In the comments section of my article “10 Ways to Reduce Your Indoor Humidity,” RoyC made an interesting point, saying, “I don’t usually run my bathroom exhaust fan during a shower when it is hot and humid out,” he wrote. I’m sure that statement shocks a lot of people. And why wouldn’t it? It goes against everything we’ve been told about controlling moisture in our homes, right? Well, maybe not everything.

You may recall that I wrote an article a few years ago on the same exact issue, except during winter instead of summer. My conclusion was that you may be able to get away with leaving the bath fan turned off during and after your shower. Even better than saving energy, though, that moisture you add to the air in the bathroom may help improve your indoor humidity in winter.

A look at relative humidity

So, back to our question about summer showers and bath fans. We all know that the relative humidity in the bathroom rises when you take a shower. Here’s a graph of my morning shower today. It didn’t hit 100 percent, but 85 percent isn’t the kind of humidity you want in your house for very long. Hence the advice to run the bath fan.

But if you’ve been reading my posts for a while, you’ve probably noticed that I’ve been trying to wean people off of thinking about humidity in terms of relative humidity. It’s a perfectly valid psychrometric quantity. The problem is that too many people misunderstand it, which leads to bad interpretations of what’s happening. So let’s look at the more relevant metric.

Understand dew point temperature

The dew point temperature is where we need to look to understand this paradox. When you turn on the bath fan, you’re exhausting air from the bathroom. You do get local removal of humidity, but bath fans do more than just remove air from the bathroom. They create a negative pressure, which results in outdoor air leaking into the house.

The graph above shows how the dew point changed during my morning shower. You can see that it went up from about 58 °F (14.5 °C) to 70 °F (21 °C). At the same time, the outdoor dew point was at 64 °F (18 °C), shown by the orange line on the graph. (Drier, cooler air just moved into the Atlanta area, and it’s going to be a beautiful week here!)

Because exhausting the humid bathroom air leads to an equal amount of outdoor air coming in, running the bath fan during the 30 minutes of elevated dew point would help dry out the house. But if this morning had been one of those days with a 75 °F (24 °C) dew point, I’d have made things worse by turning on the fan. Hmmmm.

Another thing to notice is that the dew point came down fairly quickly after I turned off the hot water. Within 30 minutes it was the same as the outdoor dew point. And here’s the thing that may surprise you. I did NOT have the bath fan turned on.

Yeah, we did get a little excess humidity in the house during that time. But the mirrors didn’t get steamed up and the walls weren’t dripping, so it was OK even in our current conditions.

The effect of air temperature

But what about the air temperature (called dry bulb by psychrometric geeks). Here’s another part of RoyC’s comment:

“When I get done mowing the lawn on a day when it is 95 °F DB and 75 °F DP outside and take a shower, my bath fan would be exhausting 75 °F saturated air (DB and DP) at best while causing additional air to be sucked into the house somewhere else at 95 °F DB and 75 °F DP. Thus, I haven’t reduced my latent load, but I have increased my sensible load.”

Even if you might be dehumidifying because the outdoor dew point is lower than the indoor dew point, you still may be adding to your air conditioning load. As he wrote, an even exchange of moisture that brings in hotter outdoor air will cause the AC to do more work.

How to make the call

This is much like my article on how to decide when it’s too humid to open the windows. You start by comparing the indoor and outdoor dew points. But you don’t have to measure the actual dew point to make a decision. Just follow these three steps:

1. Assume the dew point in your bathroom is the same as the bathroom air temperature. If the air in your bathroom is 75 °F (24 °C), that’s your maximum dew point. That happens when you hit 100 percent relative humidity. If your relative humidity is lower than 100 percent, your dew point is lower than your air temperature.
2. Look up the outdoor dew point on your weather app. (You can see in my screenshot below that the dew point here in Atlanta is 19 °C (66 °F) as I write this.) If the outdoor dew point is lower than your bathroom’s, you’ve met criterion number one.
3. Finally, check the outdoor air temperature. If it’s the same or lower than your indoor temperature, you’ve met criterion number two.

If you pass both criteria, go ahead and run the bath fan. If it’s hotter or more humid outdoors, you may not need to run the bath fan. Another consideration, of course, is how wet things get in your bathroom if you do leave the bath fan turned off. If you shower with the bathroom door open and have good air circulation, leaving the bath fan off should be fine.

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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. Photos courtesy of author.