Bath Fans Can Make Indoor Humidity Worse
Bath fans are a big topic in building science. How do you get good air flow through them? Should you insulate the duct? Is it OK to use an ERV extract vent instead? Can they work as part of a whole-house ventilation system in every climate? Today, I bring you another one: Will your bath fan make your indoor humidity worse?
The most difficult weather for humidity control
We’ve been have cloudy, humid weather since early May here in Atlanta. Some days the temperature gets high enough that the air conditioner runs enough to do some dehumidification. Other days (screenshot below), the temperature is in the 70s or low 80s Fahrenheit, and the AC doesn’t run much. As I write this, the temperature is 70 °F (21 °C) and the dew point is going to be between 63 and 66 °F (17 and 19 °C) today. That’s cool and sticky.
Those are the days when it’s hard to keep the indoor humidity below 60 percent. And when they go on and on—as you see above in our forecast for the next nine days—the indoor humidity can keep growing. (If you’re not familiar with Celsius temperatures, 23 °C equals 73 °F and 32 °C is 90 °F.)
Now, there is a way to keep the indoor humidity low, of course. You can do it with a dehumidifier, a related topic we discuss a lot in building science. But today we’re focusing on bath fans.
Bath fan controls can make things worse
When I moved into my house in 2019, I changed the bath switch to simple on/off switch to one that detects humidity and comes on when the humidity is above a threshold. You can change the threshold, but if you live in an old, leaky house, you may have to set it so high that it doesn’t do much good.
That’s what happened in my case. On the days with weather like we’re having now, it would come on a lot. I’d frequently go into the bathroom and find it running when no one had taken a shower in many hours. And all that bath fan runtime meant drawing even more humid air into the house.
So, I eventually changed the switch. Now I use the countdown timer switch on the right in the photo above. I turn it on for 10, 20, 30, or 60 minutes at a time. But if you’d prefer shorter or longer intervals, they’re available, too. Leviton sells them with maximum time intervals anywhere from 15 minutes to 12 hours.
The countdown timer switch has worked well for me, but now that I have my Zehnder energy recovery ventilator (ERV) installed, I use it, too. The ERV runs continuously at a low air flow rate, but boosts up to a higher rate when I push that boost switch below the countdown timer. When the outdoor weather is hot, humid, or cold, I use the ERV boost. When we have open-the-windows weather, I use the bath fan. This strategy works better than the humidity-sensing bath fan switch.
You may not even need bath fans some days
Then the issue of using the bath fan on hot, humid days. The air conditioner runs a lot because of the heat, so you may not need the bath fan at all, or not for long anyway. And if you happen to live in a super-humid climate, like Corpus Christi or Savannah, running the bath fan at all may bring more humid air into the house than you’re exhausting from the bathroom.
If the bathroom is at a temperature of 75 °F (24 °C), for example, and the relative humidity gets up to 100 percent, the dew point is 75 °F. In those super-humid places, the dew point often gets higher than that, so you would definitely be making the house more humid overall. See my article on this topic for more detail.
The upshot here is that the bath fan switch—and how you use it—can affect your indoor humidity in a negative way. So be careful with those bath fans and their controls.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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In your example above, you mention a relative humidity of 75 F. I think you meant 100%.
Is your bath fan located directly above the shower or away from it? Either way, have you tried putting a humidity sensor near the fan intake to see how humid the air is that is being exhausted?
Roy: The bath fan is right above the entrance to my shower, so I don’t know that an RH sensor near the fan would change things.
Thanks for pointing out the typo. I fixed it.
I wasn’t thinking of putting the sensor there for control purposes, at least not yet. I have just always wondered if the dewpoint of the air near the shower ceiling is much higher than in the rest of the bathroom. In other words, how much mixing occurs before the air goes out the vent fan.
I asked one builder in the past why he put the fan in the middle of the bathroom instead of above the shower. He said bathroom fans aren’t designed for that high humidity. I have talked to bath fan manufacturers and that is just not true.
Roy: Good point. I should try that.
I live in south central Texas and we experience some of the same conditions that you go through. A couple of years ago after replacing our two air conditioning systems we began to notice that we were having a hard time controlling the humidity as the new were the same tonnage as the old ones but they were cooling better and the air handler fans would run for several minutes after compressor shutdown. The units were doing a good job of holding temperature but we were not condensing much water as the units did not run long enough and the air handlers were just reevaporating the condensed water by running after the condensers cut off. I mostly solved the problem by replacing the thermostats with thermostats that also measure and control humidity by overcooking up to three degrees during each cooling cycle. This works well most of the time.
AC manufacturers often certify their equipment at higher indoor airflow rates and with the fan delay option to get higher rated efficiencies. Check and see if their are switch settings in the air handler that will let you reduce indoor airflow rate and turn of the fan delay. You might sacrifice a little efficiency, but you should get better dehumidification.
I have a bath fan with a humidity sensor built in right outside the enclosed shower. Most of the time, it works great, but, like you, during the summer, I have found it running when the shower has not been in use. In that case, I just turn it off until cooler temps and lower dew points come back.
And I also have my mini-split in that area set to “drying”.
Norman: What happens to the temperature and relative humidity when you set the ductless unit to dry mode?
It’s a ducted mini=split serving the sleeping half of the house. What happens is the temp gets as low as 72, which my wife loves at night, and the relative humidity stays in a more acceptable range.
I also run a ducted mini-split in a loft area in drying mode. Since this is a smaller unit, it primarily serves as a dehumidifier for the floor below.
It is not a perfect system, but it works for us and is a lot cheaper than adding a whole-house dehumidifier.
Ductless systems are typically designed to run at higher indoor airflow rates to get higher efficiency ratings. This is one of their advantages over ducted systems in terms of rating conditions because they do not have to develop an external static pressure to drive the airflow in the ducts. The “dry” mode is just a lower indoor airflow rate. This is one of the things that has to be considered when comparing ductless to ducted systems.
I had a 50cfm Broan fan in my bathroom that stopped running, so I replaced it with a 60cfm Broan retrofit kit. The 10cfm difference has been amazing. Whereas formerly the mirror would always fog up, it no longer does. Can anyone explain why?
I would guess that the 50 cfm fan was not producing 50 cfm before it quit running.
So do fully functional 50 cfm fans always prevent mirror fogging?
I doubt it. Depends on room size and shower duration among other things.
The countdown timer is the ultimate ‘low-tech, high-reliability’ fix.
I agree and I doubt that a cloud-connected AI system would do any better.
A similar issue can happen on a colder day. Opening a window to clear out steam can result in the temperature dropping below the dew point for a steamy room.
Condensation will appear on the walls of your bathroom.