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I run the exhaust fan when showering but even so, the mirror fogs up completely. Is this an indication that the fan is not sized or functioning correctly? Or is this expected behavior, as no exhaust fan could keep up with that amount of humidity put into the air that fast?

The mirror is right outside the shower, as is the exhaust fan, but I've seen this happen when the mirror is farther away from the shower and/or around a corner.

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    The purpose of the exhaust fan isn't to keep the bath moisture free while the shower is on. It's to help exhaust the moisture after the shower is off. – Chris Cudmore Jul 5 at 20:10
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Depends on:

  • The location of the fan relative to the shower and the mirror
  • The effectiveness of the fan
  • The air temperature in the room
  • The initial relative humidity of the room
  • The temperature and volume of the shower steam
  • The length of the shower
  • HVAC conditions
  • Window configuration (sunlight and airflow)

The bottom line is that it might, but it doesn't much matter as long as the fan is run long enough to reduce humidity after each shower. In my case (Minnesota), this means an hour in the winter and at least two in the summer (though maybe less when air-conditioning is active). A digital timer switch is very helpful.

  • Also ceiling height. My ceiling is so high that I've never seen the mirror fog up. Even running the shower for 30 minutes on full blast and I don't even have a fan. – HenryM Aug 6 at 18:28
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The mirror will fog when you're showering with hot water no matter how good your exhaust fan is. The hotter water can continuously create more humidity than the fan can handle while the shower is running. If the mirror stays fogged for a long period after you've turned off the shower though you may have a fan issue.

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Generally, (in the US at least) residential bath/toilet room exhaust fans are manufactured sold to meet the minimums required by the applicable building code. Historically this has been 50 cubic feet per minute irrespective of room size or fixture particulars. The purpose is general sanitation: reducing mold and diffusion of human waste vapors throughout the dwelling. Mirror fogging is not a consideration when it comes down to “proper” exhaust sizing in ordinary circumstances.

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The fog is caused by hot water continuously running when you shower. If you try to shower with lukewarm water, the fog won't be as bad as the hot water. It is really not an issue with the exhaust you just have to wait for a couple of mins for the fog to dissipate. Just make sure to keep your bathroom door open after you shower to avoid mold buildup since humidity is very high.

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