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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12 Responses to “Bathroom Exhaust Fan”

  1. 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.



  2. 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.

  3. 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???

  4. 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.

  5. 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.

  6. 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.

  7. 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?

  8. 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!

  9. 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!

  10. 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.

  11. 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.

  12. 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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