Please excuse the expression, but we used to call them ‘Fart Fans’. And it is certainly true that some pretty unsavory odors can be created in a bathroom. It is also true that those odors are volatile organic compounds or VOCs. Many people think that that is the sole purpose of a bathroom fan which is why they are commonly connected to the light switch, vent into the attic, or not allowed to run much beyond the time the bathroom is occupied.

Code requires the installation of bathroom fans in most cases now, but I have never seen anything in code that mentions the expulsion of flatulence. And why are the manufacturers bothering to make the fans so quiet and energy efficient if they’re only going to run for the short time that the bathroom is occupied? And should you keep the bathroom door closed while the fan is expelling the malignant odors?

First of all, a properly installed bathroom fan must be ducted all the way to the outside of the building. It should never, ever be vented into the attic or ceiling cavity or someplace else inside the thermal envelope of the building. If you have an existing fan that is not ducted all the way to the outside, you need to fix it so that it does. Air is drawn into the house, passes through the rooms, under doors, through cracks and holes, moves into the bathroom, is drawn through the fan, which pushes it out through the ducting and the final termination fitting back to the outside. It has to be a complete path from outside to inside and back to the outside. Just imagine those happy little air molecules flowing all that way!

That path is why leaving the bathroom door open while the fan is running will make it easier for the fan to more quickly dispel the odors.

When you shower, the steamy hot water increases the relative humidity (RH) in the bathroom. But it is interesting that as the temperature of the air rises, the relative humidity (RH) falls. The inverse is also true. So since the temperature of the air in the bathroom is warmer (because of that hot shower) than the temperature of the air in the body of the house, the relative humidity is actually going to go up when the fan is running. But wait there’s more.

As the fan runs, it also lowers the temperature of the air in the bathroom, gradually bringing the temperature down to the temperature of the air in the main body of the house. If it runs long enough, the two temperatures will eventually equalize.

What’s really important in terms of mold growth, is the dew point. There is this magical little kingdom right at the surface of the materials. If that surface temperature stays above the dew point, moisture won’t condense there. At sea level (where I live) if the temperature in my bathroom is 68 °F and the RH of the air is 40%, the dew point would be 43 °F. That means that surface temperature of the wall or ceiling would have to be 43 °F for moisture to condense out of the air. And that’s cold! As the temperature rises, the dew point also rises (at 75 °F and 45% RH the dew point would rise to 52 °F).

So the key is to run the fan long enough after the shower to reduce the temperature in the bathroom while lowering the dew point. It will take several hours of run time in most bathrooms after the shower has been turned off. Water will continue to drip down the shower walls. Towels will still be damp. All that water will still be evaporating into the air.

So, yes the bathroom fan is there to expel the mephitic odors but it is also there to prevent the formation of mold on cooler surfaces. But the duct must run all the way to the outside AND the fan must run long enough to effectively do its job. And that’s why it has to be a quiet and energy efficient fan.