After insulating over 30,000 vans for the past 9 years and partnering with over 300 professional van upfitters, we’ve learned a thing or two about van science. Here is our view on vapor barriers and moisture management.
Do I Really Need a Vapor Barrier in My Van?
When insulating your conversion van, moisture control has to be intentionally designed into your build and always kept top of mind. Believe it or not, cargo vans – just like airplanes – were engineered to shed moisture and NOT to be entirely moisture-free environments. But what does this mean for insulating your van walls? For us at Havelock Wool, it means that your van walls will never be 100% airtight and that relying on a one-way vapor barrier to vapor seal your van wall is problematic. Even with a perfect seal to the inside, there will still be points of access for moisture to get inside your walls. Rather than spending your time and money engaging in the futility of vapor barriers, we instead suggest spending your energy focusing on moisture control. An insulation like Havelock Wool will actively moderate the moisture levels inside your walls, and if you adhere to proper Van Insulation Theory this will ensure your rig stays warm in the winter, and cool in the summer.
A Dive into the Minds of Van Engineers
Cargo vans are designed differently than passenger cars. They are built to different specifications and for different purposes. One of the starkest differences is their water tightness. Cargo vans are designed to be work vehicles; continuously open to the environment and under the assumption that they will eventually get wet. Rather than making cargo vans watertight – where this moisture would get trapped and cause problems of rust, mildew, and mold, cargo vans are engineered more like airplanes. That is, they are designed to drain water. Their roof curves, wall cavities, and structural pillars are all designed for water to run down them in a controlled manner and out very tiny holes and gaps in the walls, around weather seals, and through the doors. This is the first very important point when thinking of sealing off your walls with a vapor barrier. Even if you achieve the monumental task of a perfect seal on the inside, these designed exit points for water will still allow for convection within your wall cavities – defeating the traditional purpose of a vapor barrier. So what can be done to control moisture levels in the walls of your conversion van? To answer this, let us first look at where moisture comes from when living in a metal box.
Where Does Moisture Come From in a Van?
When living in a van, moisture is generated by a number of human activities. Just by breathing, a single person exhales an average of ~2 oz of water per hour. Over the course of a day, this equates to over a liter per person! Throw in showering, cooking, drying clothes/dishes, and maybe even running a non-vented heater and it is easy to see how two people living in a van could easily generate almost a gallon of water per day of moisture!
How Much Water Can The Air Hold?
This is where things get tricky. The amount of additional water that the air can hold depends upon several factors – namely the temperature and humidity already in the air. However, the temperature in a van is not uniform throughout. The exterior metal surface will be much closer to the ambient outdoor temperature. During the day this metal skin will warm up, and at night it will cool down. It is when the exterior of the van gets cold enough that moisture, and in turn condensation, becomes a concern. Before we go deeper, let’s define some terms.
Concepts and Definitions of Moisture Variables
Absolute Humidity
The first concept to understand is absolute humidity. Absolute humidity is the total amount of water vapor in the air regardless of the temperature. Although warm air can hold more moisture than cold air, absolute humidity is not measured in terms of temperature.
Relative Humidity
Expressed as a percent, relative humidity is a measure of how much water IS in the air vs how much water the air CAN hold at a given temperature. 0% RH means there is absolutely no moisture in the air, and 100% RH means the air is saturated and can hold no more water vapor. At 100% RH, water vapor starts condensing out of the air onto cold surfaces.
Dew Point and Condensation
The dew point is the temperature the air would have to drop to in order to reach 100% relative humidity and then start condensing out. It is this condensation that is the enemy of vanlifers! This is the same process that occurs on a hot summer day when you enjoy a nice glass of ice water. You will quickly find your glass is soaking wet! Did that water come through the glass? No! It came straight out of the air. So back to the question of how much water the air can hold.
A Case Study – How Much Water Can the Air Hold?
Humidity can vary greatly from region to region and also upon the localized weather and season, but let’s assume the average relative humidity around the US is 60%. Then let’s take a regular-length Ford Transit with a cargo volume of roughly 250 cubic feet or about 7 cubic meters. At room temperature of 68F, the 250 cubic feet of air can hold a total of 140 grams of water, but is already holding 3 ounces because of the humidity already present! This means that you can only add 2 ounces of water before it starts condensing out! Put another way, you need to fully exchange the air inside air of your rig to the outside before it condenses. If you’re generating ~ 1 gallon of water per day, then you would need to recirculate your entire volume of interior air every 27 minutes. This illustrates just how important proper airflow is in your campervan. The need for high levels of air circulation only increases the colder the climate becomes or the higher the relative humidity is.
But how does this relate to moisture in your walls?
Knowing that vapor barriers are an effort in futility in a van, the relative humidity inside of your van will eventually make its way to the inside of your walls through tiny gaps and convection. If that air has a high moisture content, then this can spell trouble as it will condense inside your walls at night. However, as alluded to above, the proper solution is adequate ventilation so that this won’t be an issue.
Daily Habits for Moisture Reduction inside Your Van
Luckily there are great guidelines for reducing the amount of moisture that accumulates in your van while engaging in certain activities. The theme is the same for every activity: the more moisture an activity creates, the more airflow is recommended. Here are a few things to consider:
Van Design
Perhaps the best way to control moisture in your van is to design your van for high levels of airflow. The best airflow is going to be achieved with the help of a roof fan. Warm, moist air rises, so to get the humid air out of the van we recommend venting air out of the van, rather than venting air in. One fan is not enough though. If the van is all closed in with no other access point for air to enter, your fan will be rendered almost useless. You also need a way for air to get into your van. This can be achieved in one of two ways. First, you can open or crack a window. This way air can come in through the window and out through the roof fan. If your roof layout allows for it, a better approach is to install two air fans. One can be set to pull air in, and the other can be set to exhaust air out. This is the holy grail of van ventilation.
Airflow can also be optimized by designing your cooking area and shower location to be close to an air vent. More on that later!
Showering In a Van
It’s no doubt that showering is one of the highest moisture-producing activities we engage in while living in a van. Luckily, these showers are usually limited in length purely as a water-saving measure. Even so, whenever you shower we recommend having your roof fan venting air out on medium to high speed.
Cooking In a Van
Cooking generates water in two ways. If you are cooking on a propane or other open flame source cooktop, the fuel is combusting with oxygen and producing water vapor as a byproduct. If you are cooking with an induction cooktop, then you avoid this entirely at the expense of a more beefy electrical system. Then there is the water vapor produced via evaporation. The latter is unavoidable. If you are cooking with propane or other open flames, exhaust your fan out on a high setting. If you are using an induction stove then exhausting outwards at a medium speed will suffice.
Airflow While Inside vs Outside the Van
Anyone who has woken up with their walls and ceiling wet to the touch knows that simply breathing produces a lot of water vapor. The double whammy is that temperatures also drop at night so the dew point decreases and can increase the likelihood of condensation. To solve this, you need to leave at least one fan running during the night. Yes, even during the winter when it is below freezing outside, exhaust your fan out on the lowest speed setting.
Dry Heat – A Must for Vanlifers
Another major source of moisture can come from heating your van. Just like cooking, propane heaters with an open flame combust with oxygen in the air and produce water vapor as a byproduct. Even the smallest versions of these heaters can produce as much as 6oz of water per hour! This will start to condense on your walls in no time. For this reason, we never recommend van lifers use this style of heater for ideal moisture control.
Instead, we recommend some style of vented heater. There are many brands of heaters that can run on gas, diesel, or even propane, and they all operate upon the same basic principles. They combust fuel in a sealed chamber and vent the moist exhaust air outside, but pass the hot flame over a heat exchanger element such that cold air can be taken from inside the vehicle, heated, and distributed throughout the living area. This style of heater introduces absolutely no moisture into the inside air.
On the topic of heat, keeping your van heated during the winter can also be one of the best things you can do to keep the relative humidity down inside your van and prevent condensation. Remember, warm air can hold more moisture! Air that is 68 F can hold about twice the amount of absolute humidity than air that is just 50 F.
How Havelock Wool Manages Moisture In Your Walls
Wool is a naturally occurring fiber whose surface exhibits the unique characteristic of serving as a chemical adsorption medium. Having an astonishingly high surface area, water molecules in the air can “stick” to this surface when humidity is high, and then release themselves when the relative humidity drops below 65 percent again. This method of active moisture control means that the water is not simply sitting in droplet form in the wool fiber’s interstitial space. As long as the average relative humidity inside your van is maintained, the wool within your walls will go through a natural cycle of adsorbing water at night, and then releasing it during the day thus preventing condensation in your van walls and reducing any risk of mold and mildew.
Key Takeaway – No Need for a Vapor Barrier in Your Van When Insulating with Havelock Wool
Rather than undertaking the monumental and ultimately futile task of installing a perfect vapor barrier, the best way to prevent condensation inside your walls is an active moisture management strategy inside your van paired with an insulation that can naturally manage moderate levels of moisture. Not only will doing so keep your insulation dry, but it will also protect all of your van’s interior. Humans create a lot of moisture by breathing, showering, cooking, and heating. The theme for managing these activities is to increase the amount of ventilation as the moisture level rises. Based on this article’s example, two people living in a Ford Transit should aim to recirculate their interior volume of air every 27 minutes. The small amount of moisture that is present in your inside air will move in and out of the walls on a daily basis and will be easily managed by Havelock Wool’s ability to adsorb 30-35 percent of its weight in water without getting wet or condensing.
A couple of other things that I would add. We put in Havelock and we have done a bit of remodeling after a couple of years. The wool looked great! I made a point to not cram it in super super tight because the wool needs air circulation for moisture and temperature moderation. I used most of the wool that I ordered and I did take the time to work it into the ribs. Our primary home is a houseboat. So we used many of the same theories used in a well built boat on the van. Vinyl instead of fabric for the walls and ceiling. Cowl board (used in automotive interiors) instead of luan wood, aluminum cabinet structures with snap on vinyl enclosures instead of wood. Wood draws and holds a lot of moisture, so it’s better to eliminate as much as possible in the build. We also use a small electric space heater at floor level in conjunction with roof air conditioning and have seen a significant reduction in interior condensation at night. I love my wool and always recommend it to anyone asking for advice.
Great knowledge!Thanks for sharing!
The statement “Just by breathing, a single person exhales an average of ~2 cups of water per hour” needs to be corrected. That’s 1.5 gallons per day we’d be breathing out, and most of us don’t drink anywhere near enough to do that. Otherwise a good and informative article.
Oops! Thanks for the fact-check. All fixed now!
Much Needed Article For New Builders! Like Trena, I insulated with Havelock Wool and after 5 years have opened a wall for some upgrades. Venting while cooking along with dry diesel heat seems to have complimented my wool’s ability to regulate moisture in the walls. The wool looks and feels exactly the same as day I 1st installed it! No visual indication of surface rust on the metal wall of the van, especially at the bottom edge where moisture would most likely pool at the floor area.
Bill – Thanks so much for the feedback!
Overemphasizing the importance of active ventilation. It was -4 Fahrenheit last night and I still have the driver’s window cracked and the maxxair at drawing 10% air out. Although in the cold weather I got a square of XPS foam and put a 4-in diameter hole in it and then put that in the Max Air fan square trim piece. It restricts the ventilation but still does the job. Having the entire fan exposed would be too much when it’s that cold.
Great knowledge! Thanks Tom!
I understand the concept of wool absorbing and releasing moisture and wonder what wall, floor, ceiling coverings allow and possibly assist this process? Cotton fabrics would allow mold, wood could hold moisture and mold, vinyl doesn’t breath creating vapor barrier. I’m guessing a wool fabric? What else?
You really want a wall system that can dry out. In building science this is often referred to as a vapor open assembly. For the typical van build, wood is a great choice as it does allow for drying and in out experience does not create a mold risk provided the van has proper ventilation. Thanks!
I am using havelock wool in my van build and know it’s the best product to use