We have a master suite bathroom with a corner tub, shower stall, and a sort-of enclosure for the toilet. There's a typical switch-controlled exhaust fan above the toilet that vents out to the roof.

My wife has taken a liking to hot baths and I'm getting concerned about the humidity. The fan that's already there is in a space that's partially separated from the rest of the bathroom and I feel like it's leaving a good amount of steam hanging above the tub. Wife also complains about the fan noise.

So I'm thinking about installing another duct and fan purely for humidity. I think I'd like to put in an inline fan in the attic for minimal noise and have it automatically activated by a humidistat that's ideally located on the ceiling above the tub and shower stall (they are adjacent). I don't want a manual switch, except in the attic for maintenance. This way the wife can do her hot baths without worrying about anything, and the fan will automatically pull the steam out when it needs to.

First of all, can you guys think of any big problems with this set up? Code violations (I'll check my local code as well)? Suggestions on how to do it better?

Second, what about the humidistat location? I was thinking of putting the sensor right in the duct opening, behind the grill, where it's not visible but is essentially on the ceiling. Run the wire out through a small hole in the duct and seal it. Do you think it would be significantly affected by the airflow past it when the fan is on?

  • I think the humidistat is a good idea. Exhaust fans come in different sizes. A larger one may do the job and save you time and money, Wires in ducts is a No-No. There should not be much difference in the room humidity as the fan runs.
    – Gil
    Mar 6, 2022 at 4:10
  • @Gil why is a wire in a duct a no-no? Keep in mind that this is a low voltage wire for the humidistat sensor (likely 3V or 5V) and it's just an exhaust duct with room temperature air.
    – Egor
    Mar 6, 2022 at 6:22

1 Answer 1


That is a code requirement with very few exceptions. NEC 300.22(B) covers what you can do. You best talk to your inspector about that, here we cannot. There are exceptions for when it is necessary for the direct action upon, or sensing of, the air contained inside the duct such as smoke detectors. I also believe you must use plenum wire if it is in the duct.

  • Perhaps you could explain a bit more about what NEC 300.22 says? Mar 7, 2022 at 5:19
  • You appear to be addressing a question asked in a comment on the OP. While there's nothing wrong with that, clarifying that will help make it more obvious exactly what "that" is that is a code requirement.
    – FreeMan
    Mar 7, 2022 at 13:39

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