What You Need to Know About Bath Fans and Humidity

Wednesday, October 21st, 2015

Attic humidity from your bath fan can cause mold.First of all, let’s hear it for indoor plumbing.  Outhouses aren’t much fun, and hot showers are pretty nice.  The challenge is to deal with all that humidity – remember, we want to keep the relative humidity in our home below 55% or so to avoid mold, must, mildew, and mites.  If your bathroom mirror is fogged up, that means you’ve reached the dew-point at 100% relative humidity.

In the summertime, it’s easy – just open up the window and let the breeze blow through.  What’s that?  This house has air conditioning during the summer?  Oh.  Perhaps we need a bath fan, then.  Besides, that will help with the wintertime too, because I doubt Mr. Homer Homeowner is jumping into the shower in February with the bathroom window wide open.  So let’s stick a bath-fan in the ceiling.  What could possibly go wrong?

Before answering that ominous question, let’s leave our friend in his bathroom, step back for a moment, and look at the forces at play here.  I mentioned “relative humidity” and “dew point.”  You’ll hear those on the weather forecast a lot.  What you won’t hear is the word “psychrometrics,” which is basically the study of humidity.  It all comes down to the fact that warm air can hold more moisture than cold air can.  When warm air (imagine August sitting by the pool) comes in contact with a cold surface (the outside of your drinking glass), the air cools down and deposits its moisture on the soda-glass.  Relative humidity is the amount of moisture in the air, RELATIVE to how much it can hold.  That hazy hot and humid day by the pool may have 90% relative humidity.  That warm air is holding almost as much as it can.  When it hits your drinking glass, the air starts to cool.  Cold air can’t hold as much water, so the relative humidity starts to climb quickly… 90… 95… 100%.  That means we’ve reached the dew-point.  The temperature at which that air CAN’T HOLD that much water.  It’s where the dew on your grass in the morning comes from.  It’s why you get rain when a low-pressure system moves in.  It’s why your soda-glass sweats, and it’s why your bathroom mirror fogs up.

When a bath fan is mounted, the hole is cut larger than the fan.

OK… Back to our friend in the foggy bathroom.  The contractor came by and installed a 7” bath fan by cutting a 7.5” hole in the ceiling, and mounting the fan in place.  Unfortunately, now we’ve got a ¼” gap all around the fan housing.  We’ve got all this warm humid air in the bathroom -- and warm air rises.  All that humidity is going to find its way up into the attic where it can find a cold surface to condense on… and that means mold and rot.  The bath fan housing should always be sealed to the drywall.

Let’s see WHERE this contractor vented the fan – surely he didn’t just vent the fan straight into the attic, right?A bath fan needs to be vented to the exterior of the house -- the gable end, for instance  The correct options here are: 1. Up through the roof, 2. Out the side gable end, and 3. Down through the soffit vent.  Number three can cause trouble, though, so beware.  Remember, this is warm humid air which wants to rise.  If we exhaust it from the attic through a vent pointed downwards, that air is going to float back up again.  Is there an eave or soffit vent nearby where the humidity can get back into the attic after its quick trip outdoors? 

Next, let’s look at HOW it’s vented.  You COULD use a piece of flexible mylar or vinyl hose, but those tend to break down with time, and really aren’t ideal for your bath fan vent.  What you really want in a northern climate is an INSULATED rigid metal vent-pipe.An uninsulated bath fan vent LOOKS fine at first glance. A closer inspection shows that mold is growing inside this uninsulated bath fan vent. It doesn’t impede air-flow, the seams can be sealed with mastic, and the insulation keeps mold from forming on the inside of the pipe and potentially dripping back into your house – Yuck.  Even if it doesn’t drip down, every time you open the bathroom door, you create a momentary vacuum which sucks air (and mold spores) down from your bath fan.Mold can form inside your bath fan vent if it isn't insulated

OK.  Our happy homeowner now has a properly vented, properly installed bath fan.  The newer models are silent, and move at least 75 CFM of air (that’s Cubic Feet per Minute, by the way).  Let’s install a timer switch too, so that it’s easy to run the fan both DURING the shower, and for about 20 minutes after, as well.

P.S.  Here’s a fun way to learn about psychrometrics.  (yes, that’s me losing the game early on.)




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