Humidity is one of the biggest threats to your RV.  Not only can it cause serious damage to your rig, it can lead to mold growth which can become a serious health issue for anyone spending time inside.  While humidity can be an issue year round, it becomes a much bigger issue as the outside temperatures fall below about 55 degrees.  In this post, we’ll look at where all that water comes from, why it’s such an issue in a RV and what we can do to control it.

Why is humidity such a big deal in an RV?

Humidity is measured in percentages.   If you considered the air in your RV to be a cup, at 50% humidity the cup is half full (because as an RV’er you are by definition optimistic).  At 100%, the cup is full and if you add any more it will start to overflow.  In the case of air and humidity, “overflowing” means the water will condense on the surfaces around the RV.   RV’s have several characteristics that make controlling humidity particularly difficult while making the effects of that humidity more severe.

RV’s are a very small living space.  The smaller the space, the smaller the volume or air.  With that smaller volume of air it is easier to cause large swings in the percent of water in the air.  If you evaporated a gallon of water in the air outside it will make no difference in the measured humidity.  If you do the same inside the RV, you will see a substantial increase in the humidity.  On the plus side, this can work in your favor.  As you’ll see, humidity in the RV can also be lowered fairly quickly.

RV’s are typically poorly insulated.  The lack of insulation in the walls and the use of single pane glass in most RV windows leads to cool surfaces on the inside of the RV when the weather turns cold.  Like a cold glass of beverage on a humid summer day, water will condense on those cold surfaces if the humidity in the RV is too high.

How much is too much humidity?

Optimal comfort levels for indoor spaces during the winter is 40 – 50% humidity for a standard home.  Because of the issues listed above (particularly the insulation issue), I’ve found that you want to be closer to the 40% side to keep humidity level issues at a minimum.

How can I tell if I have a humidity problem?

Our Acu-Rite Weather Station gives temps and humidity inside and out along with a lot of other useful weather information.

Generally speaking, if you have a problem you’ll probably know it.  Condensation will appear on the insides of the windows.  As it gets worse, it begins to pool in the tracks under the windows.  Condensation will also form on the inside surfaces of outside walls, particularly on the walls and ceilings of slide-outs.   You may also notice water pooling at the base of walls along the floors, particularly in areas where there is poor air circulation.

One other area is in the shower.  When you take a shower, if you notice excessive amounts of condensation on the ceiling, walls or skylight then the humidity may be too high.

If you’re not seeing these signs yet and want to keep ahead of the game, there are several good weather systems on the market that can measure inside humidity.  As an added bonus, many of these will also wirelessly measure temperature and humidity outside too.

What are the primary sources of humidity in an RV?

Water in your RV comes primarily from four sources – showers, cooking, people and combustion.

The biggest source is steam from showers.  A good, long, hot show may get your day off on the right foot, but it will add more humidity to your RV then any other single event.  To counter this you have two options.  First, you can shower at the shower house instead if you have access to one.  This will cut back significantly on the amount of water you put into the air.  If you prefer (or need) to continue showering in the RV, it’s best to close the bathroom doors and open the roof cap and run the exhaust fan.  This will exhaust the humid air from the bathroom with air drawn from the rest of the RV.

Cooking makes a lot humidity.  To minimize its effects, use the range hood to exhaust the humid air outside.  Don’t forget to make sure that the hood exhaust vent is open – there is a flap on there that locks down for when you travel.  You have to go outside to open it, otherwise your hood fan will just be blowing against a closed door.

Combustion of propane for cooking or heating also creates water.  Propane burns to make CO2 + H2O.  The stove is the biggest contributor in this area.  The furnace and water heater also produce a lot, but most (if not all) of it should be blown out the exhaust port to the outside.

People also contribute quite a bit if humidity.  Breathing and sweating both put a significant amount of water into the air.  This may sound crazy, but keep in mind that you’re in a very small space so the amount of air you breath each time is a decent amount of the total air available.  This problem is multiplied by the number of people in the RV.

What can I do to keep humidity down without spending any money?

1.) Shower elsewhere, or shower with the room vented outside.
2.) Use the range hood while cooking.
3.) Run the stove only as much as needed or use electrical heating appliances.
4.) Crack open a window.
5.) Keep the inside temp cooler.

Those things aren’t working, what can I do now?

It’s time to start using some sort of dehumidifier.  There are several types of dehumidifiers on the market.  The three most common types in the RV industry are desiccant, Peltier and compressor dehumidifiers.  These break down into two methods of removing water from the air.  Desiccant dehumidifiers use chemicals to pull the water out, while Peltier and compressor dehumidifiers use electricity.

Desiccant dehumidifiers contain a chemical which water sticks to.  As soon as you expose them to air, they begin drawing water from the air and begin losing their ability to draw more.  Eventually, they will become saturated and will no longer work.  Once that happens they need to be regenerated.  Depending on the unit, regeneration may require plugging it in (heating the media to dry it back out), chemical processing, or replacing the media all together.

In the RV world, the Eva-Dry desiccant dehumidifiers are popular.  These are small, lightweight and silent operating dehumidifiers that you place where you want them to work.  The beads in the middle will change color (yellow=dry, green=saturated) as it absorbs water.  When the beads are saturated, you plug it in to a 110 outlet (the plug is built-in on the back) and it will regenerate itself in about 24 hours.   These are small (about 6″ x 6″) and best used to control humidity in confined spaces like a pantry or wardrobe closet.   We have a couple of these and while they do change color it’s hard to tell if they really do anything meaningful.

The bigger issue I have with these is the whole process of regeneration, or how they work in general.  The water in the air goes from the air into the beads.  To get it out of the beads you plug it in and it heats the beads and evaporates the water back into the air.  The obvious issue is that we are using this to keep the water out of the air.   It seems to me that unless you plug it in outside (not recommended), you’re putting the humidity right back into the RV airspace.  This is different then the other two types of dehumidifiers which collect the water from the air into a container in liquid form where it can be dumped down the drain.

Clearly these are too small for whole RV (or even whole room) humidity control.  There are desiccant type units that are designed for that purpose, but they tend to be more expensive and still need to be regenerated.

For constant use in larger spaces I would recommend a Peltier or compressor type unit.

Peltier Dehumidifiers.

Peltier dehumidifiers use an interesting electric principle know as the Peltier effect.  Without getting into the physics, an electrical current can be applied between two metal plates.  That current cause one plate to get warm, and the other to get cold.  It does this with no moving parts.  On the cold side, water from the air will condense and drip into a collection pan – dehumidifying the air.  Add a fan to circulate the surrounding air over the cooling element and you have a Peltier Dehumidifier.

The process is very quiet (only the circulation fan makes noise), and energy efficient.  Unfortunately, it isn’t very powerful so it’s typically limited to low volume units.  The largest ones I was able to find were rated for up to 1200 sq feet of space.  While that sounds like plenty for an RV that is 350 sq feet or smaller, I’m not sure that would be the case for a couple of reasons.  RVs are long and narrow with a lot of airflow choke points.  These have small fans which would have a hard time moving the air around enough to provide coverage for the entire RV.  Also, when you have humidity issues in an RV it tends to be a constant high level of humidity that all the sources I mentioned above are trying to maintain.  I just don’t think one of these units has the output (or tank capacity) to handle this.

They may work well if you add one to a room at each end of the RV and maybe another in the middle.  This would not be much cheaper and would require more maintenance (emptying 3 units more often) then buying a single larger compressor type unit.

Compressor Dehumidifiers.

Compressor dehumidifiers are the type that you or your grandmother may have had in their basement.  It is basically an air conditioner running in reverse.  They are fairly quiet, fairly efficient, can run constantly and can handle dehumidifying an entire RV with a single unit.

We went with the Homelabs 4 gallon unit (30 pint) which we ordered from Amazon for about $140.00.  For slightly more money, they offer models up to 9 gallon capacity.  As you go up in size, the footprint gets larger, the weight increases and the energy usage goes up.  We bought what we thought would be the smallest one that would do the job efficiently.

These units are very highly rated.  The 4 gallon unit is fairly small (15″ x 10″ x 19″) and weighs only about 35 lbs, which is the perfect size to fit on the end of our kitchen island when we are using it.  In fact, its small enough that it fits in the kid’s closet when not in use.  When running, the noise is just the fan blowing so it’s not too bad.

The water container is at the bottom and is easy to remove, empty and reinstall without spilling any water.  It also has a gravity drain port which allows it to gravity drain to the outside or the grey tank automatically if you have some way to set it up above plumbing that goes either of those two places.  This model does not have a drain pump so it cannot automatically lift the water up to a sink.

The controls are very easy to use.  The LED display shows the target humidity when setting the target, or the current humidity when the machine is running.  The target humidity is set using the left and right arrows under the LED.  It can be set to run as needed, continuously, or for specific amounts of time.  It also has a “Turbo” mode which increases the fan speed to drop the humidity in the room faster.  In that mode its a bit noisier and the extra blowing is distracting in a small space so we rarely use it.

Performance wise, it can drop the humidity in our 37 foot travel trailer from 75% down to 40% in a few hours and it has no problem keeping the humidity between 36% and 40% all winter.

The tank capacity is near perfect.  We empty it once in the morning and once in the evening (every 12 hours).

Since adding this unit we no longer have any issues with humidity in our RV and we highly recommend it.






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