I have a primary bathroom which was just remodeled. Our GC put in a 110 CFM fan tied to a 4" duct. The exhaust should be taken to an external vent.


Theoretically, the fan is the appropriate size for our bathroom but we have such a problem with humidity and condensation even after leaving the fan on and opening the bathroom door and window. Nothing seems to work.

Could there be problems with the fan? With the duct work? The fan turns on and seems to be moving air (at least according to the piece of tissue I held to the vent to test). What can I look into to identify the problem and possible solution?

ETA: I live in Upstate NY. It's relatively warm and humid right now, but not bad and this was a problem before the warm and humid summer.

  • 2
    "The exhaust should be taken to an external vent." You need to verify this.
    – JACK
    Jul 6, 2023 at 17:11
  • @JACK that's true, but even if the OP's fan vents to the attic when it shouldn't, it should still be removing the moisture from the bathroom.
    – Huesmann
    Jul 7, 2023 at 14:18
  • @AMC, this isn't a solution for you, but if you need something to ameliorate your situation while you solve the real problem with the vent fan, get yourself a portable dehumidifier and run it with a timer, e.g. start it when you turn the shower on and set it to run for some period of time after you stop the water.
    – Huesmann
    Jul 7, 2023 at 14:22

1 Answer 1


It is very possible that the termination of the duct work out a wall or on the roof is clogged or blocked.

If you can reach it safely, check that any screen is open and free from lint, debris or possibly even nests from bugs or other animals.

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