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I want my bathroom door open while showering, and I don't want to have to turn on the fan. My husband wants the door closed and the fan on. Our bathroom seems to accumulate mold regardless of which approach we take.

Which approach, his or mine, is the right approach to take and why?

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The fan is better, as it actively moves air out of the room. However, if you're getting mold with the fan on, then it's either undersized or not working properly. Keep in mind you may need to let the fan run for a time period after you actually finish the shower to get all the humidity out of the room. You can get fans that speed up and down depending on the level of humidity, and even turn off when it is below a certain amount.

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Part of the problem may be that with the door closed the fan is not able to pull in fresh air so it is not able to properly vent the room. I say run the fan AND open the door. – Craig Apr 16 '13 at 21:29
The fan should run for 15-30 minutes with the door open after the shower is turned off to completely clear out the excess moisture. Also make sure the fan vents directly to the outdoors. If it vents to the attic, it won't work as well. – Evan Johnson Apr 16 '13 at 21:37

The first thing to verify that the fan is able to both pull air in, and push it out. Turn the fan on with the door closed. After it has run for a couple of minutes open the door. If you notice a change in the noise of the fan the room is too tightly sealed for the fan to pull air in. At the least you will want to leave the door open a crack. Also make sure the exit point or anywhere along the vent isn't blocked.

Secondly you'll want to verify that your fan is designed to move sufficient air. Remove the fan cover and check the specification tag for its rated cfm (cubic feet per minute). Measure your bathroom, multiply length x width x height to find the number of cubic feet (ft3) in the room. (If the room is 10' by 10' and has an 8' ceiling your volume is 800 ft3.) About eight complete exchanges per hour is the minimum you'd want for good circulation. For the example room you'd need a fan that can move 6400 ft3 in an hour. 6400 ft3 / 60 = 106.6 cfm. The fan rating must exceed this to provide adequate ventilation.

Turns and curves in the exhaust pipe slow the flow of air necessitating a larger fan. Also allow the fan to run for several minutes after the shower to ensure all excess humidity is removed.

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