My bathroom measures in at 289 cubic feet. Dividing that by 7.5 and rounding up gives me a target fan capacity of 40 CFM. However, there are many more fans available at 50 CFM.

I read this post, Are there drawbacks to installing a stronger bath fan?, stating that a fan moving more air than necessary can inhibit appliance fumes from venting properly, not the mention the HVAC efficiency hit from expelling conditioned air.

My question is this: will increasing from 40 to 50 CFM create such problems, or is that in the realm of a much bigger increase?

1 Answer 1


An extra 10 CFM won't hurt you at all.... If anything, it will get rid of the unwanted moisture faster. I'd wager that article was meant for much larger bathroom fans.

  • Thanks @JACK. How much higher than target do you reckon would create problems? Jul 21, 2020 at 22:10
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    @zwiebelspaetzle Well, let me just say I have a bathroom about the same size as yours and I put in a 120 CFM fan. I like a lot of noise when I'm in there. I actually would have gone higher but then I would have had to change the duct. When you think about it, they don't stay on for a long time.
    – JACK
    Jul 21, 2020 at 22:25

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