I was visiting one of our local RV dealerships when I ran into a friend of mine. He and his wife were looking at a new fifth wheel trailer to replace their 8-year-old Montana.
As we were talking, he mentioned that he wanted the thermal pane windows on their new RV. He was tired of having to take a towel and wipe the condensation off his single pane windows that accumulated when they were camping in cool or cold weather.
He seemed determined to order a trailer with this window option, so I decided, “Not to go there” and withheld my opinion on double pane insulated glass windows in a RV.
You see, I don’t believe they are the solution for eliminating condensation. In fact, I believe the single pane window that “sweats” in cool weather to be a good thing.
Single pane windows are the least insulated part of an RV. It is where the most heat loss occurs and will be the coldest surface when outside temperatures drop and we turn the heat in our RV on.
As occupants living in an RV we are naturally going to create humidity in the air. We do this by breathing, cooking, bathing, washing dishes, and more just by being inside.
Unless this excess moisture is reduced by something like a dehumidifier or a lot of ventilation, it is going to condense when the air moves across the cold glass window. This system of condensation actually acts to remove excess moisture from the air. If you elect to occasionally wipe the windows down with a dry towel and then take the towel outside, you will effectively be removing the excess moisture from your environment. This is, of course, a lot of trouble but it actually can produce a good thing.
Let’s turn our attention to another aspect of how an RV is built for a moment. This information will, I hope, give some insight to how a nuisance problem like wiping water off of the inside of a window can keep you from having even more serious problems later on.
The average camping trailer’s walls are only 2” thick. The outside may be covered with aluminum siding or a solid fiberglass surface. Trailer’s with a fiberglass outer surface usually have a substrate made of wood or composition hardboard behind the fiberglass coating.
The walls may be framed with either wooden or aluminum studs. The inside wall is either wood or composition hardboard (I call it paper wood) covered with a thin vinyl wall covering. The space between the inner and outer wall is filled with fiberglass batting as an insulation material. (NOTE: Some trailers use an entirely different construction method – check with the manufacturer to discover how the walls are built.)
Your inside walls are not really all that well sealed. There are small openings around switches, electrical outlets, window frames, door frames, floor and ceiling joints, cabinets, cord hatches, etc. If you happen to put any screws, nails, or picture hangers in these walls you add another leakage point.
Now visualize this: If the cold windows cannot condense the moisture in the air so that it can be removed, the moist air will inevitably move into the inner wall cavity. There it will meet the cold outer wall and condense back to water just as it does on the windows. But, this time you cannot remove the water with a towel. It is in the wall where there is poor air circulation and will be absorbed by the wall substrate, insulation and perhaps wooden framing. Over a short period, this moisture will begin to grow mold and even cause wood to rot and cause wood or paperboard walls to de-laminate and buckle.
It becomes an insidious down hill ride. The moisture has to go somewhere. If it can’t condense on the windows it will condense inside the walls.
Of course, there are some stopgap measures that you can take to prevent condensation and moisture migration. Number one is to reduce the addition of moisture inside the camper by putting wet towels from bathing outside to dry (even if they freeze!). Use the vent fan when bathing. Avoid cooking on the LPG burners or with the oven.
I have a small electric dehumidifier that we keep in the camper during cold weather. It does seem a little odd because we typically use a dehumidifier in our homes during the summer months – and a humidifier in the winter to add moisture. But, a RV is NOT built like a typical house.
I use the windows as my meter for what to set the dehumidifier dial on. If I see water beginning to form on the windows, I turn the dehumidifier up enough to stop the condensation.
So, you see, what may seem like a solution to one problem can create an even bigger problem.
Now, that said, a RV with thermal pane windows and a dehumidifier inside during the winter would be a nice combination. We could reduce heat loss from the windows and also remove moisture! Having an inside humidity meter can help to keep the air from becoming too dry. About 30% humidity is a good figure to shoot for. Higher will cause condensation, lower will dry out your sinus passages.
There is one RV wall construction method that I really like more than the ones I described above. If the inner and outer wall surfaces are vacuum bonded to a solid polystyrene inner wall to make a solid, no air space wall the problem of moisture trapped inside the wall is virtually eliminated. This wall construction method is becoming increasingly popular – especially on lightweight models since it eliminates a considerable amount of structural framing.
Another material making it’s way into RV construction is called azdel. It is a plastic-wood composite that is moisture proof and lightweight. When used as a backing on outer fiberglass laminated walls and inside walls, it reduces the problems caused by inner wall moisture, mold and rot – especially if the RV also has an aluminum frame.
Unfortunately, I learned all of this the “hard way”. No one forewarned me of the condensation problems that could develop in the inside of an outside wall. As moisture began to collect in the wall, the resultant damage was not immediately visible. Then, one day I cut a slot into the wall for an outside mounted radio and speakers. That was when I discovered in horror rotting wood and mold inside the walls on a three-year-old camper. Obviously, we no longer own that camper. It was repaired by removing the outer aluminum skin and insulation. The rotted wood and insulation was replaced and the mold professionally removed. When the trailer was sold, the buyer had a full disclosure of the repairs.
I have found that running a medium size refrigeration style dehumidifier year round continuously works best in my 37 foot diesel pusher. I get about 3/4 gallons per day in summer and over a gallon a day in the winter. I have not had problems with mold since I started doing that.
First, very good job on your article.
Second, I’ve been looking through a lot of models of RV and I can only find fibreglass backers made of luan. Do you know of any models that has “compsition hardoard” that you mention or “paperboard” as a fibreglass backer.
Thank you in advance,
trailer’s with a fiberglass outer surface usually have a substrate made of wood or composition hardboard behind the fiberglass coating.
Wow… I do use the fans and try to keep the humidity down in the 5th wheel. This is a great article and quite an eye opener. Who would think that such damage could be going on. YOu can bet that we will be getting a small dehumidifier for our rig.
Thanks a million.
Howdy Prof. Randy 95,
Thank you for the 5***** article….
Virtually all RV roof-top air conditioners have a thermostatic control module that prevents the compressor from running if the outside temperature is below 60 degrees. This protects the A/C from dangerous freeze-up. In the summer the A/C DOES work as a dehumidifier – and an extremely efficient one at that!
You are 100% right on the difference in volume and moisture build up between a camper and house. The thin walls add to the problem, especially in 40 degree and below temperatures (a lot will depend on the dew point and current humidity).
One thing I did not mention and should have: The new small-space dehumidifiers that use a Peltier or thermoelectric cooler rather than a conventional refrigeration compressor WILL NOT provide adequate dehumidification inside an occupied camper during cold weather. Neither will the canisters using desiccant crystals – they simply do not have the capacity needed..
Thanks for your excellent add-on information.
The other element to keep in mind when looking at humidity in an RV vs. a house is the volume of air: an RV is orders of magnitude smaller in volume than a house (e.g. a travel trailer with 25 of living space is really about 25x8x7, or about 1,400 cu. ft. compared to a 24×40 two-storey house with a basement, which can be about 23,000+ cu. ft). Two people in 1,400 cu ft put out a lot more water vapor (relatively speaking) than those same two people in 23,000 cu. ft. of house. A dehumidifier for an RV makes a lot of sense in the winter. In the summer, the AC would presumably act as something as a dehumidifier, but I’m not sure you would want to run it in the winter :-).