When it gets cold and dark outside, the last thing you want is dank, cold, and dark inside. But an RV does not present an easy task for environmental control, especially of humidity. So condensation on the inside can be a problem in an RV when it gets cold outside. This condensation can be as bad as a water leak when it comes to the possibility of damage to the RV.
In order to get condensation, two factors have to work together. The first is evaporating moisture into the air and the second is changing the temperature of that air enough to cause the moisture to condense back out of it. In between there is the movement of air caused by various mechanisms such as convection and moving objects (such as people).
Perhaps the most important sources of moisture inside the RV are washing, especially with warm or hot water, and combustion such as in the use of the stove and refrigerator or auxiliary heaters. People also tend to contribute quite a bit of moisture as well. If the air is cold, you can see your breath as its moisture condenses. When the air isn't cold, the moisture in your breath contributes to the humidity.
Since an RV is a confined and small space it won't take a lot of moisture added to the air within it to load up the air with moisture. Activities that add moisture also tend to make the air warmer and warmer air will hold more moisture.
There are obvious places where condensation will occur such as on single pane windows as the glass is not a good insulator and will be near outside temperatures. The frames of windows, if made of metal, may also prompt wetness. Condensation can also occur in other places, too. Have you ever looked at the metal parts around the refrigerator burner and wondered why they looked so rusty (or your automobile tail pipe!). Also inspect those hidden areas in your RV where the air is relatively still behind a couch or cabinet near the corner of the floor and an outside wall?
Ventilate! To reduce or prevent condensation you have to remove moisture. This means moving air that is likely to be nearing its dew point to somewhere else so it can increase its temperature or mix with air that has a bit less water vapor in it..
When washing in the kitchen or bathroom, crack the window or turn on the vent. Yes you may waste some heat but you will also greatly reduce the moisture in the RV.
Keep absorbing (think rot) or reacting (think rust) materials away from places where condensation may occur. Water soaking into wood or cloth over time may cause rot or other unpleasant deterioration. If things do get wet, make sure they get dried through and through as soon as possible.
Provide a means for any water that does condense to escape. Provide some sort of drain to allow any condensed water to run off to someplace where it can dry out or not cause harm.
The typical RV rooftop air conditioner refrigerates air by passing it over cold pipes. Your refrigerator works by putting cold pipes in the insulated box. In both these cases, water can condense on the cold pipes and may even freeze on the pipes. Air conditioners usually just drip this water down the side of the RV, which may cause unsightly stains. Refrigerators will usually freeze condensed water so that you will need to defrost your refrigerator occasionally.
The swamp cooler or evaporative cooler works in just the opposite way. In dry climates where the air is warm and the humidity is low, you can put water on a pad and blow air through it. The water will evaporate which causes the air to become cooler and more humid.
To evaporate water, energy must be added to it. This energy comes from the surrounding air, which makes this air cooler. To condense water, energy must be taken away from it. This means that water condensing on something is trying to warm that something up.
Water is normally a liquid or solid at temperatures we can live in but it evaporates to a gas up to a point where the water vapor tends to push back and limit further evaporation. The two main factors that limit how much water vapor can be evaporated are temperature and the amount of water vapor currently in the air. Meteorologists measure both of these factors to be able to determine what the moisture in the air (water vapor) might do. Relative humidity is the amount of water vapor in the air as a percent of the total amount possible. The dew point is the temperature at which the water in the air would hit maximum possible amounts and start to condense. For any chunk of air, its relative humidity will depend upon its temperature while its dew point will remain constant.
You might note that a very cold, dry winter day might have a high relative humidity. This is because cold air cannot hold much water vapor so what it little has is a high percentage of what it can hold. When this air is moved into your RV and heated, its relative humidity will become very low as the now warmed up air can hold a lot more moisture. Whether the air is hot or cold, its dew point will stay the same. When the temperature gets down to the dew point temperature, you begin to see things like fog or pogonip.
Condensation can also cause problems for the RV mechanic. Condensation can occur inside the wheel bearings if they are not regularly exercised. This can cause rust and bearing failure. The smell they put in propane can also condense in the gas pipes. If this condensate gets into the regulator it may fail. So mount your regulator near the top of the tanks so that anything that condenses in the tubes feeding it will drain back to the propane tanks.
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