If you listen to the arguments on home inspector forums you would discover it apparently is not easy.
What does the code say about it?
R303.3 Bathrooms (2018 IRC): Bathrooms, water closet compartments and other similar rooms shall be provided with aggregate glazing area in windows of not less than 3 square feet, one half of which shall be openable.
Exception: The glazed areas shall not be required where artificial light and a local exhaust system are provided. The minimum local exhaust rates shall be determined in accordance with Section M1505. Exhaust air from the space shall be exhausted directly to the outdoors.
Per code, an exhaust fan is “not required.” However, as soon as you have a bathroom with no window, or a window that is not big enough, mechanical ventilation would be required.
Some states (like Washington State) have amended the code to require mechanical ventilation in all bathrooms regardless of opening windows.
So why do home inspectors still argue about it?
Because home inspectors like to argue?
Well that is true enough, but I think the deeper problem is what looks like a lack of interest in what the fan is for.
An exhaust fan has a couple of jobs. One is to exhaust moisture from the room, but it is also to exhaust odors from the room. If its only job was to vent moisture, we would not be required to install them in windowless bathrooms that have only a toilet and a sink.
An opening window is NOT very effective in reducing odors in a bathroom—while a fan can be.
An open window can be VERY effective in reducing humidity levels in a bathroom–even more effective than a good exhaust fan. To understand why this is, we need to look at the building science a bit.
Some ask, “When it is zero outdoors, who in their right mind is going to open a window while they shower?
However, if you try opening the window, you will find cooling the room to the point of being uncomfortably uncomfortable just does not happen. Not nearly as uncomfortable as that first step into the stream of the shower. Does running an exhaust fan make the room cold? No, and yet the direction of flow is the same as with the window open.
The biggest difference is the cold opening in the wall, that we call a window, creates a path of radiational cooling as our bodies give up heat to the cold. It is that same feeling of standing next to a single pane window when it is very cold outside. The window is not throwing cold at you, you are giving up your heat to the cold. This can be largely mitigated by an air permeable privacy curtain. After all, aren’t privacy concerns what keeps us from opening the window to begin with?
Under the principles of warm air moves to cooler air and high moisture moves to areas of lower moisture, the cooler exterior air works to literally suck that warm moist air out of the bathroom—and generally it does it much quicker than an exhaust fan. (It is actually “pushing” its way to the exterior, but you get the idea.)
An important thing to keep in mind with all of this is that there must be a good temperature difference. Being cooler and dryer outside and warmer and wetter inside will do the trick.
Keep in mind that “cooler and dryer” does not mean that the relative humidity is not higher. Because cooler air cannot contain as much moisture, the relative humidity could be quite high and still easily absorb the high temperature, higher humidity air mixing with it from the bathroom.
If you live in a mostly “cooling climate” you will likely have a much harder time venting that warm moist air to the outdoors by opening the window. An exhaust fan that pulls air-conditioned cooler/dryer air into the room under the bathroom door will do the job.
Likely it would be quicker to leave the bathroom door open and all that warm moist air would be sucked (pushed) into the bigger volume of cooler dryer air inside the home. But just like, “who the heck is going to leave the window open in the winter,” who is going to leave the bathroom door open when they take a shower? While the open kitchen concept is a real thing, I do not think the open bathroom concept is going to gain any traction. People that have homes that do have bathrooms open to the master bathroom, can get away without using exhaust fans at all—especially in primarily cooling climates. In heating climates, the more tight the house becomes the less forgiving not using the exhaust fan becomes.
If you live in a primarily “cooling climate” you will be stuck with a good exhaust fan–and keep that window closed.
I want to keep this article simple and will not introduce lots of test numbers, because there are SO MANY variables. It is not meant to be a scientific treatise, just a rough explanation of how things work.
What climate do you live in?
If you are in a heating climate, or anytime it is colder outside than it is inside, you can use a window effectively to lower moisture levels in the bathroom.
If you are in a cooling climate or anytime it is warmer outside than inside, you should not expect the open window to lower moisture levels in the bathroom effectively.
If you are in a mixed climate (and at one time or another we all live in a mixed climate), you must be aware of the direction of the temperature differential. In some ways you must be smarter if you live in a mixed climate. In most locations in the continental USA you may need to use your brain at least seasonally.
Something else you can do to reduce condensation and humidity in the bathroom is to raise the air temp considerably before you take a shower. Any condensation that happens will be vented out much quicker whether using a vent fan or a window. Artificially increasing the temperature differential can have a dramatic effect.
People ask me, “But won’t running a fan or leaving windows open waste a lot of energy.”
No. Pretty simple answer, but still “no.”
The amount of electrical energy a fan uses, or additional costs in heating the space, is almost insignificant compared to the importance of getting rid of moisture. While air temperature “can” go down in the venting process, the wall/ceiling/floor temperature, and the temperature of other things in the bathroom change even less and work to maintain balance because of their thermal mass. Any changes up or down in air temperature of the room is quickly heated or cooled by the thermal mass of the walls/ceiling/floor. Warm steamy air also acts to heat the room more than necessary as well. The adverse moisture conditions are just not sustained long enough to require a window to be open long enough or a fan run long enough to contribute to significant additional energy costs.
While home inspectors may be in a fog about vent fans, you can yourself watch how the condensation on your mirror (or windows in the room) behaves with just the fan on, or just the window open, or from doing nothing.
In my own testing, opening the window is 3 times as effective as a fully functional 110 CFM exhaust fan. At 100 watts, it costs about .01 cents for an hour–so the cost of running the fan is not an important consideration.
A huge advantage of a fan, while it is a mechanical device that can fail, is “control.” You can put it on a timer for an hour and forget about it.
Most human beings won’t remember to either open the window or close the window when necessary. Maybe we need timer controlled windows. Humans basically cannot be trusted to do this important job.
Another aspect of this, is that an exhaust fan will not keep up with the amount of moisture that can be introduced to the bathroom when teenagers take long showers. A timer that lets the fan run for a good 60 minutes after the shower is over is essential to dry the walls/ceiling/mirror etc. Choosing either the window or fan option, the mirror will always fog up at least a little unless you warm the room above 80° F.
The open window will fog up very little if at all–and is that not where mold grows very often on the closed window–especially if there are curtains? Poor circulation around the window encourages mold growth because of increased drying time.
So far, we have mostly been talking about opening a window or running a fan after a shower is over. But really, either the fan or the window should be used during the shower to reduce the amount of time after the shower the window would need to be left open or for the fan to be operated.
Perhaps a better protocol would be to keep the window open during the shower, close it when we are done, and then run the fan for 30 minutes.
It is important to keep in mind these are very rough guidelines. The bigger the temperature difference between indoors and outdoors the better venting will be with open windows in predominately heating climates. In a cooling climate–god help you. Seriously it works the same way but mostly because of the air-conditioned dryer air that gets pulled into the bathroom. So, leave the door open and the window closed.
Until such a time when better humans exist, it is my opinion, a timer controlled exhaust fan is necessary in every bathroom where moisture is produced. I think it is fun to know how the science of using windows as a means of ventilation supports them being opened or closed depending on one’s climate.
Another consideration that could be a post all in itself is whether vents should terminate in the attic or not. The code says to terminate them at the exterior of the building. In heating climates, or where attic temps spend a lot of time being colder than indoor temps, it is NOT a good idea to vent into the attic. That is where the warm moisture laden vented air will condense on cold surfaces and lead to mold growth. If you are in a mostly cooling climate you might find that venting the wet/cooler air into the humid/hot attic might function to lower attic moisture levels. Again, just follow the science.
Last but not least, make sure the fan unit and exterior cap are cleaned annually.
By Charles Buell, Real Estate Inspections in Seattle
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