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If a bathroom has no windows and doesn't get completely dry after a shower (neither do the towels, and the ceiling starts to grow black spots of mold), can a regular fan dry everything after the shower?

Which of these is more efficient? (forget about the other rooms that will get more humid)

  1. Placing the fan in the doorway, pointing to the outside the bathroom.
  2. Placing the fan in the doorway, pointing to the inside, oscillating.
  3. Placing the fan inside the bathroom, far from the doorway, pointing the doorway.

Why is the exhaust fan better than a direct wind into the surfaces? When we want to dry our hair, we point a blow dryer to it, and not a "humidity sucker".

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    how much time? ... that cannot be predicted by anyone here – jsotola Dec 1 '20 at 23:56
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    the only way anything will dry in the bathroom is if you continually remove the humid air from the bathroom – jsotola Dec 1 '20 at 23:58
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    @Harper-ReinstateMonica whose code? Which country? The OP hasn't said where they are. – Chris H Dec 2 '20 at 12:01
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    @ThreePhaseEel I think there's an equivalent rule here, but that's not really the point. The OP could be in Outer Mongolia for all we know, and plenty of experienced users go straight to assuming that US rules are relevant, when they should know better. – Chris H Dec 2 '20 at 13:22
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    @Harper-ReinstateMonica I know what to look for: The OP stating a location. They don't. Your guesswork may be amazing, but it's still guesswork. I'm fairly sure they not in Outer Mongolia, but we do have quite a lot of users from Eastern Europe, for example, and in some of the former Soviet states historic building styles are very different. – Chris H Dec 3 '20 at 7:51
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You need a dehumidifier.

https://en.wikipedia.org/wiki/Dehumidifier

If there is no way to blow the humid air out, run it thru a dehumidifier. You can keep it in the bathroom running on a timer or roll it into the shower to run after you wash up, and let the collected water go down the drain.

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    This is the only solution that doesn't involve venting somehow. It also works just outside the bathroom if you prop the door open. I do this with my wet-room en-suite sometimes – Chris H Dec 2 '20 at 12:04
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    I had this same problem and this is the solution I used. I left the humidifier on constantly with a preset limit. It would kick on when the humidity was too high and off when it was low enough. I got one that had a pump on it which would automatically drain in the regular sink drain. It wasn't pretty, but did the job. – Ben Dec 3 '20 at 14:05
  • Dehumidifiers are expensive to operate--on the order of US$30/month--and they're expensive to own as they don't tend to last more than a few years. They're also noisy, bulky, and need regular service if they're not on a drain. I'd consider this a last resort. – isherwood Dec 3 '20 at 15:09
  • @isherwood If all you need to do is dry out a single room, whether one-time or daily, a portable dehumidifier is much cheaper to operate than running the whole-house HVAC. I live in a very humid climate, so I have a portable dehumidifier that I use nonstop that pulls at least 10L of water out of the air every day. It has a removable bucket that I pour down the drain or use to water the plants. – shoover Dec 3 '20 at 16:31
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    @isherwood, agreed with most of your downsides, but it won't cost $30 per month just to dehumidify the bathroom after a shower; that figure would be more for whole house dehumidification in a humid climate. Also, you may have had bad luck but if you get a quality one, they should last much more than a few years. They work on the exact same principles as refrigerators and air conditioners, so it's reasonable to expect them to last at least a decade or two. – Nate S. Dec 3 '20 at 17:45
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When the things in a bathroom dry the water is not simply disappearing, rather it is converting from liquid you can see and feel on the surfaces and towels into vapor in the air. In other words, visibly wet surfaces are exchanged for palpably humid air.

A portable fan will circulate air inside the room and will accelerate the process but it will be hampered by one important factor: after a shower the air in the bathroom will already have a high humidity level. The air won't be able to hold much more water vapor, and so the towels and ceiling will just sit there in front of the fan and remain wet.

You'll need to find a way to dissipate the humid air out of that bathroom. That's why US building codes require a bathroom to have an operable window or an exhaust fan (or both).

Since your bathroom has neither, you could use the portable fan to blow air from the adjacent room into the bathroom. Whether it'll be enough, or how long it'll take, is anybody's guess.

The best remedy is to add a proper exhaust fan. A useful way of limiting the problem is to reduce the temperature and duration of showers. Shaking or brushing as much water off the body as possible before toweling will help too. If things are really dire you could take the towels to another place to dry (and even towel off the walls or ceiling, and bring those towels elsewhere to dry also).

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    Moving the towels to another room will A) help them dry faster by not being in the humid air, and B) help the bathroom dry faster by not being another source of moisture that has to be dissipated. – FreeMan Dec 2 '20 at 13:55
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    Blowing in air from adjacent rooms is the most straightforward option, but has the downside that you're now moving the humid air through the rest of the house, which could potentially cause problems elsewhere - this is why it's unacceptable to have a bathroom exhaust fan vent into the attic or any other interior space. Dissipating the humid air in a large space will work OK, but venting the bathroom into a confined hallway, for example, may just cause problems in the hallway. – Nuclear Hoagie Dec 2 '20 at 15:13
  • @NuclearHoagie - In the northern US, in winter, moving humidity into other parts of the house is helpful. – Hot Licks Dec 3 '20 at 22:51
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Our current bathroom does not have a vent fan or heat, so we've been using a floor fan for years.

The model we have has a stand that sits flat and allows the fan face to be tilted up at various angles. We set the fan in the doorway and tilt it toward the intersection of the back wall and ceiling.

The key is to start the fan before you start the shower or whatever creates steam. Even when the bathroom is cold in winter, getting the steam out of the cold room before it condenses greatly helps keep down moisture in the bathroom.

Fortunately for us, the hallway and the rest of the house has central heat, so the released moisture is actually good for keeping the humidity up during the winter.

Our fan blocks the doorway, so it is a trip hazard that needs to be removed as soon as we're done showering.

As always, YMMV, but the solution can work and has worked for us for years.

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I see another couple of options here.

An exhaust fan

I have seen this appliance in a lot of modern buildings. Well, even a number of blind bathrooms have a fan for removing smell. But I am referring to fans dedicated to dehumidification, which are insatlled in all rooms for this purpose (and smell too, of course).

The fan will pull air from inside the bathroom to outside the apartment. You keep the fan running for a few hours and you get a decent level of humidity. Air from the rest of the apartment will pour through the door, which is not obviously tight-sealed.

Mind that this must be designed properly as it must run within the walls. If your bathroom is on a perimetral wall, you might need construction permit, or at least a lot of work to make the hole.

The pros are that you can keep the door shut and the power usage is minimum. While this is similar to have a ventilator running, it's more discreet and you don't have to keep the door open.

A chemical dehumidifier

You can DIY using salt. In this tutorial I found quickly I think they are using common NaCl. Sodium chloride is the same chemical entity that you use in the kitchen for your food, but, as comments suggested, you would prefer road salt as it's definitely cheaper because not approved for feeding.

At the DIY shop I found another type of product, along with the proper case, that is based on CaCl2, which is a bit more irritating/poisonous when in contact with eyes or such.

Salt dehumidifier

The first bag of salt (1kg) this year was depleted in 3 days. Additional refills took longer to deplete. This one is half-way and running since 5 days, fyi

The pro is that you don't use power (electricity or gas) at all, but I am not sure how eco-friendly this is

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    I would go for road salt or rock salt over kitchen salt. (It's all NaCl, of course, but kitchen salt designed for use in food is going to be much more expensive at the necessary quantities.) – Darrel Hoffman Dec 3 '20 at 17:06
  • Of course, I just wanted to highlight that it is the same chemical entity you find in kitchen salt – usr-local-ΕΨΗΕΛΩΝ Dec 3 '20 at 17:09
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Yes, in the same way that fans dry a mopped floor faster. Using a towel would be much faster, though.

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    This is simply not correct. If there is already condensation, and mould implies that there is, then that means that the air is saturated, so moving it around inside the same room will NOT help. – Mike Brockington Dec 3 '20 at 11:00
  • The reason a fan dries a mopped floor faster is that the water on the floor evaporates, and the air around the floor starts to saturate with water (i.e., it can't hold any more). A fan will push dryer air into that area much faster than normal, making it dry faster. In a bathroom where there is very little volume of air and a hot shower is running, all of the air will visibly fog up with water vapor. There is simply no dry air for the fan to circulate. – Redwolf Programs Dec 3 '20 at 17:05
  • @RedwolfPrograms I was assuming the shower is over and the door is open... "can a regular fan dry everything after the shower?" – Josh Dec 4 '20 at 2:32
  • @MikeBrockington Probably not saturated air coming in through an open door, assuming it's not some tropical location... Maybe your assumption is the one that's 'simply not correct'. – Josh Dec 4 '20 at 2:34
  • @Josh I guess that does make sense, sorry if my comment seemed rude/overly explanatory. – Redwolf Programs Dec 4 '20 at 2:39

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