[NOTE: I ve blogged all these how-to posts before, but am gonna re-run them on occasion, since folks may have missed them, and many of them have been updated or expanded since they were first posted.]
There is one weather condition about which a van-dweller can do very little: when it is hot outside for a few days, you will sweat. Unfortunately, there is also another weather condition about which a van-dweller can do very little: vans are notorious for producing heavy condensation in cool weather, particularly in the winter. It s hard enough dealing with a long period of rain, when you unavoidably drag water inside the van and get everything wet. But it s even worse all winter long when there is so much moisture and condensation inside the van that it is practically raining from your roof.
Time for some more sciencey stuff:
As we know, the amount of water vapor that a given mass of air can hold (its humidity ) is determined by its temperature warm air can hold more water vapor than colder air can. And once the humidity of an air mass reaches 100%, it cannot hold any more water vapor.
So, what happens inside your van in wintertime? No matter where you are, the inside of the van will become warmer than the outside, simply because it will be heated by the sun. If you have any sort of heater in the van, that also warms the interior.
Further, your normal daily activities will also tend to increase the humidity level inside the van. Cooking, washing up, doing laundry all of these release water vapor. Even while you are asleep, you are constantly exhaling water vapor in your breath around a pint or so of water every night (equivalent to a 16-ounce Coke bottle full). And so, as long as your van is sealed up against the outside cold, you will be inevitably producing an atmosphere inside that is warm and humid.
When warm humid air meets a cold surface (such as the metal sides and roof of your van in winter), the result is condensation . As the warm air cools, it loses its ability to hold water vapor, and at a particular point, known as the dew point , it will begin to release water vapor as tiny droplets of liquid water which will coat the colder surface with which the air is in contact. The greater the temperature difference between the warm inside and the colder outside, the more water droplets will be deposited, and the higher the humidity inside, the higher the dew point will be and the easier it is to produce condensation. The result is a thin film of water that will form on the inside of your van: on the windows as a fog, and on the metal sides and roof as water droplets. If enough water droplets form, they will run together and begin to drip and your van will sweat .
This is bad news. Although the metal surfaces of your van have been painted to protect them, there are always tiny cracks and flaws in the paint, which this layer of water will find and penetrate. When metal and water meet, the result, sooner or later, is rust . And rust is a van-killer: the majority of campervans and RVs probably do not die from mechanical breakdown they die from rust damage.
Further, these wet, warm and humid conditions are perfect for the growth of mold and mildew inside the van, which will not only attack your carpets, clothing, and bedroll, but can present a health hazard to you. It therefore becomes a matter of importance to protect the interior of your van from condensation. And unfortunately, your options for this are limited.
There is only one good way to deal with condensation in the van, and that is through ventilation and airflow. By exchanging air between the interior of the van and the exterior, you are dumping the excess humidity outside before it can condense. And as a side effect you will be lowering the temperature inside the van and making it more equal to that outside, which lowers the level of condensation.
The extent of airflow needed to prevent condensation depends on the temperature and humidity, but it is not a trivial amount. At minimum, you will have to at least crack open your windows to allow an exchange of air between inside and outside. A better option is to run a ceiling ventilation fan to continually dump humid air to the outside. You will know you have sufficient ventilation when the condensation goes away and does not reappear.
Good ventilation is particularly important whenever you are doing anything inside the van that generates water vapor. This includes heating or boiling water while cooking, washing up or doing dishes in a sink, doing laundry and, especially, drying wet clothing. If you are using a propane heater inside the van, these also put out a considerable amount of water vapor when they burn (almost one pint of water for every pint of fuel), and good ventilation is an absolute necessity (not only to help prevent condensation, but also for safety reasons).
Many van-dwellers, however, will not like the necessity of allowing winter air inside their van (after all, it s cold outside and people want to stay warm), and, with insufficient ventilation, will instead search for different ways to absorb the moisture inside their living space before it can condense on the walls. And so, in any van-dwelling forum when the subject of condensation comes up, there will always be a few people who will mention these cures . Sadly, the harsh truth is that none of them works very well.
The first option that usually comes up are vapor barriers . Usually installed underneath the insulation, these are waterproof plastic sheets which are attached directly to the metal walls and ceiling. In theory, they prevent any condensation from reaching the metal and thereby prevent rust and corrosion. In reality, they are never completely vapor-proof and will always allow at least some moisture to penetrate. My own view is that if the van has sufficient ventilation, then vapor barriers are simply unnecessary. (It may be of limited use in areas such as the Pacific Northwest, where constant cool and humid conditions make ventilation almost futile. The brutal reality though is that if your local climate is chilly and damp, or if it rains a lot, there is virtually nothing that you can do to prevent condensation from becoming a constant problem.)
Another option that usually comes up are the commercially-available moisture-absorbers , available in hardware stores, which are intended for use in keeping mold out of closets and clothing chests. These come in either small packets or sometimes in 4-pound buckets. They usually contain powdered calcium chloride, which absorbs water: over time as it is used up, the powder is converted into a caustic liquid that remains inside the packaging and is disposed of once it is exhausted. The idea is that by hanging some of these packets around the inside the van, you can absorb the excess humidity and prevent condensation from forming.
In reality, these packets are only intended for small areas like closets or drawers, and to cover a volume the size of a van interior, you would need a lot of them. Further, since they absorb only small amounts of water, they will be quickly overwhelmed and saturated, and you will need to replace them often. I once experimented with them by distributing an entire box of these packets onto the dashboard overnight and sealing up the van. In the morning, there was condensation all over the windows, including the area literally an inch away from the pile of absorbers. They simply don t work well enough. The same goes for clay cat litter, silica-gel packets, or any of the other moisture-absorbers that people have tried. Either you will be changing them out constantly, or they will require such a large amount as to be impractical.
Some van-dwellers will also point to the small electric-powered dehumidifiers that are available in stores: these use pumps to draw air in and extract the water vapor, condensing it into liquid water which can then be dumped. But these too will not be a practical solution for most people. Any dehumidifier large enough to effectively reduce the condensation inside a van will be large and use a significant amount of electricity.
So, once again, the only really effective solution to condensation is to provide good ventilation, with enough airflow to remove the humid air.
My solution is even simpler, though since I am mobile and can move with the weather, I go south for the winter where it s warm outside, and I never have to deal with the problem.