alex Stadtner says :

    You have to know and understand where the moisture is coming from that is supporting microbial growth. If it’s a leak in the plumbing or roof, an exhaust fan probably won’t do anything for your situation. If, however, the problem is condensation due to excessive humidity from bathing in the bathroom, then installing a sufficiently sized exhaust fan may be enough to prohibit future mold growth. Good luck!

  29. Sharon Reams says :

    We all are facing the problem with growing mold in the bathroom. There is a solution to reduce the mold well but the question is how effective can be a exhaust fan. You mentioned the testing ways quite easily and understandable to all hopefully. Thanks for sharing the tips.

  30. Jay says :

    I am having a battle with my builder in a new home. I say the fan is too small for the square footage (110 sq feet) and a ceiling height of 9 feet. Builder points at IRC 2015 as 50 CFM (he put in an 80) but I still get fogging meaning the fan is not pulling hard enough for the size of the bathroom, The HVI says I should be at 130CFM then add 50 for shower and 50 for bathtub. This takes me to 200 or a bit more. The builder refuses to do nothing as he has met IRC code which states as a minimal use a 50 CFM …minimal being key. The IRC code is ridiculous as it gives no calculations for the bathroom size…just makes a single statement that in bathroom the fan is a minimal 50CFM. I want to get better pull which means a larger fan and a larger duct system plus roof cap. Not a cheap effort but I can’t seem to get the builder or the HVAC people to come up with an analysis of my specific bathroom….one can not generalize on one fan size for all bathrooms. I am looking for a smoke test tool (safe system) to run a smoke test to prove to them their fan is undersized and will only pull very weakly from one direction (French door openings on bottom of each door) and will not pull any more. I believe a larger fan will have a wider pull area in the bathroom. I ran a quick test with some smoke from incense to attempt to record the smoke… just went vertical at every place in the bathroom except very close to fan.

    Additionally I live in Central Texas which is loaded with “spores” and all kinds of stuff that is bad for folks with allergies.

    I installed a timer and allow the present fan run 15-20 minutes after leaving bathroom.

    Builder has no idea about suggested cycles per hour for ensuring the air has been swapped out and all the exhaust system has had time to dry out up all the way through the cap.



  31. Kevin Cordes, CMI, CIE says :

    Hi Krista,
    If the exhaust fan was cleaned and is still not working properly, then yes you may need to replace it. You should also make sure that the exhaust fan is ducted directly outdoors. Frequent cleaning of bathroom surfaces to remove moisture, dust, and/or biofilms will also help to limit the chance of surface mold growth in the bathroom.

  32. Krista Novak says :

    If your fan does not work properly using the TP test, what should you do? Can you clean it? Get it serviced? Does it need to be replaced? Ours is 20 yrs old and we leave it on for up to 2 hours after showering and we still have moisture in our skylight. Moisture on the skylight is somewhat new, so we think it might be the fan. We vacuumed out the visible dust but what next???

  33. choosefan says :

    Great information like this blog, A bathroom fan is a mechanical device which is used for the purpose of ventilation. It works like an exhaust fan.

  34. Kevin Pham says :

    This is a wonderful article, Given so much info in it, These type of articles keeps the users interest in the website, and keep on sharing more … good luck.

  35. Kevin Cordes, CMI, CIE says :

    Hi Kevin, thank you for the comment. Yes, cracking the door or a window will increase passive ventilation in the bathroom and help to control condensation. As for sizing the fan, a general rule is one cfm per one square foot of space in the bathroom. But if the bathroom is greater than 100 square feet, than additional cfm will be required. Also, the bottom of bathroom doors should have at least a 3/4″ clearance to provide enough makeup air, and bathrooms with greater than an 8 foot ceiling may need additional ventilation.The fan will work better if it is located directly above the shower and/or tub.

    Hope this helps. Let us know if you have any more questions.

  36. Kevin says :

    Great tips, I also have a timer and works great. Over several months, we noticed bathroom was fogging up. Fan was old so upgraded from 50cfm to 70cfm and checked duct, no obstruction, hold toilet paper to it…but still bathroom fogs. Better if crack door open a bit. How do you size a fan? Any other thoughts?

  37. Thanks, Alex, for the heads-up about leftover water adding to humidity. I never made that connection before. But it’s another great reason to remove all the moisture we can.

    Most of my customers squeegee down their own showers, creating more time for me to dust and vacuum. One customer even towel-dries her tub, after every bath she takes. Now they’ll have this added incentive!

  38. Alex Stadtner says :

    Hi Regina,
    Thanks again for the note. The squeegee is a great addition! I use that at home to reduce the liquid water… which will otherwise evaporate slowly over hours and add to interior humidity. Thanks for the follow up!

  39. Alex Stadtner says :

    You got it! A timer switch is an easy and inexpensive retrofit. You can program it to turn off 15 minutes after you toggle the switch. Good luck keeping that bathroom dry, and thanks for writing in.

  40. Really appreciate your TP tip! I’ll direct my customers here. Every shower should have a shower squeegee, too. And towel for mopping up any leftover wetness.

  41. Thanks for the exhaust fan testing tip. Mine both work. Yeah! Now to remember to leave them on for 15 minutes.

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