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How to Properly Vent Bath Fans

Monday, August 31st, 2015 by Ben Kershner


Poorly vented bath fans cause attic mold

Bath fans are supposed to be vented to the outdoors.  Why?  Because when hot and cold mix, you get water.

What I mean is that warm air can hold a lot of humidity -- especially the air in your bathroom after a hot steamy shower.  It fogs up the mirror because the mirror is colder than the air -- when the warm air hits a cold surface, the air cools down -- and since cold air can't hold much humidity, it drops its water in the form of condensation on the mirror surface, just like the outer surface of a cold drinking glass.

What would happen if you pumped that steamy air into your attic in the middle of winter?  That's right.  Condensation would form on the coldest surfaces, which is usually the underside of the roof, where it soaks into the wood causing rot and mold.

So, of course we can't have that.

Instead let's add a flexible vinyl or mylar duct to deliver the air right up next to an attic vent, so the humid air can escape.  Sound good?  Umm... No.  We're talking about AIR.  Unless your bath vent duct goes ALL THE WAY OUT of the attic, you're still mixing hot and cold INSIDE your attic (where you don't want water).

OK, so now we're venting the bath fan all the way up and out of the attic, but we're using a paper-thin material that breaks down with time and with exposure to extreme temperatures (like in an attic).  The corrugated shape of the vent duct slows down the air moving inside, making the fan work harder to move the save volume or air...  This means higher pressures inside the vent duct... meaning that leaks could be an issue.  Let’s replace that flimsy duct with a rigid metal pipe.  It won't leak; the smooth surface allows for more airflow with less wear-and-tear on the fan; and it will last forever.There's ugly black mold growing inside this bath fan vent

...except for one last problem (maybe).

Are you in New York?  Any northern climate?  Is it cold in the wintertime?  The duct ITSELF is a cold surface where we can get condensation!  If the inside of your vent is dusty, we're talking about mold INSIDE the bath fan vent, dripping down out of your fan.  yeeauck!  

Luckily, there are two easy solutions for this.  

If you live in the south, just make sure you run your bath fan for at least 15 minutes whenever you turn it on.  The extra time will warm up the vent and eventually evaporate out any condensation that appeared BEFORE mold can start to grow.

Properly vented vath fans consist of insulated rigid pipe, properly sealed.

If you live in the north, Your vent duct should be insulated, of course.  The insulation means that the vent duct can warm up more quickly.  Just like everyone else, you should also make sure to run the fan for at least 15 minutes each time you turn it on, just to make sure the condensation inside the vent has a chance to evaporate.  

Is all this required by building code?  Not yet!  Your brand new house is very likely to have improperly vented bath fans.  But don't worry.  It's an easy fix.  Talk to your local contractor (like True Energy Solutions) and they can fix it while they're working on your insulation.

About the author

Ben Kershner

Ben Kershner is a home energy advisor at True Energy Solutions, and is a regular contributor to our website. He lives with his wife and family in Greece, NY.

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