Condensation And Moisture In A Van | Why It Happens And How To Control It


Condensation And Moisture In A Van | Why It Happens And How To Control It

An Essay By:



Condensation and moisture are pretty much inevitable when living the vanlife. While it is normal occasionally and at a certain level, precautions should be taken during the van conversion by selecting the appropriate materials & techniques, but care should also be taken after the conversion in the day-to-day.

Denying this issue could turn your van into a fungus incubator… Sharing your home with mold colonies could lead to serious health issues, no jokes. And once the molds invite themselves to the party, it’s almost impossible to kick them out definitely.

And on the short term, living in high humidity environment is uncomfortable. That’s plenty of reasons to do something about it! 

Disclosure: This post contains affiliate links, which means that if you click a product link and buy anything from the merchant (Amazon, eBay, etc.) we will receive a commission fee. The price you pay remains the same, affiliate link or not.


1- Prologue


An aircraft is basically a big van with wings.

No, really, we mean it: it’s a big box made of metal and has humans in it. So they share the same moisture problem and a lot of money is invested in understanding and controlling moisture as it’s leading to multiple problems: it “rains” on passengers and electronic equipment, condensation is trapped on sheet metal (skin) and create corrosion issues, gallons of water accumulate in insulation (a.k.a. blankets) and at the bottom of the fuselage (there are drain valves, but they sometimes get plugged with dirt). So aircraft manufacturers spend big money in R&D to understand and control nuisance moisture.

Here’s an very interesting and highly recommended article published by Boeing:

Controlling Nuisance Moisture in Commercial Airplanes

This passage is an absolute classic. You shall memorize it:

Because moist air will inevitably come in contact with cold structure, condensation cannot be eliminated. As a result, Boeing chose to evaluate potential moisture-control systems that can help […] minimize condensation, minimize dripping onto equipment and into the passenger cabin, maximize liquid drainage, optimize evaporative drying from wet surfaces and insulation blankets.

2- Theory

Don't worry we'll do this quick.

2.1 - Concepts


2.2 - Think Inside The Box

This is a box filled with air: 

  • It’s sealed, so nothing can come in / out.
  • It has a given amount of water in it, under vapor phase (AH).
  • Air has a given relative humidity (RH).

If temperature is increased:

  • AH: Remains the same (no water was added into the box)
  • RH: Lowers (hot air can contain more water vapor)

If temperature is lowered:

  • AH: Remains the same (no water was added into the box)
  • RH: Increases (cold air can contain less water vapor)


Reality Check (no heat source)

  • If our FarOutVan is left alone for a few days (no humans, no heat, no ventilation), it should act like our box above right?
  • With the help of the awesome SensorPush sensors (, we plotted this graph which is exactly in line with the theory above!

Reality Check (Webasto)

  • Now let’s fire up the Webasto and see what happens…
  • Yep, it works! Let’s continue with more theory then.

Neat, we just found a way to lower the relative humidity (RH): HEAT!

Don’t celebrate too fast, because these will add more water to air (and therefore increase AH):

  • Humans and pets (breathing and sweating).
  • Cooking.
  • Damp clothes.
  • etc.

Dang, now what?

Increasing temperature will lower the RH, but it’ll get too hot in that box…

  • What about exchanging the air from inside with fresh air from outside?
  • It’s a good way of getting rid of water that was added by humans, cooking, etc.

And even better...

Ventilation + heat will lowers the AH, RH and keep the ambiant temperature to an acceptable level!


Wait... we created a...



Back to our box

Condensation happens even if ambiant air is below 100% RH…

  • Here, we have a nice ambiant T° (68F) and RH (50%).
  • At 68F and 50%, the Dew Point (TD) is 48F. It means decreasing the air to 48F will bring RH to 100% (and vapor will turn into liquid on surfaces).
  • Because the T° outside is cold, it lowers the surface below Dew Point.
  • The closer the air gets to the cold surfaces, the cooler it gets, the higher RH gets.
  • T° of air in contact with cold surfaces is below Dew Point (TD), so there is condensation.

So what?

The solution is to add insulation, so that the T° of the surfaces (inside) are above the Dew Point!

  • The dew point is now somewhere inside the insulation; if the insulation is perfectly impermeable to water vapor, we eliminated the condensation issue!
  • We’re still in the “THEORY” section, so that works. In the real world, it’s hard to achieve… 

3- How To Control Moisture And Condensation In A Van

3.1 - Moisture Sources

These increase the absolute humidity level by evaporating water in the air:

Respiration and Transpiration
Propane Combustion
Drying Stuff


1L to 3L of water is added each day in the air, contributing to increase  MOISTURE CONTENT.

3.2- Mythbusters

We’d like to start with ideas that are frequently mentioned in discussion forums or Facebook groups. Please make the Internet a better place by NOT RECOMMENDING these! 😉

Moisture absorbants

3.3- Ventilation

Ventilation is critical to control moisture and condensation. It replaces the “polluted” inside air (charged with moisture, CO2, carbon monoxide, cooking smells, etc.) with “new” fresh outside air. A functional ventilation system must have exhaust(s) and intake(s).


3.3.1- EXHAUST



Hot air rise towards the ceiling. Remember that hot air is able to contains far more water vapor than cold air? While the hot air near the ceiling might be at an acceptable relative humidity (RH), it contains more water vapor by weight. By having the fan in exhaust mode (catching ceiling’s hot air and pulling it outside), more water is thrown out of the van (as well as other polluants)! Therefore, we always run the roof fan in exhaust mode.


The Maxxfan has an integrated cover that prevent rain from entering the van, even when the fan is running. On the other hand, the Fantastic is basically like a big hole-in-the-roof: its cover must be closed when raining. To compensate, the Fantastic has a sensor and will automatically close when it detects water. But it’s when it rains that controlling moisture and condensation is critical! From our experience, not being able to ventilate when it rains doesn’t make sense. For this reason, we highly recommend the Maxxfan.

3.3.2- INTAKE(S)

The volume of air “removed” by the roof fan must be replaced by the same volume of fresh air, otherwise the ventilation system doesn’t work. In most cases, there is no fan to push the air inside: as the exhaust fan creates a negative pressure inside the van, air is “sucked” into the van by any intake available (windows, holes, cracks, etc.) This is referred as passive intakes.

Keep in mind that a passive intake offers resistance to air flow; too small and your intake won’t be effective (so your ventilation system)! Try it: close all intakes (windows, holes ,etc.) and you will hear the roof fan “force” as it cannot pull air outside.

Think of surface area: ideally the intake should have a similar area as the exhaust. For example, the Maxxfan exhaust is 12in diameter; that’s 113in2 surface area. It’s quite large, so a window is ideal as a passive intake. In comparison, a hole of 4″ diameter has 12.5in2 and a 4″ square has 16in2 


It’s also important to note that several small intakes won’t be as efficient as one larger intake (even if the total surface area equals the same), as each one of them offers more air flow restriction. Same with a pipe: this adds restriction too.


If the roof fan is located in the back of the van, cracking the driver and passenger windows open is a good passive intake as the air circulates throughout almost the entire length of the van. Consider adding Rain Guards to prevent rain from entering through the windows:


Not a sustainable ventilation method when parked, but when driving it is an excellent way of venting the interior of the van!


Adding floor vents helps optimize air circulation, as fresh air enters through the floor and exits through the roof. That’s the strategy we chose for our van conversion and it’s doing what its supposed to (to a certain extent; read “On Second Thought” in each article respectively for our long-term review):

3.4- Heat

While ventilation is the most important moisture-control system, dry heat take it to the next level: by adding dry heat, we’re pretty much creating a drying machine out of our van!



Like it or not, combustion adds water vapor to air. Here’s propane combustion equation (H2O means water…):

C3H8 + 5O2 –> 3CO2 + 4H2O

Burning propane at a rate of 10,000 BTU per hour adds 450 mL of water per hour in the air; that’s a LOT!

Portable propane heaters are not vented, meaning the combustion is internal to the van:


Mr Heater Buddy

9,000 BTU/h of heat=
405 mL/h of water!

Olympian Wave 8

8,000 BTU/h of heat =
360 mL/h of water!


If combustion produces water, how can we achieve dry heat then? With vented heaters!

Vented heaters are physically located inside the van (or outside sometimes), but combustion is external: air for combustion is taken from outside, burnt, then rejected outside. The heat from the combustion is transferred inside via a heat exchanger. Air from outside / inside is never mixed together. Vented heaters:



7,000 BTU/h of heat =
0 mL/h of water!

Both Webasto and Espar have been around for decades. And with an extensive dealer network in North-America, you can't go wrong!


7,500 BTU/h of heat =
0 mL/h of water!

Both Webasto and Espar have been around for decades. And with an extensive dealer network in North-America, you can't go wrong!


7,500 BTU/h of heat =
0 mL/h of water!

A cheaper alternative to Webasto/Espar. Based in beautiful British Columbia, Snugger offers service throughout North-America.


6,500 BTU/h of heat =
0 mL/h of water!

A super robust, reliable and affordable solution if you prefer propane to diesel/gas. The installation is slightly easier too!

Webasto vs Propex

Can't decide between fuel (diesel/gas) or propane? We installed both in our van, so we can help make up your mind!

3.5- Thermal Insulation

We’ve seen that surfaces below dew point temperature generate condensation. That is generally the case with windows (worst), metal (bad) and beer mugs (very good).

Thermal insulation helps control condensation by keeping interior surfaces above dew point temperature. We could easily achieve the perfect insulation with our theoretical box (from “theory” section), but achieving perfect insulation with a van is a bit like chasing unicorns…



  • Temperature varies through the walls (interior finish, insulation, metal). It gets colder as it’s getting closer to the exterior.
  • Therefore, relative humidity (RH) varies through the walls as well. The colder the air gets, the higher the RH.
  • Condensation happens at the dew point (RH: 100%). Here, in our “perfect insulation”, the dew point is located inside the impermeable insulation; moist air from inside the van cannot reach the location of where the dew point is. In fact, moist air is in contact with the interior finish which is above the dew point; there is no condensation!

Note that metal is totally impermeable to water vapor: it’s a vapor barrier. Moisture cannot migrate through the metal.




Soooo, here’s the bad news: nothing’s perfect in this life. And as a reminder:

Because moist air will inevitably come in contact with cold structure, condensation cannot be eliminated. As a result, Boeing chose to evaluate potential moisture-control systems that can help […] minimize condensation, minimize dripping onto equipment and into the passenger cabin, maximize liquid drainage, optimize evaporative drying from wet surfaces and insulation blankets.


So why is it that “moist air will inevitably come in contact with cold structure”?

Looking at how a van structure is made, we get our answer…

We’re dealing with surfaces that are far less than ideal:

  • Hollow regions are impossible to insulate perfectly (even with expanding foam). Frames and structure create thermal bridges (thermal bridges are defined under section 3.5.3). As a result, air is cooled down below dew point locally and condensation happen in isolated places.
  • Uneven geometry creates air gaps where condensation could form and remain trapped.
  • Drain holes and openings in frames/pillars allow moist air from outside to infiltrate the insulation layers.
  • Minor leaks (from roof and side plastic trims) are a reality. As a result, liquid water can penetrate the insulation layers. (we had reports of small leaks on all platforms: Transit, Sprinter, ProMaster)


There’s no doubt in our mind: moist air will inevitably come in contact with cold structure. Denying that fact could lead us to take wrong decision about insulation. Condensation cannot be totally eliminated, so our approach is to mitigate it (just like in that Boeing’s article we keep talking about!). Before deciding on an insulation strategy, let’s look at our options:


Closed Cell Foam Insulation

Closed cell foam insulation have excellent R-value. 

Spray Foam



Polyiso Rigid Board



XPS Rigid Board



NRCA’s most recent recommendations are to assume that polyisocyanurate has R-5.6 / in. when designing for warm climates and R-5.0 / in. when designing for cold climates.
Polyiso Apparent R Values Temperature Effect
The graph above shows the effect of temperature on apparent R-value of different polyiso products for 4" thickness (expected R-22.4)

Hydrophobic Insulation (doesn’t absorb water)

Hydrophobic material doesn’t absorb and retain moisture. Try it: soak it in water. It does get wet, but water or moisture doesn’t get absorbed into the material and therefore it dries out quite fast.




Bottom word: it’s a tried-and-true product that gives a added value to higher-end builds.




Sheep Wool

Sheep wool is the new kid on the block, so long term data is not really available except for what the manufacturer claims:

“A chemical-free passive filtration insulating material that manages moisture and reduces sound”

It’s not exactly hydrophobic as it “manages” moisture: it absorbs moisture when ambiant air is above 65% RH and releases it below that. The manufacturer claims it prevent condensation on cold surfaces. 

Hydrophilic Insulation (absorbs and retains moisture)

No need to say hydrophilic materials are a big NO NO! Some people choose hydrophilic insulation and package it in trash bags to prevent moisture, only to find years later that the trash bags were damaged during installation or after (due to vibration on a metal edge, for example)… 

Radiant Barrier

Radiant barrier purpose is exactly like the name suggests: a radiant barrier! By itself and without any air gap between the metal surface (in other words, if applied directly against the metal), it has pretty much no R-value. That being said, it’s a good material for window covers as it will prevent your van from turning into a oven in summer (but it won’t do much in sub-freezing temperatures).

Thermal Bridges

Metal is an excellent heat (or cold, same thing) conductor. You can choose the insulation with the best R-value, but leaving any surface of metal exposed to interior ambiant air (with no insulation) will create a path for cold (or heat) to sneak inside the van; it’s called a thermal bridge.

Why would one leave exposed metal then? Covering some areas with insulation would reduce the living space too much; frames are a good example. To minimize the effect of thermal bridges, we covered any exposed metal with LOW-E EZ COOL. LOW-E EZ COOL insulation has a flexible foam core sandwiched between aluminum faces. It’s primarily a radiant barrier, but the foam core provide some R-value too. As opposed to Reflectix, it can withstands some pressure without damage and is more silent.


Bottom word: while it shouldn’t be elected as main insulation type, LOW-E EZ COOL is a good way of covering thermal bridges.

Vapor Barrier or not?

The purpose of a vapor barrier is to prevent moist air (from inside the van) of migrating towards cold surfaces. The idea is that moist air from respiration, cooking, drying gear, etc. won’t reach cold surfaces and therefore that eliminates condensation issues. However if, for any reason, moist air makes its way past the vapor barrier, it would be very hard to dry that moist air because it would be sandwiched between two vapor barrier layers (remember that metal is a vapor barrier too).

Even if one could achieve the perfect vapor barrier (which is unlikely, sorry), we’ve seen that there are paths for outside air to infiltrate and there are potential leak points as well. Therefore we believe that moist air will inevitably come in contact with cold structure, so our approach is to let the insulation layers “breathe”.


Here is our take on insulating a van: let it breathe. It doesn’t have the highest R-value and it doesn’t completely eliminate condensation issues, but nothing’s perfect: the best solution is the best compromise!

  • THINSULATE covers most large surfaces.
  • LOW-E EZ COOL covers frames and protruding structure to prevent thermal bridges. Since it’s a good radiant barrier we partially covered the Thinsulate, but not completely because it’s also a vapor barrier!
  • Vapor Barrier: nope.

Don’t get us wrong:

It’s titled “Our Insulation Strategy”. It means we believe there are other ways to obtain good results by using different materials and techniques! 


Windows ARE the weakest link. One can spend a fortune on getting the highest R-value in the walls, but something has to be done about the windows as well!

NOTE: when using the window covers and it’s cold outside, you WILL get condensation on the windows. Indeed, window covers minimize the heat transfer from inside the van to outside; but the small air gap between the window and the cover gets very cold; way below the dew point. So condensation or frost is inevitable. We normally remove the window covers 15-20 minutes before driving and dry them off with a chamois cloth: Buy on Amazon.

Window Shades

Building your window shades from a radiant material is a cheap and good solution for summer or mild cool temps. Reflectix is the material everyone uses out there, but having worked with both EZ-COOL and Reflectix, we would use EZ-COOL as it is more resistant and it doesn’t make that annoying “foil” sound.

Insulated Window Covers

If you are snowchasers like us, insulated window covers are a game changer. They make the interior of the van more comfortable by reducing heat loss significantly (no more cold drafts near the windows!). We made our own from Thinsulate and EZ-COOL; we documented their fabrication in detail!

3.6- Dehumidifier

A dehumidifier catches water vapor present in the air, turns it into liquid and collects it into a container for easy disposal. In other words, it removes moisture from the air.

Are dehumidifier the holy grail of condensation and moisture control for vans? Keep reading!


3.6.1- How Dehumidifiers Work

By now, we know for a fact that a cold surface condenses water. A dehumidifier is nothing more than an air conditioning unit designed and optimized for collecting water: instead on focusing on blowing cold air into the room, it focuses on refrigerating the surfaces located within the unit, so water condenses on it. 

There are two types of dehumidifiers: Compressor and Thermo-Electric.

Compressor Dehumidifier
(Commonly found in houses)

Same principle as a refrigerator (including the 12V NovaKool fridge we have in our van!) and an air conditioning system: this type of dehumidifier uses a compressor to “pump” heat

Thermo-Electric Dehumidifier
(a.k.a. Peltier)

A Peltier dehumidifier uses thermo-electric principle to convert electricity difference into a temperature difference.


Compact Peltier dehumidifier (no compressor) with 16 oz. capacity water tank. Make sure to buy the 12V adapter too, as it’s more efficient than using your inverter. CURRENT DRAW = 2A (at 12V).

3.6.2- Dehumidifiers help with moisture, but are not the ultimate solution to condensation

If you flew in an aircraft before, you know that the air is so dry it’s very uncomfortable. Yet the Boeing article above confirms aircrafts have condensation issues… what the? Remember that condensation issues are mostly because of cold surfaces. At 36,000ft the outside air is around -70F, so the fuselage skin is very, very cold; the dry air is cool down so much that condensation happens no matter what.

Now, our van doesn’t experience -70F outside temperatures, but it does get cold out there during winter. Cold enough to lower windows and metal temperatures below dew point even if RH is very low.

Let’s play with parameters.

We start with 40% RH and 71F interior temp (which is typical). Condensation happens if windows (or metal) reach 46 F (or lower); that’s relatively high.


Interior RH 40%, Interior Temperature 71F = Dew Point 46F

With the help of a dehumidifier, let’s bring the RH down to 30%, which is the lowest RH that humans are comfortable with. And still 71F interior temp: condensation happens if windows (or metal) reach 38 F (or lower). An outside temperature of 32F will most likely bring a window down 38F. We still have condensation…

Interior RH 30%, Interior Temperature 71F = Dew Point 38F

If we lower the RH down to 15% (which is not really possible in the real-world if people live in it), we still have condensation if windows (or else) get down to 21F (which you will most likely encounter if you are skiers like us!). 


Interior RH 15%, Interior Temperature 71F = Dew Point 21F

Bottom word: 

A dehumidifier helps with moisture, but won’t completely eliminate condensation. It’s a good idea to use one when parked and plugged to shore power, but it draws a substantial amount of power for those who are off-the-grid (knowing that condensation is most likely to happen when solar power is at its lowest)…

Real-World Check:

We observe the most condensation (on the windows) during winter when it’s cold outside (sub-freezing); because we heat with a vented heater (Webasto), our RH is normally in the 30-35% range. Our ambiant air is already dry, a dehumidifier wouldn’t help much; the issue really is with the cold surfaces.

4- Life After The Conversion


We did it! The van conversion is over. We chose the best insulation strategy and materials to our knowledge. But the fight is not over! We’ve been living in our van full time since August 2017, and we’re still controlling the moisture and condensation in our day-to-day life:

4.1- Cooking

Cooking during winter is far from ideal: it adds a lot of moisture (from food and from propane combustion, especially the oven) and since it’s so cold outside, ventilation means a lot of heat loss. It is what it is: whatever the temperature outside, ventilation is the key! When we cook, we usually keep our side window opened and run the roof fan.  

  • Above 35F outside, it’s no big deal.
  • Below 25F outside, it gets chilly in the van for a short while. But the Webasto keeps us comfy anyway.
  • Down to -15F, the cold draft between the opened window and the roof fan can be pretty brutal. The Webasto runs full-speed! But when we’re done cooking, we close the window and the floor vent is the only passive intake. After the meal we normally close all intakes.

4.2- Climate

Here’s the worst possible scenario that we’ve encountered: after a day skiing at Crystal Resort (3900 ft at the base) during a relatively warm (28F) and very wet day, we hung our gear to dry and we drove down immediately to a lower elevation so it would be warmer. It was snowing at the resort, but raining at lower elevation.  We cooked and we rested for the night. 

Let’s think about it for a second:

Nothing wrong with cooking and drying our gear, but we definitely should have stayed at higher elevation where it was colder (more heat from the Webasto = lower RH) and where outside air was dryer (less moisture coming in from the air intake). Now we know!

4.3- Activities

The main reason we live in our van is for riding our mountain bikes and our snowboards more often. 

We were very concerned with drying our ski gear in the van at first, but we’re happy to report that our gear is fully dry in about 2 hours (a little more for the 2 pairs of boots). With the Webasto working hard when it’s cold outside, the air inside is dry and comfortable. Nice!

Winter Vanlife

Traction, electricity, finding water and camping spots, staying warm, controlling moisture, drying gear, etc.

It’s actually worst after a day riding our bikes in the wet (think Pacific North West): since it’s warm and moist outside, we can’t really use the Webasto to full capacity or it gets too hot. So gear takes longer to dry. Huh!


4.4- Location

Oh, we probably should have mentioned this at the start of this essay… but look, if you use your van mostly in any of the yellow-ish places below, don’t overthink this moisture and condensation thing! You’re fine, it’s dry enough there…

If, on the other hand, you plan on spending extended time during fall/winter in the Pacific North West (Coast of British Columbia, Washington, Oregon) then this article is definitely for you! Actually, we got the idea and wrote this article while we were here during the rainy months… This amount of water in California would probably create a supra-landslide and make the whole state totally flat 😛

Google Earth North America Dry Regions

5- On Second Thought


We completed our van conversion in August 2017 and we are living full time in the van since then. While we feel confident about our material choices and we would do exactly the same if we had to start over, time will tell if we nailed it or not. It will be interesting to strip down the van after 10-15 years of usage…

That's it folks, hope that helps!

Thanks For Reading.


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about us

Nice To Meet You.

Hello! We’re Isabelle and Antoine 🙂 In 2017 we sold our house (and everything in it), quit our engineering careers and moved into our self built campervan. We’ve been on the road since then and every day is an opportunity for a new adventure; we’re chasing our dreams and hopefully it inspires others to do the same!

49 thoughts on “Condensation And Moisture In A Van | Why It Happens And How To Control It”

  1. Excellente explication du phénomène de l’humidité et de la condensation. Important à bien maîtriser dans la conception et la vie quotidienne en VR.

  2. This site is a gold mine! You are really making a huge difference in our plans for our van and it’s going to be so much better thanbks to you!

    I was wondering if you had an opinion on using a wood stove as a solution for heat and humidity.

    I can deal with the hassle of getting wood and cleaning the ashes but I wouldn’t want something that adds water in the air.

    Looking at somethink like the Cubic cub mini stove.

    I’m also up in Vancouver so we are expecting a ton of humidity. 🙂

    • For full time living, I honestly wouldn’t want to deal with wood. Sometime (very often actually) you just want heat QUICK without hassle. And what if you have to stay in a residential street or something? I wouldn’t use the wood stove there…

      Personally we’re not fan of wood stove for vans (even if we LOVE fireplace), but if that’s what you want, we see many vans with chimney. We have no experience with them so can’t really comment.


  3. Thank you for the excellent information. Did you ever have problems with snow building up on the roof and blocking the maxxfan when you would get 1-2 feet over night? Would you ever consider putting a maxxfan on the back vertical wall of the van to address this issue? I am planning my first build for a 2o20 transit on order! I want it to be a winter focused ski van!

    • Of course we have to shovel the roof on really big dump, cause the Maxxfan (and solar panels) are buried under. No big deal, it’s kindof fun.
      I wouldn’t install the fan on a vertical wall, it belongs on the roof (for proper ventilation and installation considerations, plus you’ll get tons of dust inside when driving if installed on the back door).


  4. Great overall site for van conversions. I am in the planning stage now and have found your site invaluable. I don’t see any mention of wanting an air conditioner. It seems this is only possible when using shore power but do you find it necessary when in hot climates?

  5. Hi, like others above, I really appreciate the “why,” and hearing your thought process in coming to decisions- thank you!

    One thing is puzzling me: you didn’t address mold speficially. The risk of mold growing is the main reason I would care about condensation. Is the mold risk automatically factored into your thinking and product selection (simply by way of it being so closely tied to reducing moisture and condensation), or did you not factor that in? (I am really allergic to mold, so that is a big consideration for me!) – Thanks!

    • Moisture and condensation are mostly responsible of creating mold; choice of materials also have an impact. Our final solution take account of all of that; we think it’s the best way to prevent mold!

  6. Hi,
    Very well thought out article — thanks!

    A couple of comments…
    You downrate polyiso vs XPS quite a bit because it loses some R value at cold temperatures, but this test data shows that polyiso does better than XPS even at quite cold temperatures when you account of the fact that the average temperature of the polyiso is warmer than the outside temperature.

    A downside for XPS is that the maximum service temperature is only 165F. The van skin may exceed this temperature in the sun on summer days — especially for colors other than white.

    Another downside for XPS is that it is blown with a very powerful greenhouse gas, which over the years diffuses out. I believe that they are working on this problem and will eventually use a less harmful blowing gas, but I do not think this has happened yet.


  7. An extremely interesting and informative article. I’m a heating engineer, and I’d like to add a couple of thoughts to an already excellent piece. Everything you write is true and valid, so these are just a couple of extra things to consider.

    1. Air intake at low level is not always a good idea. Thermal comfort is seriously compromised if you have cold air around your feet; less so if that cold air is around your head. (Basically, human beings like to have cool heads and warm feet, and hate having cold feet and hot heads.) Cold air coming in from outside at floor level will tend to ‘pool’ on the floor and make you feel cold no matter how hot the air is at high level. Cold air coming in at high level will mix with the warm air at high level as it falls towards the floor. So you might want to consider having both air exhaust and air intake at high level, but at opposite ends of the van.

    2. A flueless fuel-burning heater will heat the space very effectively, but will add moisture. A balanced-flue or ‘room-sealed’ heater such as a Webasto will heat the space, but will not add moisture. That’s already better. But an open-flued heater, such as a wood-burning stove with a chimney through the roof, will BOTH heat AND ventilate the space very effectively, and that is ideal.

    (Very often, a cold place like a snowy mountain will have lots of dead wood available to burn in that stove, and you can probably cook on the stove too, as well as produce hot water for washing. You could equally use an open-flued gas heater, although wood burners are more common in vehicles.)

    You do have to ensure adequate ventilation at all times with an open-flued heater, and fitting a carbon monoxide alarm or two in the van would be a sensible precaution, but technically it is a pretty-much ideal solution from the point of view of controlling condensation. Again, I’m not saying it is necessarily better than other options overall, and they can take up more space than other options, but I don’t think a discussion on heating and ventilation is complete without including that very old, but still very functional combined heating and ventilation system.

  8. Very useful information that is nicely backed up with research from Boeing and your own and others’ experiences. Thank you!

  9. Great article, thanks!!! …Hey, but I was wondering, did you put some type of sound deadening matt on inside metal walls before insulating? Like the stuff by Noico, that’s supposed to be petroleum free?? Thanks for the info, much appreciated. 😉

  10. Thanks for posting this review on moisture, it was a great read! I’ve been living in a tiny home and this will be my 3rd winter. I have a propane Ashley direct vent heater installed and use shore power to run a dehumidifier, it pulls about 8-10litres of water out of the trailer every 3 days and I use the water for washing dishes:) I do 80% of my cooking outside and that keeps the majority of moisture out of the trailer. Did you consider installing a dehumidifier in the van? I start my van build this summer!

    • In fact, because the Webasto produces dry air, ambiant air is quite dry in the van (in the 30-50% range according to our weather station: The issue really is about the cold surfaces during winter. I don’t think a dehumidifier would solve the issue AND it draws too much current when we’re off-the-grid (which is always the case).

      Have a good one!

  11. Could you use either of your heaters as a fresh air source? It would save an extra hole in the floor if so. It looks like the Webasto Air Top 2k has a fresh air / recirculate mode, and some controllers can run in ventilation (fan only) mode. I have been ready to buy one for while and am having a hard time confirming which heater model(s) and controller(s) can do this, or if newer units have these features but not older ones…?

    • Hi James,
      Vented heaters (Webasto, Espar, Propex) don’t use outside air as fresh air source; the heat the air INSIDE the van. Using outside air wouldn’t work as:
      1) The temperature sensor is located in the air intake of the heater: if using outside air, the heater couldn’t detect what’s the temperature inside the van and adjust speed accordingly.
      2) It would suck air from the exhaust and inject carbon monoxide inside the van
      3) In extreme cold, it would take too much time to heat the van.

      Hope that helps!

  12. Have you explored spray cork for the initial lining of vans? It looks interesting and I’ve seen it used in sailboats. It would certainly deal with thermal bridging and get into those hard to get at spots. I also wonder what it would do for acoustic insulation?

  13. Nice synopsis of a really irritating worry.

    The Boeing article was a good read. Three things caught my eye: Evaporation is required to dry wet insulation blankets. Inspections of aging airplanes revealed that corrosion in the crown area is extremely unusual…. Fiberglass batting treated with an enhanced hydrophobic coating does not reduce water retention.

    Those pictures of water stains on the overhead bins were impressive. I assume that has never happened to your overhead bins.

    Is there a small little place in your van walls where you could pop off the wall and insulation and look directly at the moisture? That would be interesting and informative, especially if you did it when it is -15C out. (Maybe behind a speaker?) Sometimes I wonder that we over think a problem that might not exist. (Wouldn’t that be nice?) When I scrapped my old van (280000+ miles, 1978-2001), the inner sides/roof of the van looked the same as the day I covered them (with fiberglass/vapor barrier/1/4 inch plywood, I know, I know, don’t say it). But that vehicle was not used 24/7 like yours.

    The biggest daily problem I remember is frost/water on the front windshield in the morning. Removing it was a pain and, in the process, water always pooled at the forward edge of the dash (really hard to even see it!!), and led to rust holes (over some 15 years). (Does your window covering stop that? It must control it a little.) The other places were around the rear/side windows where sub freezing metal was in contact with van air. This moisture would in time rust the hinges/closers on the windows that open as vents.

    One thing in a house that is like a van are sloping roofs that have the inner ceilings nailed to the same trusses. When those are insulated, there has to be an air channel between the insulation and the outside roof, with air coming in the soffit and going out the roof ridge (It’s to control moisture). But it’s hard to imagine how to do that in a van. First, we want every cm of insulation, and second, who wants to put more holes in vehicle that goes 70 mph?

    I wonder how hot the side of your van gets when it’s directly facing the sun (even in subfreezing temps). If there is not much wind, they might get fairly warm, maybe warn enuf to drive the water back into the interior of the van i. e. evaporation. So maybe we don’t really want a vapor barrier in the end? I don’t know. (Of course, we all know how much sun there is in the NW in the winter…)

    Cheers, Don

  14. Excellent article! One of the best I’ve read, thank you for taking the time to write so clearly and with all the fun and informative infographics! I am building out my transit and your web site has been super useful, THANK YOU AGAIN!!!
    P.S. I live in Enumclaw, WA, just outside Crystal Mountain, so if you need anything, let me know!

  15. Nice analysis and summary! Particularly appreciate the Boeing analysis – moisture happens, now deal with it!

    Couple of questions/comments:

    1. Why use reflective foam as a thermal barrier on the walls without an air gap? I’m using 1/8″ closed cell as my thermal barrier, as it’s cheaper and I couldn’t see any value to the aluminum facing when glued against metal without an air gap.
    2. Organic insulation such as denim and wool provides a nutrient base for mold and mildew. I’ve also heard stories that insects find it a nice home 🙂
    3. You’ve got this covered with your setup, but for others without good internal airflow, a supplemental wall mounted fan to keep air moving will move moisture dense air out of dead zones to be expelled by the exhaust fan. In Costa Rica, most rooms have a ceiling mounted fan going 24×7 – not for cooling, but to keep mold from growing on the walls.

    • Hi David,
      Closed cell foam is just fine for thermal barrier, I agree! The reason we went with ez-cool is 1) we had it in stock because we used it for our window covers. 2) we extended the ez-cool over the Thinsulate (but without completely covering it) so we get some radiant barrier properties too.

      Thanks for your input! Much appreciated!

  16. Partly as a safety matter I think that you should repeat in this article that unvented heaters are only designed to be used with a window open or other ventilation. They are also not approved for use while sleeping. This is stated clearly in the owners manual for the Mr. Buddy and other unvented heaters. People do die using these heaters in small unventilated places, unfortunately the dead people have a hard time writing blog posts about their experiences so the reviews and experiences in the internet are skewed towards experiences from the living (sarcasm intended :).


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