Do You Really Need to Run the Bath Fan in Winter?

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Condensation in the bathroom after the morning shower

You may have heard or read somewhere that you should run your bathroom exhaust fan whenever you take a shower and then let it run for a while after you're done with the shower. Showers increase the humidity in the bathroom. Sometimes it gets high enough to cause condensation to appear on the mirror and other surfaces in the bathroom. And that can result in mold growth. So you should always run your bath fan when you shower. Or so they say.

Reasons to run the bathroom exhaust fan

Bath fans are a really nice thing to have. I know. I lived without one in my 48 year old condo for years before finally remodeling and installing an exhaust fan two years ago. They do indeed remove moisture. I took that photo above one day last summer after intentionally leaving the fan off so I could get a good picture of condensation. (Yeah, I'm like that.)

But they also remove odors. Bathrooms have been known to be smelly on occasion. You know, with all the candles and incense and hairspray and stuff. Right? Oh, and then there's the cleaning products. And, in some bathrooms (not mine!), a pile of dirty laundry. Seems like there's something else, too, but anyway, you get the point. Bath fans are good for removing odors.

Another reason to run the bath fan is if it's part of your whole-house ventilation system. You can get controls to run them continuously or a certain number of minutes per hour. Some fans have the controls built in. Either way, if the bath fan is part of your whole-house ventilation, you don't want to turn it off in winter. (By the way, bath fans don't have to be part of exhaust-only whole-house ventilation. You can pair them with supply fans to have balanced ventilation.)

You may not need to run the bath fan for your morning shower in winter

Now, let's explore the humidity side of things. Taking a shower increases the amount of water vapor in the air. But that may not be a bad thing. What's bad about dumping a lot of moisture into bathroom air is when it gets into the porous materials, like drywall, and keeps them wet. When they stay wet long enough, they can start growing mold. Not good.

Let's look at another aspect of what wintertime does in many homes. What's the relative humidity in your home when it's cold outdoors? Remember: Cold air is dry air. It's not the furnace drying out the air in your home. It's cold, dry air leaking in, or being pulled in by the mechanical systems and stack effect.

So if your home's air is really dry, not running the bath fan when you shower may be a good thing. Are you running a humidifier to combat dry indoor air? All the more reason not to run the bath fan.

Think about this: You take a shower, which increases the humidity in the bathroom. So you turn on the bath fan, using energy to suck out the humid air. Then you use more energy to heat the air that has to be made up by the air exhausted from the bathroom. Then you use more energy to run a humidifier because your indoor air is too dry. Hmmmm. Something doesn't add up here.

Moisture migration after a shower

I live in a 48 year old condo. I've done some work to make it more airtight but not enough yet. (OK, if you really want to know why, it's because our condo association pays the gas bill and I'm behind on retirement savings. Skin lotion is cheaper than replacing the atmospheric combustion appliances. And I have a low-level carbon monoxide detector. There. Now you know.)

When it gets cold, which Floridians think happens way too often and Vermonters believe almost never happens, our indoor air gets pretty dry. We've had some cold snaps this winter, and here's a couple of graphs of data from one of them. The outdoor temperature on this particular day started off at about 25° F and got up to 47° F. (We got 2" of snow later that night.)

The first graph shows the temperature and relative humidity in the bathroom, up high near the ceiling, and the temperature and relative humidity at the central return vent in the living room. Several interesting points jump out when you look at the graph but let's focus on the humidity part. It's not hard to see when I took a 5 minute shower. The relative humidity spiked in the bathroom, going up t almost 80%. And then it started coming down as soon as I turned off the water. The relative humidity drops back to about where it started in about two hours.

Relative humidity spike during a morning shower

The second graph shows the same thing except with dew point instead of relative humidity. Here you can see the real longer term effect of the shower. Although the relative humidity dropped to about the same level it had been before my shower, the dew point stayed elevated. If you understand my previous rants about relative humidity versus dew point, you know the reason. The relative humidity dropped to the same level, but the temperature also was higher. The rising dew point shows that we actually had more water vapor in the air.

A morning shower's effect on dew point

When is it OK to leave the bath fan off?

If you think it might help in your home to capture that moisture, here are a few pointers to help guide you.

  • If the air in your home is really dry, you may be able to shower without the bath fan running.
  • If you run a humidifier, why are you removing humidity from the bathroom?
  • If you are attentive to what's happening in your home, this may be worth a try.

Beware that adding moisture to a home in winter can cause problems. Remember that article I wrote about two rules for preventing humidity damage? If you've got humid air, you need to keep it away from cold surfaces. If you've got cold surfaces, you need to keep humid air away from them.

There are two keys for making this work for you instead of against you:

  1. Watch for excessive condensation. When I took the shower for the above graphs, the condensation disappeared pretty quickly because the overall humidity in the home was low.
  2. This works best if you leave the bathroom door open during the shower. Not everyone can do that so you might need to run the fan at least while you're in the shower. But if you get out and open the door quickly, you probably can turn the fan off quickly. Watch the condensation. Once it dissipates, you can turn the fan off.

If you decide to experiment with this, be sure to keep an eye out for condensation that lingers and any surfaces in the bathroom that might grow mold. If you can keep those two things under control, you should be fine leaving the bath fan off when you shower in winter. Even if you do run the bath fan for humidity control, the disappearance of condensation is a pretty good guide to when you can turn it off.

 

Related Articles

What Is the Best Indoor Relative Humidity in Winter?

Cold Air Is Dry Air

Does a Gas Furnace Dry Out the Air in Your Home?

5 Reasons Bath Fans Have Such Poor Air Flow

Installing an Exhaust Fan During a Bathroom Remodel

 

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Comments

In my arid climate, I almost never run the bath fan when taking a shower. Likewise with the range hood fan when cooking with water. We can use all the moisture we can get!

In general, 45% is a reasonable upper limit for relative humidity during cold weather, to stay out of trouble. That allows for elevated RH in nooks and crannies where cooler temperatures will naturally raise local RH levels.

The tighter the home, the more likely skipping bath exhaust can lead to trouble. For example, if your home is super-sealed with a low-volume ERV ventilation system, it's a bad idea to skip the fan, or rely on the ERV as primary bath exhaust. An ERV will dutifully recycle around half the shower moisture back to the house.

As an aside, those with positive pressure ventilation systems (CFIS) should be careful not to allow indoor RH to get too high in cold weather (unless walls have an adequate layer of exterior insulation). Even 45% RH air will condense if it gets pushed into wall cavities where it can reach cold sheathing, hidden from detection until the damage is done.

Allison
Bailes

David, that's an excellent point about tight homes holding onto the moisture. And yes, 45% RH at 70° F is a dew point of 48° F. It's quite likely that sheathing can be below that temperature when it doesn't have exterior insulation.

Allison,

Two quick questions.

No 1: Regarding winter time condensation. I walked through a new townhome development in Vinings (I-285 & Atlanta Rd) and these particular buildings have flat roofs due to the skyline view of downtown Atlanta. It looks like the builder is going to use batt insulation underneath the roof decking so I couldn't help but wonder about the potential for winter time condensation forming on the underside of sheathing via voids between the batts. I'm guessing that the builder isn't going to use spray foam because the flex duct has already been installed (btw..you have to see the contortions in flex duct, I have photos but can't post them).

No 2: Some of these unit will have a partially enclosed as living space on the roof so the builder adds a small dedicated HVAC system in a closet where access is via and exterior louvered* door. Is that a good idea? Is that even code in Ga?

*There's a small louvered section at the bottom half of the door.

Is there any research on whether bathroom fans actually prevent condensation? I mean of course they reduce it, by removing some moisture laden air, but I don't believe I've ever taken a shower in a bathroom for which running the fan stopped condensation occurring.

Ok, so I didn't see condensation in Arizona during the winter, but it definitely didn't matter if the fan was running or not (I believe that dewpoint was single digits that day).

Hmmm, thinking a bit more I might be focusing on mirrors too much. Presumably the area that people are worried about most is drywall and other porous surfaces? But hey, the question stands: under what range of circumstances (shower type/rate/length, cfm of fan, temperature of drywall, baseline humidity in the house) is condensation prevented. Then: how often do those circumstances occur? I suspect it's not frequently (or almost never for humid parts of the country?)

Allison
Bailes

James, if there is research on using bath fans to prevent condensation, I'm not aware of it. But I don't think preventing condensation should be the goal. Keeping that moisture from building up in porous materials is the more important objective, as you surmised.

But if you wanted to prevent all condensation for some reason, you could do it. You'd just need to keep the temperatures of all the surfaces above the dew point. In my graphs above, you can see that the dew point went up to about 70° F when the RH peaked at 80%. Since it was about 27° F outdoors at that time and the thermostat was set at about 70° F, yes, there was some condensation. But as you see in the graphs, the dew point dropped quickly and the condensation evaporated.

I live in Colorado at 5,300 feet. It's dry. Really dry--split fingertip, knuckles, occasional bloody nose. So, in the winter, I vent my electric clothes dryer indoors to capture the otherwise-wasted heat and humidity. My thermostat indicates the indoor humidity increases after several loads from about 25% to about 45%. I can barely see out of the ground-floor windows for all the condensation. But, the condensation dissipates in an hour or less and the indoor humidity is right back to about 25%. I'm assuming that I'm not doing any damage, but I don't really know. Thoughts?

Allison
Bailes

Briscoe, if you've got condensation lasting an hour, I'd be leary of doing this. You could be getting a lot of moisture inside the walls and growing mold in there. Better would be to get your home's air leakage down to keep from getting so much dry air inisde. If you still have a problem with dry after that, a humidifier that adds moisture at a lower rate, without producing condensation, would be the way to go.

@Briscoe, it's never a good idea to allow bulk moisture (i.e., water) to accumulate on interior surfaces. Those window sills and frames are certainly at risk, depending on materials. Older non-low-e windows effectively limit indoor RH in winter, as noted in most humidifier instructions. The fast drying time and return to 25% RH indicates how leaky the enclosure is, and eliminates whatever benefit you were hoping to achieve by venting the dryer indoors.

In general, older, leaky homes are naturally less likely to experience moisture problems, but all bets are off when you dump that much moisture into the house!

George Kerr recently passed away, so we can't order the CO detector off his website. :( RIP George.

Allison
Bailes

Thanks for letting me know, Peter. I hadn't heard. George was one of the most dedicated people I think I've ever met. He was absolutely tireless in promoting the need for low-level CO monitoring.

It's a two-way street though, right? Wouldn't whatever moisture crept into the walls start coming right back out of the walls as soon as the humidity of the indoor air dropped lower? Incidentally, after considerable professional effort and my own, the house is about as tight as I'm going to be able to get it at .30 (CFM50/nACH). We have a humidifier attached to the air-handler, but it's only working when the blower fan is running (when the furnace is running). I don't want to run the blower fan more independently, because I suspect the fan is just as energy-inefficient as the 1998 furnace (80.0 AFUE). As you can imagine, venting the dryer indoors for a few hours a week doesn't help much with the low average humidity, it's mostly that I really don't like shooting all that spendy heat and humidity right out into cold Colorado winter.

Briscoe - in your situation it might be even better to consider drying racks. My wife has been using them for decades and we do not live in a particularly dry area - she just likes the feel of the clothes better. With your dry interior it may only take a short while until your clothes are dry w/o any energy used - and you get the washing moisture in a more metered way.

I would be interested in your observation on using a fan in summer as well, particularly in hot-humid climates. The conditions I am thinking of are something like 65F dewpoint indoor air, being sucked out of the house via exhaust fan. If outdoor air is 75F dewpoint, are we helping or hurting?

Mark, that is a good point. I have heard of houses in humid climates with humidity-controlled bathroom exhaust fans where the fans would never shut off in the summer, probably for the reason you stated. If the humidity sensor for controlling the fan was at the ceiling, near the exhaust fan, it might work properly. Unfortunately, the sensor is usually located at the fan switch.

@Mark, the 'steam' rising from the shower has a dew point higher than 75F. But to your point, when outdoor dew point is that high, it would make sense to turn off the fan as you exit the shower.

So glad that you distinguished between ventilation for local moisture control and whole house ventilation for indoor air quality. They are definitely not the same thing and may have two different -- and on occasion opposing -- objectives. However, well designed and operated it is possible to use a single system to get both.

On a related issue, what about walk-in showers with no doors? I have seen that more frequently in new construction, and recently bought a house with a door-less shower in the master bath. What a stupid idea! First of all, there is a constant recirculation of air between the shower and the rest of the bathroom, so it feels uncomfortably cool while you are in the shower. Second of all, this humid air fills the whole bathroom from floor to ceiling, so I am sure that the incremental humidity load on the house is much higher. I couldn't stand it, so I had a glass shower door installed, and everything was better--more comfortable (warmer) showers and much less condensation in the bathroom.

Also, I keep a humidity sensor in my bathroom. I have found that if the RH in the bathroom is less than 30% before the shower, I can skip running the exhaust fan at all, and there is no apparent condensation anywhere after the shower.

RoyC - this is a funny comment. We're buying a new house and it has the same open shower concept in the master bathroom. It looks beautiful. You realize that you're suppose to run the shower for 30+ minutes to actually warm up the entire bathroom before you take your shower during the winter. I'm also learning towards never ever using that for the intended purpose which is to take a shower. It would be too much to clean afterwards. There's only one good reason to use that shower in the master bathroom, and let's just say I can't share that reason in a public forum. ;)

Peter, that leads to yet another issue. We only take showers. Our current shower has two shower heads and could accommodate two people . . . However, both my wife and I hate baths. Our current house, as well as our previous two houses had whirlpool tubs that we never used. I have seen studies that few people use tubs today, but everyone thinks that you still need them for resale value.

As for shower cleaning practices, I have come up with a door opening strategy for that. I generally think that the door should be left open after a shower so that it dries as quickly as possibly and reduces the chance of mold growth. However, every time the water in a shower evaporates, it leaves some hard water residue behind. Thus, if my wife is going to take a shower soon after me, I leave the door closed so that it doesn't dry out since it takes about 24 hours for mold growth to start on a wet surface. My wife goes even farther and uses a squeegee on the walls after her shower. Since I am usually the first person in the shower each morning, this works well.

One more comment. Where should the exhaust fan be located? Obviously in the ceiling, but I claim that it should be directly over the shower stall. Why would you not get as close to the source as possible? I have rarely seen that done, and have asked some builders why they don't install it over the shower, and they claim that it is too humid and will shorten the life of the fan! I have asked at least one exhaust fan manufacturer about that, and he said that it is not a problem.

For years I kept a small electric heater in the bathroom and would use it while showering in the winter. And leave the fan off in most cases. That reduced or eliminated condensation on walls and made the bathroom a little more comfortable. Also, it made the air in the bathroom more "energetic", ensuring quicker drying when the door was opened.

Now I have a house with radiators, which is even better! (As long as the boiler has run recently.)

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