Condensation in RV

[sourdough]

Just wanted to pass this by folks. I'm sure most experienced RVrs know this, as do I, but.....

Sitting here in FL we've had some serious rain in the last couple of days. I always keep my vents open when here due to humidity. I run my AC during the day when it gets too warm/humid.....and have to turn on the heater at times when the nighttime gets cool. Today, I was working in the kitchen and felt a drop of water hit my head...?? Looked up and the surround for my ac was covered in water and dripping (not bad but a drop is too much). We had lost power for about 2+hours and the RV was very warm and humid. We had opened all windows and were trying to stay as comfortable as possible. Power came on, closed windows and turned on the main AC to cool off....after a while (don't remember how long) I got the drop on my head. Opened the AC, checked it out and it was obvious it was just condensation due to the humidity; in the 2 months we've been here, doing the same thing, that hasn't happened. Checked the AC etc. and decided what we've been doing wasn't enough to keep the condensation down. Called Lowes and bought a dehumidifier to pick up in the morning. Asked DW if our roof vents were open....NO! She had shut them during the downpour. I said open the vents to see what happens. That was 2 hours ago and......no condensation on the AC. I'm looking forward to having a dehumidifier to take up more space while it looks "pretty". This post is primarily to reinforce the difference it can make to have your vents open even in winter to let the moisture out....what a week (not to mention stolen CCs, water heaters etc)!! Thinking I need a break

Oh, as a side note. We have the MaxAir vent covers and the reason she shut the vents was because she felt a bit of water coming through...maybe they're not as good as we think?.......

[66joej]

Good info sourdough! We have maxx vent covers on the vents and keep them open all the time rain or shine.

[sourdough]

Originally Posted by 66joej

Good info sourdough! We have maxx vent covers on the vents and keep them open all the time rain or shine.

We had the vent covers installed when we bought the trailer. I have always kept them open thinking they eliminated the "splashing" of water through the vents into the RV. DW put her hand under them during the storm and felt a droplet of rain hit her hand. She had me come over and I felt another droplet and left (realize this was a real downpour). She closed the vent to prevent water intrusion. Ultimately it may have been an overreaction but I can't tolerate water coming into the RV from any outside source when I'm going to bed and I thought the Maxair covers stopped that... it looks like they would from the outside but obviously not.

[JRTJH]

A few things to consider:

Air inside an RV is like a sponge. It will absorb moisture and hold it. Leaving a roof vent open will allow the "sponge effect" to continue until the vent is closed.

If the A/C is turned on, it will make the air inside the roof ducting "colder" than the cabin air.

Cold air will cause the moisture in the "warmer air" to condense or collect on the colder surfaces. This will cause the ducting/air collector to "attract and hold" water and cause it to drip when it's full.

Turning on the A/C to "dry the air" will cause the "overly humidified air" to condense on the colder surfaces (probably inside the ducting). Dry air will attract moisture, so as the A/C "dries the air" the roof vent will provide replacement water in the form of "outside humidity" to be absorbed by the dryer "conditioned air".

Leaving a roof vent open is like turning on a "water source" to resupply the inside air as it dries. Water will "sponge" from outside the RV through the vent and be absorbed in the cabin air.

So, open a roof vent to "freshen the air" or to allow for excessive humidity to escape (bathroom vent fan, etc), otherwise, leave the vents closed for maximum humidity control and for best A/C performance.

[Tbos]

This past summer I would get condensation around my AC vents when the AC was running and the vents were open. Recently I closed our vents since it's in storage and I have multiple dehumidifier bags and tubs in it. I have the covers over my vents but we gat these soaking fogs that get trough every crevice or crack. I'll check it next week again to see how it's doing.


2016 Passport GT 2810BHS, 2014 Silverado 1500 LT in Deep Forest Green

[MattHelm21]

Exactly my experience John. The AC is always on in my rig especially when its raining to remove the humidity. My vents only get used when it is cool and dry out. There was also the thread that mentioned that for maximum moisture removal, the AC should be sized so it has to work to keep things cool. If it is oversized it won't be as efficient at removing moisture. I can't speak to the oversized aspect but my last unit was undersized and it really kept things dry.

Additionally, a problem I've already experienced in one of our new air conditioners, the duct divider between the output and input sections of the unit had shifted out of place allowing the humid return air to be pumped right back into the ducting where as you said, the moisture condenses right away due to the cold. If a drip comes from here, I would begin looking at the AC unit. My previous rig had a Dometic that never did this while this unit has the Coleman(Airxcel?) Mach units and the divider has shifted twice.

[sourdough]

Thanks all. I have a Dometic and I've opened it up and the condensation is coming from it being cold and the humidity. I opened the roof vent a few hours ago and I have zero condensation now.

A thought: does anyone/everyone use a dehumidifier when in humid climates (coast)? I never have and have never had this issue when here but thought I might need to do it. If the consensus is no one does, everyone just moderates the humidity with various measures, then I'll cancel the order along with my drive into Tallahassee tomorrow - a tremendously frustrating experience. Looking forward to your thoughts.

[MattHelm21]

Except for a few days after being shut up all winter, I've never used a dehumidifier. That said, I wouldn't hesitate to use one if I noticed a condensation problem in the rig. The Sears model I did use those few times outputs heat as a byproduct.

[Tbos]

I've got a friend that runs one in his all the time. When camping and using the heater I will run a small dehumidifier. Seem to help some especially in areas that don't have vents.


2016 Passport GT 2810BHS, 2014 Silverado 1500 LT in Deep Forest Green

[Javi]

Originally Posted by sourdough

Thanks all. I have a Dometic and I've opened it up and the condensation is coming from it being cold and the humidity. I opened the roof vent a few hours ago and I have zero condensation now.

A thought: does anyone/everyone use a dehumidifier when in humid climates (coast)? I never have and have never had this issue when here but thought I might need to do it. If the consensus is no one does, everyone just moderates the humidity with various measures, then I'll cancel the order along with my drive into Tallahassee tomorrow - a tremendously frustrating experience. Looking forward to your thoughts.

Both roof vents open and if we're in the trailer the Maxxfan fan is on low...

[Ken / Claudia]

John, I am attempting to follow your post and understand it since you do have great advice. Maybe different from where I live than in Southern states. What I do is leave vents open almost all the time in storage or in use. Does that cause more moisture problems? or is it just when using the A/C and having vents open. When I use the A/C I close vents part or full just to keep cooler air inside. Never thought about more or less moisture regarding the vents and AC use.

[JRTJH]

Usually, when it rains, the air is completely saturated with water and can't hold any more. As the air cools, it releases that moisture as rain or dew/fog. The air inside an RV seldom gets "that saturated", but also holds water, largely depending on air temperature. When the A/C is running, the air is cooler and drier, so the humidity inside is less than it is outside the RV. As the air inside the RV cools, it can hold less moisture, so the A/C actually "dries" the inside air. But, as that air warms up while in the RV, it can absorb more water from the surrounding air.

Leaving a vent open or leaving a window open will allow the outside "warm, wet" air to migrate into the RV. As that outside air "warms" the inside air, it also transfers moisture to it, sort of like a sponge transfers moisture from a wet surface to the body of the sponge.

The problem with "wet air" inside an RV is the potential for condensation to occur on surfaces that are below the dew point (temp where water will condense on the surface). There are two different issues with "humid air" inside an RV. One is in the "bowels of the RV ceiling or attic" and one is in the "people space". As you blow "cold dry air" through the ductwork, the ducting will get cold. If its temperature falls below the dew point, water inside the "dead air space" in the ceiling will condense on the colder duct surface. That condensation will either drip into the insulation or run down the ductwork to the lowest point. If it's a vent, then it will drip into the RV interior. If it wets the insulation, then you can imagine the potential for mold, mildew or damage to the luan ceiling in the RV. The water that condenses on the ducting in that "closed ceiling space" is resupplied by migration of wet air from inside the RV as well as from leaks/cracks and absorption through the luan ceiling.

But, the second, and I think biggest problem with humidity, in my opinion, is what we experience because we're sitting in it "inside the RV". When you leave a roof vent open, it makes the A/C work harder trying to first, cool the air coming into the vent but also to dry the moisture from the air that's coming in through that vent. It'll eventually catch up and lower the interior humidity, but the cold air blowing from the vent will cool the surfaces of the trailer upon which that air blows. When the surfaces are colder than the air coming in through the vent, that moisture will tend to condense on the walls, oven door, or wherever that cold air is blowing. That means the water never gets to the A/C coil where it "should be collected" as the air flows through the A/C, rather it stays in the RV "interior space" where it makes the air uncomfortably wet and muggy or fogs the windows.

If I remember correctly (it's been a long, long time), the recommended "summer/AC" humidity is below 40% and the recommended "winter/furnace" humidity is 40% - 60%. There's a lot more that goes into comfort, like the ability to evaporate sweat from the skin, facilitating cooling and keeping that sweat "on the skin" to prevent drying and irritation in the winter. So, it's not to say those percentages are "hard and firm" but rather "people are most comfortable and heat/cooling efficiency" is optimized at those humidity levels.

Think of it along the lines of leaving your refrigerator door "slightly cracked"... Not only will the refrigerator run longer to recool the "lost air", but you'll notice frost, then ice forming along the edges of the door where the cracked door opening is located.

ADDED: Leaving the vents open in an "uncooled/unheated" RV while in storage is a good idea. First, as the temperature rises, the temperature and humidity are "normalized" or equaled both inside and outside the RV. Then, as the temperature cools in the evening, it also cools inside the RV. That allows the excess moisture to migrate through the vents and the water is "drawn out of the RV." If the vent's were closed, the interior of the RV would remain "wet" (if it was stored with high humidity) and that water would condense on the cooler interior surfaces inside the RV. That would lead to water damage as the heating/cooling cycles continue (high daytime temps/cooler night temps). The RV will absorb moisture during the day when it is warm and release it at night when the walls, floor and ceiling are cooler. Some call it "sweating".....

[sourdough]

I used to always do what John says and leave the vents/windows everything closed and it always worked, but I was usually in pretty dry climates. In the past here in FL I've noticed small amounts of condensation on the AC unit in the ceiling; not dripping, but a sheen more or less. I opened the vents (started in 2014 and it seemed to help). Prior to this event we lost power for over 2 hours, it was cloudy and the humidity was over 90%...and it was warm. When the power came back on I turned on the AC and closed the windows leaving the vents open. Right after we got the AC on it began to rain hard and that's when DW closed the vents and we had the dripping from the AC shroud.

In the sequence of events above I do believe leaving the vents open probably hindered progress on getting things cooled down and contributed to even more humidity in the air. By the time I reopened the vents the air had been dried out pretty much and probably why things seemed to get drier.

Upon inspection last evening I found small traces of what appears to be mold on the outside of the vent - nothing anywhere inside the AC unit, just right on the edges of the output grate. We've had as much, or more, humidity here this winter than we do in the spring/summer; it's just cooler and you don't feel it as much. I bought the dehumidifier today and will be putting it to work this evening to try to remedy the situation. I'm also leaving the windows/vents etc. all closed so I don't complicate the problem. I suspect this will take care of it and I'll have to bring it with me when I come here every year.

[Ken / Claudia]

Thanks John, good information. Including the reference to the refrigerator helps make the point as I and maybe most with kids can relate to.

[Jay Pat]

I don't keep track of every situation.
Last month, we traveled to AZ.
Had condensation that lead to drops of water puddling on bottom of window.
Next night, pulled out the electric dehumidifier and condensation was greatly reduced to very small minor droplets. No puddling.
We had rainy conditions and near low 30's some nights.
Can't run heater and a/c at the same time. Heater only was used.
Hope this is helpful.
Pat

[66joej]

Also thanks from me too. I believe we have some very good and knowledgeable people on this forum and John is right there at the top. Always giving excellent and most times the solution to our problems. Don't sell your Keystone!