Dealing with moisture in your motorhome

[tbutler]

Water vapor collecting on the interior of a motor home can cause problems from fogged windows to mold and delamination of wall and roof panels. Preventing this accumulation of moisture inside the motor home is the purpose of this article. I have seen discussions of this problem with varying levels of understanding. I hope this helps to advance the understanding of water vapor problems in motor homes.

There are several reasons that moisture control in a motor home is more difficult than in a sticks and bricks house. Size is a big factor. Inside my Class A motor home, with about a 360 square foot floor plan, I have less than 2500 cubic feet of air. In a small home with a 1000 square foot floor plan there are about 8000 cubic feet of air. More air allows more water vapor to be absorbed before noticing a problem. The smaller your motor home, the more extreme this factor becomes. The other major factor is the difference in insulation. Even the best motor homes have marginal insulation compared to houses. If the insulation is lacking or thin in some areas of the motor home, those areas will be most strongly affected by moisture accumulation.

As a result, normal activities you do in a house now cause problems in the motor home. Activities which account for most of the problems are cooking, washing and showering. These activities involve heated water which enters the air in quantity. This moisture will be removed from the air by any cold surface in the motor home. In our motor home we have pretty good insulation and sealed double pane windows everywhere but the windshield. So the first place we see moisture accumulate is the windshield. I have read of complaints of others finding moisture on outside walls behind the couch or inside cabinets. The latter two are likely cold because heated air in the motor home is not circulating to those locations. As a result, the wall surface there is colder than other surfaces in the motor home. Water vapor however does not need forced air to move it from one place to another. Water vapor will spread evenly throughout any mass of air, moving from areas of high concentration (like the shower or pot on a stove) to areas of low concentration, a process called diffusion.

Since cold surfaces take water vapor out of the air, converting it back to water, areas near cold surfaces will be areas of low concentration of water vapor and it will continue to migrate to these locations and water will continue to condense until the level of water vapor in the entire motor home has dropped or the cold surface has warmed.

As a result of the above, the time when you have the most serious problems with water vapor is during cold weather. The solution to the problem is simple though not entirely obvious. Any time water vapor sources are active in the house, showering, cooking, even washing dishes, the water vapor must be actively removed from the motor home. By actively removing water vapor, I really mean using the ventilation fans to remove water vapor. Any time you use a vent fan, you must have an open window or other vent to allow outside air into the motor home. Now, if this is only a problem during cold weather that means you must allow some cold air into the motor home as the vent fan pushes out the moist air. Even if it is raining outside, the air you bring in will have much less moisture than the air being exhausted. Cold air can't hold as much water vapor (this is why cold surfaces remove water vapor from the cold air near their surface).

So there you are, taking a shower on a cold day and I am telling you to turn on the vent fan and open a window! If you do this, your windows and windshield won't fog up, moisture won't collect on walls or ceiling. Put a small electric heater in the bathroom while you shower to deal with the incoming cold air. The furnace or heat pump will take care of the cold air in the rest of the house. Yes, it costs more to heat that cold air but you only need to do so during the brief period while cooking, washing or showering.

An additional step you can take is to dry the shower after use. If you step out of the shower and leave it wet, that water will evaporate into the air and will contribute to any moisture condensing on windows and walls. I use a squeegee to wipe the water from the walls and door and then sweep the water from the floor down the drain. If you want to keep the shower really clean, you can wipe any remaining moisture off the walls and door and you won't have any water spots. But this moisture wiped off with a cloth or sponge will enter the air in the motor home fairly quickly so it won't reduce the moisture problem. That is why I use the squeegee first. Water down the drain is not going to come back into the air in any significant quantity.

Finally, I want to mention problems caused by water vapor in warm weather. If you are running your air conditioner, you should be following the same rules above to save money rather than to maintain your motor home. The water that drips from your air conditioner is water that condenses from the air in your motor home as it is passed over the condenser coils of the air conditioner. Water vapor carries heat energy. When water evaporates it removes heat (think of what you feel like when you get out of the swimming pool). To condense the water out of the air in the air conditioner takes energy. Heat from the water vapor is transferred to the coils as it changes back into water. This makes the air conditioner work harder (or longer). If you aren't paying an electric bill I guess you wouldn't care about the cost of operating your air conditioner but the park owner will be concerned about that cost and it is factored into the rate you pay for your site. Ventilating to remove excess moisture from the air while showering or cooking, even if you are drawing in warm outside air, is more efficient than having the air conditioner do it. In very few cases would the incoming air contain more heat than the heat contained in the water vapor you are removing with the vent fan.

A few basic facts. Water moves heat energy around very efficiently. A gram of water (a very small quantity - about 30 drops) requires 1 calorie (a unit of heat energy) to raise its temperature 1 degree Celsius. When you melt one gram of ice at 0 C and change it into one gram of water at 0 C it requires 80 calories. This is why ice is so efficient at keeping your drink cold. As the ice melts it absorbs a large amount of heat, 80 calories, for each gram of ice melted. That same 80 calories would raise the temperature of one gram of your drink 80 degrees Celsius. Now, as spectacular as this seems, the numbers are even more amazing for the water to water vapor change. Changing one gram of water to one gram of water vapor takes 540 calories. This is true whether boiling water on a stove or evaporating water from your skin when you get out of the swimming pool. No wonder water cools you so nicely as it evaporates. So water vapor carries this huge amount of energy with it. When it condenses on the windows of your motor home it is warming them, that heat is then transferred to the cold air outside. When water vapor condenses on your drink glass it transfers that heat energy to your drink. This is why even a thin insulating layer on a glass will efficiently help keep the drink cold. One gram of water vapor on the side of your drink glass will warm one gram of your drink 540 degrees Celsius! Or another way of saying that is to say one gram of water condensing on the outside of your drink glass will raise the temperature of 540 grams (about 10 ounces) of your drink one degree Celsius!

[pnixon235]

Ok Henry, I'm guessing "PE" or HVAC Tech. Lots of good information here. Thanks!

[734greg]

Good article concerning moisture inside the motorhome. My unit shower has always been squeegeed and wiped dry after every shower in an effort to control the moisture. Any acitivities that use water inside the coach requires opening top vents or windows in my place. Thanks again.

Bob Gregory

F296139

"Full Timer" and love it.

[tbutler]

Ok Henry, I'm guessing "PE" or HVAC Tech. Lots of good information here. Thanks!

Nope! Just your run of the mill science teacher!

[wolfe10]

And for those who store their coaches in areas with hight humidity and have 120 VAC available, a small house-type dehumidifier will cure/prevent the mold and mildew "musty smell" .

Set the dehumidifier on the kitchen counter with the drain into the sink Just set humidistat at 45-50%.

They use very little electricity, because once humidity is reduced, they run very little.

We have used them for years on our sailboats and RV's here in humid south Texas.

Check at Home Depot, Sears, etc for their smallest unit that allows you to select humidity setting.

Brett Wolfe

[Roodriver]

Now I know the forum is in trouble with two science teachers!

[xctraveler]

I guess I will have to add the squeegee routine to the open vents went showering.

Thank you for the great explanation.

[mavric65@aol.com]

Nope! Just your run of the mill science teacher!

I would like to know if a dehumidifier wouid help ... I have been loking at a small portable unit

Thanks Mike

[tbutler]

I would like to know if a dehumidifier wouid help ... I have been loking at a small portable unit

Thanks Mike

Mike,

Dehumidifiers work better than the chemical driers which are effective only in small spaces such as cabinets or small closet spaces. I used a chemical desiccant in the cabinet over our bed when I had a leak in a slide out. It kept the moisture at a low level in that space until I could get repairs done. As my article points out, the key to controlling moisture in your motor home is ventilation. Allow the moist air to escape by ventilating, with a fan if necessary. A fan is certainly necessary when cooking or showering but may also be necessary when you have large numbers of people in the motor home. This is especially true if they have damp clothing or are breathing heavily. Bring in outside air which will be lower in moisture content and, if necessary, warm it to stay comfortable. This is the most effective way to deal with moisture. If you really don't want to open windows and use a vent fan, then use a dehumidifier.

[mavric65@aol.com]

Mike,

Dehumidifiers work better than the chemical driers which are effective only in small spaces such as cabinets or small closet spaces. I used a chemical desiccant in the cabinet over our bed when I had a leak in a slide out. It kept the moisture at a low level in that space until I could get repairs done. As my article points out, the key to controlling moisture in your motor home is ventilation. Allow the moist air to escape by ventilating, with a fan if necessary. A fan is certainly necessary when cooking or showering but may also be necessary when you have large numbers of people in the motor home. This is especially true if they have damp clothing or are breathing heavily. Bring in outside air which will be lower in moisture content and, if necessary, warm it to stay comfortable. This is the most effective way to deal with moisture. If you really don't want to open windows and use a vent fan, then use a dehumidifier.

Thanks for the input i will try that next time we use our coach

Mike & Christina

[jomarfl]

Great article and we use the open vent options. You don't need the vents wide open, just a small opening with a few cranks and run the fan and you good to go.

[mavric65@aol.com]

Thanks for the input i will try that next time we use our coach

Mike & Christina

Well i took your advise on our last trip and left the windows cracked and i still bought a small dehumidifier and put it in the bathroom which is in the middle of the coach and wallah!!!!!!! no moisture and the dehumidifier collected about a quart of water also

Thanks for the help

Mike & Christina

[tbutler]

Mike and Christina,

Glad to hear your results were good. As the weather gets colder, you'll find these same principles work but you'll have to be more aggressive with them. Turn up the ventilation when at times when cooking or showering to keep the moisture from accumulating in the air. The colder the surfaces of the motor home the easier it is to get condensation on them. Enjoy your travels.

[Karensue4789@sbcglobal.net]

There is a product available in a simple Hang up form called "DAMP RID" and it is amazing. It will suck up the moisture in any room you need to get rid of that damp musty smell. We hang them in the closet, the bathroom, the kitchen, under the coach in the Bay compartments and they will amaze you. They remove the moisture from the air and get rid of the musty smell and I think you will be pleased with this product. It is available at most Ace Hardware Stores, Wal-Mart, K-Mart etc.

[wolfe10]

There is a product available in a simple Hang up form called "DAMP RID" and it is amazing. It will suck up the moisture in any room you need to get rid of that damp musty smell. We hang them in the closet, the bathroom, the kitchen, under the coach in the Bay compartments and they will amaze you. They remove the moisture from the air and get rid of the musty smell and I think you will be pleased with this product. It is available at most Ace Hardware Stores, Wal-Mart, K-Mart etc.

The "Chemical-based absorbers ARE fine for closets and small areas, but you do have to dry them out periodically. Said another way, they will only absorb so much (measured in ounces or pints TOTAL capacity).

That is why the "real" dehumidifiers that are compressor driven are so much better if you store in a humid climate with access to shore power. Our LG dehumidifier (from Home Depot for under $200) removes 35 pints PER DAY. What happens in an RV-sized space is that it runs for a couple of hours and removes 1-2 quarts, then runs less than 10% of the time, since there is no outside source of moisture.

We just set ours on the galley counter and it drains into the sink/gray tank.

Brett Wolfe

[logintl@yahoo.com]

Interesting article...tks. I am fortunate to garage my MH (metal roof & sides, etc) with full 50amp electric but, it gets hot and humid in Georgia and I actually set my A/C at 80F. I have not noticed any moisture but worry about the additional wear on the AC units.....would appreciate comments re; the AC use at 80F. Tks, JMM

[wolfe10]

Interesting article...tks. I am fortunate to garage my MH (metal roof & sides, etc) with full 50amp electric but, it gets hot and humid in Georgia and I actually set my A/C at 80F. I have not noticed any moisture but worry about the additional wear on the AC units.....would appreciate comments re; the AC use at 80F. Tks, JMM

JJM,

Welcome to the FMCA Forum.

We live in a similar climate-- south Texas a couple of miles from the Gulf of Mexico. Plenty of humidity and heat.

We do this in both our stick home and RV when not in use: Use a 45 pint dehumidifier in RV and home. A/C OFF. We have found that we have far less of the "musty smell" than setting the A/C on a reasonably high temp. AND electrical consumption is lower.

This works fine unless you want cool temps in the RV. If you only go in the RV occasionally, just turn off the dehumidifier and turn on the A/C. It will cool quickly, as no BTU's are wasted on condensing moisture.

Brett Wolfe

[logintl@yahoo.com]

JJM,

Tks Brett....since we live in area's with similar climates (heat and humidity) I was wondering why you have a 45 pint dehumidifier, seems a bit large for MH (aprox 400-500 sq ft). Is it possible that you fill the 45 pts in a 24 hour period ? I see in earlier posts that a smaller unit was mentioned (16 oz) but, that appears to be quite small for the 400-500 sq ft space. tks, JMM

[wolfe10]

JJM,

Tks Brett....since we live in area's with similar climates (heat and humidity) I was wondering why you have a 45 pint dehumidifier, seems a bit large for MH (approx 400-500 sq ft). Is it possible that you fill the 45 pts in a 24 hour period ? I see in earlier posts that a smaller unit was mentioned (16 oz) but, that appears to be quite small for the 400-500 sq ft space. tks, JMM

JMM,

We do have a smaller one that we use in the sail boat and motorhome. But, it is on the boat right now. So the 45 pint shares duty on the RV and home. The small one is much easier to stow while sailing that the big one. And, yes the small one would be easier to handle on the coach as well and has plenty of capacity.

The 45 pint is certainly overkill for the motorhome, but since it also does duty in the house when we leave in the coach, the size is OK.

And irrespective of size, the newer/better dehumidifiers have a humidity settings. So if set to 45% humidity, they will drop humidity to that level and then run very little unless outside air is introduced. The smaller ones take a little longer, the large ones a little less time, but no big deal either way.

And by setting them on the kitchen sink (home and coach) they drain into the sink, so no buckets to empty.

Brett Wolfe

[chrisastream]

I want to thank you so much for that article, I was having lots of moisture on my front windows, to the point of having to put towels on the dash. I was going into storage in Texas in January and there was lots of freezing rain. Of course this is probably too late for that but I feel that I can understand the condensation in autos as well now.

I had been using the moisture crystals and they do work but I'm not sure how well. So far I haven't seen any signs of mole; don't want to either.

Thanks again

[DoggyDaddy]

Great.... One more item, a dehumidifier, to put on my wish list for Santa.

[dossb@mac.com]

In the NW (Puget Sound area West of Seattle) our moisture problem usually comes in the winter with the rains. We live in our RV year around. Would you guys recommend a dehumidifier or just keep the windows cracked with fans running? Any help would be appreciated.

Just as a side note, my wife is not excited about having a dehumidifier taking up counter space and "looking ugly" with a hose draining into the sink.

[dennis.l@mccanna.ws]

Very good article, informative. Would like to add that if you have the space available a de-humidifier will keep the humidity down to resonable levels, which is what we use. Of course if venting you are bringing in cooler drier air (from the open window) but then you have to heat the occupied space back up. Either way we are sucking up utilities to keep the humidity down.

[Fairfranky]

On 9/24/2009 at 9:01 AM, dennis.l@mccanna.ws said:

Very good article, informative. Would like to add that if you have the space available a de-humidifier will keep the humidity down to resonable levels, which is what we use. Of course if venting you are bringing in cooler drier air (from the open window) but then you have to heat the occupied space back up. Either way we are sucking up utilities to keep the humidity down.

Absolutely agree and sorry for posting here. But it's still solves our problems

[PaHunter]

Instead of a dehumidifier we use a air dryer, used on boats to remove moisture. They are made by Davis, use very little power and no water to worry about.