Insulation Testing – For Van Conversions

This page covers some testing of the insulation types commonly used to insulate van conversions.  It seems like there are so many claims and opinions on the best type of insulation to use in van conversions, that some actual data might be helpful to people trying to decide (the other factor is that I’m Covid bored and really needed a project to work on).

The testing is mostly aimed at moisture management and condensation, but also does a rough comparison of thermal performance.  I’m also starting a mold test to see if any of the tested insulations are susceptible to mold growth — results on the mold test are a few weeks out.

The moisture management parameters measured include: condensation formation, humidity levels in the insulation, moisture pickup over a typical van night, and dry out time after a van night.

Van Moisture Situation

Some thoughts on the moisture situation in camper vans.

People generate moisture through breathing and perspiration — from about 40 grams per hour sleeping up to 90 grams per hour doing light work.  So,two people might add 3000 grams of water to the van in a full day/night.  A dog would add some more.  Cooking or even making coffee or tea contributes moisture, this can be up to 3000 grams per day, but likely less in most van settings.  Other sources include showers (if so equipped) and drying out wet cloths.  Non-vented propane heaters also add moisture if used.

So, how much moisture does it take to get the van relative humidity up to (say) 70%?
A typical van volume is about 630 cubic ft, which is 47 lbs of air.  The water weight with 70F temperature and 70% relative humidity is 5 grams per lb of air.  So, the total water weight in the van for 70F and 70%RH is about 240 grams.

Vans have a metal outer skin that is a near perfect vapor barrier, so none of the moisture gets out via that route.

So, it seems like with people etc. generating several thousand grams of moisture per day and it only taking 240 grams of water weight to get the van up to 70% humidity, that: 1) there needs to be good ventilation in the van to get rid of the excess water, and 2) that even with good ventilation there are likely to be extended periods when the relative humidity in the van will be high.

If the van air is at 70F with 70% RH, then the dew point is 60F.  This means that if the van air gets to a surface (eg the van skin or outer part of insulation) that is below 60F, then the water vapor will condense into liquid water.  This is potentially a bad thing from the point of view of rust or mold.

This makes the van a challenging environment for moisture management.


The setup consists of a box that is about  4ft high by 5 ft wide by 3 ft deep. The front face of the box is glazed with 1/8 inch plexiglass and the rest of box is constructed with rigid insulation board.  The Plexiglass is used in place of the van sheet metal so that any condensation can be directly observed. The plexiglass face is divided into four bays, each of which can accommodate an insulation sample, but normally one bay is not insulated to give a comparison to un-insulated van skin. Each bay is 14 inches wide and 48 inches high.

The box has a heater (light bulbs) and a humidifier which are run by a controller so that humidity and temperature inside the box can be controlled.  The box is located in my garage/shop and the temperature is roughly regulated by opening doors to cool or turning on the heater if it gets too cold.

Heater and humidifier in insulation test box

The heater and humidifier are plugged into this controller in order to keep the temperature and humidity inside the box at a constant level.


Insulation test box from the front with insulation samples installed

The four bays from left to right: Polyiso, Thinsulate, Wool and no insulation (just Plexiglass).

The Plexiglass is sealed to the wood frame with with silicone to prevent air leakage.

The insulation panels from back side.
The Polyiso panel (right) is sealed around the edges with Great Stuff urethane foam to try to keep water vapor from migrating to Plexiglass side of the panel.  The center part of the Polyiso panel is removable so that it can be weighed for moisture pickup during the test.

The Thinsulate and Wool samples are held in place with spring wire to keep them snug (but not compressed) to the Plexiglass.  This allows them to be quickly removed for weighing.

The parameters that are measured and logged during the test are:

  • Temperature and relative humidity inside the box
  • Temperature and humidity outside the box
  • Humidity in the middle of each insulation sample
  • Initial and final weights of each insulation sample (to measure moisture accumulation)
  • Thermal camera pictures for rough assessment of thermal performance

Generally a test run starts by weighing and then installing each insulation sample in its own bay.  Then the inside of box temperature and humidity are set on the controller.  The test is then run for a time roughly equal to a night in an RV.  Photos and IR pictures are taken once in a while during the test to record condensation levels and see if there are any changes in thermal performance.  At the end of this period, the insulation samples are weighed to get water pickup, and then quickly reinstalled. Then the humidifier is turned off while the heater is left on, and ventilation is increased to get a warm and dry environment in the box for drying.  The test then continues until all of the insulation samples have dried out, and a final weight of each sample is taken.

Test 1 – Two layers Insulation

In this first test, the temperature inside the enclosure was maintained at 70F, and the relative humidity at 70%.  The outside the box temperatures varied from about 39F  up to about 45F.

The enclosure was kept at these conditions for 15 hours while monitoring condensation and humidity levels in the insulation.  Then the humidifier was turned off and ventilation increased while still maintaining 70F inside the enclosure to see how long it takes the insulation to dry out.

The 4 bays were configured as:

  • No insulation (just plexiglass)
  • Wool: 2 batts thick for total thickness of 3.5 inches
  • 3M Thinsulate: 2 thicknesses for total thickness of 3.375 inches
  • Polyisocyanurate: 2 boards for a total thickness of 2 inches

One reason for testing with two thicknesses of each insulation was to be able to log the humidity exactly half way into the insulation stackup.  This is hard to do with a single thickness.

Test Timeline:

Some events along the test timeline are noted below…

0 hour

Install the 3 batts.
Place humidity sensor at mid thickness of each batt.
Turn on heater only set to 70F – no humidifier for the first part of test.

0.6 hour
The enclosure is run for a while with the heater on but humidifier off in order let heat flows stabilized for an IR picture with everything dry.

IR picture of front of enclosure with all batts dry.

IR picture from front.

Plexiglass (right) is bright because its losing quite a bit of heat.
Wool, Thinsulate and Polyiso much cooler (darker) as they are insulating.  Polyiso is a bit darker indicating that it is insulating a bit better.

The temperatures in the IR pics are the outer surface of the Plexiglass (IR does not penetrate Plexiglass, so you are not seeing into the enclosure).  The colder (darker) these temperatures are, the less heat is being lost from the enclosure to the room.  The plexiglass with no insulation is much warmer (lighter) than the panels with insulation because its losing heat much faster.

Note that all of the IR pictures are adjusted for a temperature range of 35F to 55F, so they can be compared to each other.

Humidity in enclosure at this point is about 30% (dry).

0.8 hour
Turn on humidifier, which is set to get to and maintain 70% RH.
It takes about 2 hrs to get humidity up to 70%.

With 70F temperature and 70% RH, the dew point is about 60F, so as air containing water vapor makes its way into the insulation, condensation will begin to form at the point where the insulation temperature reaches 60F.

2.5 hour
First visible condensation on Plexiglass with no insulation.
No visible condensation yet for any of the insulation panels.

4 hours

About 1.5  hours after humidity reached 70%.

Plexiglass only panel showing more condensation.
Wool panel showing no condensation.
Thinsulate showing some light condensation.
Polyiso showing no condensation.


9.5 hours

About 7 hours after humidity reached 70%.

Plexiglass only panel showing more condensation.
Wool panel showing no visible condensation.
Thinsulate showing significant condensation (but less than bare Plexiglass)
Polyiso showing no condensation.

Plexiglass and Wool at 9.5 hours

Thinsulate and Polyiso at 9.5 hours

Note that the Polyiso panel shows a little condensation where I cut it to allow the center part to be removed for weighing and did not tape over the cut carefully.

15 hours – max condensation

About 13 hours after humidity reached 70%.

Bare Plexiglass panel showing considerable condensation and beginning to drain down.
Wool panel showing significant condensation, but less than Thinsulate.
Thinsulate showing significant condensation, but less than bare Plexiglass.
Polyiso showing no condensation.

Plexiglass and Wool after 15 hours


Thinsulate and Polyiso at 15 hours

Thermal camera picture at 15 hours.

I was thinking that the condensation in the outer layers of the fibrous insulation might effect its R value, but the thermal picture does not show much of any change from the initial one, or much difference between the Polyiso (impermeable to water vapor) and the Thinsulate or Wool.

16.1 Hours – start dry out
Turned off humidifier and begin the drying out part of test.

The test continues with humidifier off, but heater on and more ventilation to bring the inside humidity down and see how long it takes for the different types of insulation to dry out.

At this point, the insulation panels are quickly taken out, weighed and then reinstalled to see how much moisture the batts have picked up.

  • Wool batt felt damp to touch on outer surface,
    Weight went from 768 g to 804 g (+36 g of water pickup)
  • Thinsulate felt damper than wool,
    Weight went from 640 g to 651 g (+11 g of water pickup)
  • Polyiso felt dry,
    Weight went from 270 g to 270 g (+0 g of water pickup)

Even though the wool batt picked up quite a bit more moisture, it felt a bit dryer than the Thinsulate batt.  Perhaps due to the wool absorbing some of the water into its fibers.  This could be a pro or a con depending on how you think about it.

24 hours – end of test

Final insulation panel weights:

  • Wool batt felt dry, but weighed in at 782 g, so it still had 14 g more moisture in it than when it was fully dry at start of test.
  • Thinsulate still felt a bit damp to touch and there were still a few (not many) drops of condensation on the glazing.  The Thinsulate batt weight 641 g, so almost down to its dry weight at start of test.
  • Polyiso felt and looked dry and weighed the same 270 g as it did at start of test.


Insulation Sample Weight Gain

Each of the insulation samples was weighed at the start (dry), at the end of condensation test (wet), and after a dry out period.

InsulationStarting Weight (grams)End humid period
Humid period gain
End drying period
Gain over test


At the end of an 8 hours  of drying, the Wool sample still retained about 40% of its water gain from the previous night. This means that if there there are multiple days of condensing conditions in the van, that the wool will start each day with less of its water absorbing capacity intact.

Humidity and Temperature Logs For Test

I inserted an humidity logger sensor in the middle of each of the insulation samples to log the relative humidity inside the insulation sample over the full test.

For the wool, it was between the two thicknesses (batts) of the wool, for Thinsulate it was between the two layers of the Thinsulate, and for the Polyiso it was between the two sheets of Polyiso.


Relative humidity level inside the Thinsulate

At about 2 hours the humidifier is turned on in the enclosure, and the RH in the Thinsulate climbs pretty quickly up to about 78% and stays there until the dry out part of the test starts.  Note that as the temperature of air goes down, its relative humidity goes up even though the absolute amount of water vapor in the air does not change.  So, as van air works its way out toward the van skin, the RH keeps going up until it reaches 100% and condensation starts.

The dry out part of the test starts at hour 16.  The big change at about hour 20 was caused by increasing the ventilation in the box to speed up drying.


Humidity level inside wool insulation

The wool insulation has a more gradual increase in RH at mid batt than the Thinsulate.  This might be due to the Wool absorbing some of the water vapor into its fibers.


Humidity inside Polyiso

For the Polyiso, I cut a little plug out of the inside polyiso board, placed the humidity sensor in the hole, and them reinstalled the plug with tape.  I suspect this does not seal the sensor in all that well, and the humidity it reads is just some air that leaks into the sensor area.  Since the Polyiso is essentially impermeable to water vapor, I’m not much to be learned from this plot.

Enclosure Humidity/Temperature:

This plot shows the humidity and temperature near the middle of the enclosure.

Blue line is relative humidity inside the enclosure (%)
Black line is temperature inside the enclosure (F)
Green line is temperature outside the enclosure (F)

The first 0.6 hours, the humidifier is off and humidity is down around 30%.
Then the humidifier is turned on and in a bit over an hour, the RH goes up to the target level of 70%.

At about 6 PM, the humidifier is turned on and remains on until about 8 AM the next morning.

The change at about 8 PM is from my accidentally turning off the heater for a while.

The change at about 8 AM on the 2nd day is the humidfier was turned off — that is, the start of the drying period.  The change at about 11 AM is introducing more ventilation into the enclosure to lower humidity and speed up drying.


First, I would encourage everyone to read over the test details and see what they mean to you in your situation.

Beyond that, here are a few things that seem pretty obvious…

  • Judging by thermal pictures, all of the insulation types tested do insulate!  And, it appears that for the level of condensation in this test, the insulating properties (R values) are not strongly effected by the condensation (which surprised me).
  • Both of the fiber insulations (wool and Thinsulate) do allow water vapor to penetrate and condense both in the outer layers of insulation and on the van wall.  The Polyiso with sealing around the edges did not let water vapor penetrate and does not condense water on either its outer or inner face.
  • The wool appears to have less visible condensation than the Thinsulate – probably due to some of the water being absorbed into the wool itself.  Havelock calls this moisture management, but others would say that having insulation absorb water into its fibers is just not a good thing.  Take your choice 🙂


Beyond what’s tested here, there are other things to consider:

  • Ease of installation
  • Cost
  • Chemical or allergy sensitivity
  • Noise reduction properties


I’ve started a mold test.  I know zip about mold, but talked to a mold expert at MSU, and she suggested this simple test as a start:  In a separate container for each insulation, add some dirt and plant matter (which will contain a variety of mold spores), then add the insulation sample to each container.  Using a spray bottle with non-chlorinated water, moisten each container and place the containers in a warm spot — keep moist over the length of the test.  See if mold forms on the insulation samples — this may take several weeks.

Thanks very much to Dr. Cripps at MSU for suggesting this test.

Since some of the insulation types produce moist warm temperatures, there is at least the possibility of mold growing.  Both Thinsulate and Havelock wool offer arguments that mold will not grow on their insulation, but other people commenting on this are skeptical.


Left picture is one of the three containers that has some dirt/plant and an insulation sample. Right picture shows the temperature controlled box with the three samples in it.  Temperature is set to 77F.

I do realize that in addition to oxygen, moisture and warm temperatures, that mold requires food to grow, but from what I’ve read the food can be as subtle as organic dust particles, oil from your hands, dead skin cells…  And, these may be present even if the insulation itself is not digestible by the mold?

If you have any thoughts on how the mold test might be made more effective, I’m all ears.

Follow On

First, if you see anything in the testing that looks like a screw up, please let me know.

I’ve not taken apart the insulation test chamber yet, so if you have ideas for other (practical) tests that might be helpful, let me know in the Comments below, or on the ProMaster Forum.

I’ll report on how the mold test comes out in a few weeks.

I do plan to set up the humidity/temperature logger in the van for our next van trip just to see what the actual humidity levels are on a real trip.

Comments and Questions

Gary November 28, 2020





  1. Gary you are the man. Thanks for this helpful test and using science when so many opinions are running rampant.

  2. Have you heard about the insulated windows from Arctic Tern? I used them and they seem to perform very well. Even covered/insulated the windshield will become a mini-amazon of condensate while the added windows might have a light fog.

    They are pricy (but you get retractable insect screen and privacy/insulation shade in the deal) but it would be great to see what their performance is in your humidity box, embedded, say, in a polyiso “wall”. Maybe get a loaner?


    • Hi Larry,
      I have been following the Arctic Tern windows and reading comments from people who have them. They seem to have a lot of happy campers.

      If I were doing my van again, I’d definitely consider using them. The only thing I am still a bit hesitant about is the plastic glazing scratching — although I’ve not yet seen anyone complain about that.

      That would be a neat test — I’ll think about dropping them an email.


    • Hi,

      I guess I would say it was inconclusive. I did not see any mold on any of the samples, but I was not consistent in keeping the samples damp.

      Rethinking how to do the test better.


  3. I have been reading a multitude of Blogs about different types of insulation Pros and Cons but can’t find any feedback on getting a Company that specializes in spray foaming containers , basements and other out buildings . I have completely gutted my 1977 Gruman P30 all Aluminum Van and would appreciate any guidance on this subject .
    Thanks Yvon

    • Hi Yvon,
      There are commercial outfits that will spray foam urethane foam — I would try to find one that does vehicles as there is a risk of distorting the sheet metal if the spray is used layers that are too thick. Make sure your contractor is familiar with this problem and knows how to avoid it.

      You can buy DIY foam kits — that’s what I did for my van:
      Its fairly easy to do and you can use multiple thin layers to avoid the panel distortion.

      I’d also consider using Rigid Polyiso insulation board adhered to the walls with Great Stuff urethane foam in a can. See down this page a ways:

      With the flat walls on the Gruman, the Polyiso would go up quickly and easily. If it were my conversion, I’d go with the Polyiso.

      Would love to hear how your conversion comes out — always liked the Grumans.


  4. Amazing how detailed and thorough your experiments are.
    I wonder about any condensation between the polyiso and van wall (little as it may be) – could that freeze and create larger gaps, possibly compromising the polyiso attachment to the van wall? I may not be as good an installer of polyiso, and if some condensation can get between the polio’s and van wall…

    • Hi Dave,
      If you start with an air pocket under the polyiso that is 6 inches by 6 inches by half an inch thick, and the air in the pocket starts at 70F with 70% humidity, and you condense out all of the water in the air pocket, the volume of the water is 0.0026 cubic inch. A very very tiny fraction of the volume of the air pocket. So, as long as an air pocket is sealed off and no new moist air can get into it, I don’t think ice is a problem.
      If the polyiso install allows van air to freely circulate behind it, them maybe the ice could be a problem. But, I know some people make a point of installing the polyiso and leaving air access to the back side and I’ve not heard of any of them having a problem.

      On the test, just a small bead of Great Stuff around the periphery of the Polyiso was enough to seal it and prevent condensation on the Plexiglass.
      If installing Polyiso myself, I would use a grid pattern of Great Stuff on the back of the Polyiso such that when the Polyiso was pressed against the van skin the Great Stuff would cover almost all of the Polyiso surface, and then do a bead of Great Stuff around the full periphery.

  5. Very nice for someone to start trying to get empirical evidence on this most contentious of issues.

    In a real van the skin of the van is curved. Thus any rigid insulation board will not sit flush with the van skin. It would be interesting to see a follow up where an airgap is introduced between the polyiso and the simulated van skin.

    • Hi,
      I guess that depends on the particular van.

      The ProMaster wall panels and ceiling are pretty flat and for 1 inch Polyiso it is pretty easy to bend the Polyiso to the curvature. Most people use “spring sticks” to hold the Polyiso against the van skin while the Great Stuff cures and glues the panels into the curved shape.
      This may be harder on other vans — not sure?

      But, I do think that as long as there is a good bead of Great Stuff all the way around the periphery of the Polyiso panel that some trapped air behind the panel will not have enough moisture to produce any visible condensation. I went through the numbers on this, and as far as I can remember, a cup of air contains less than half a drop of water. It think it takes a porous insulation with a flow of water vapor laden air flowing through it to get visible condensation.
      But, I’ll see if I can work in an air gap on one of the tests.


  6. Very informative and interesting. Thanks

    You didn’t observe condensation with the polyiso. Is there any air gap between the Polyiso and plexiglass? That information would tell us a little bit more about the importance of avoiding air gaps.

    Ideas for others tests:

    It would be very interesting to add a period of low humidity at the end of your cycle to monitor how well the insulation materials are drying out.

    The often put forward advantage of Thinsulate is the fact that it is hydrophobic (doesn’t absorb moisture). It should dry out quickly. The chemical formula of Thinsulate is very close to many other petroleum based polymers such as Polyester fibers that are used in garments and that can be bought at any good fabric store at cheap price. That would be very interesting if you would add a panel to compare the performances of both Thinsulate and Polyester.

    • Hi Dennis,

      There was no air gap between the Polyiso and the plexiglass, but I think the key to the no condensation was the bead of Great Stuff around the periphery of the Polyiso. That is, any air pockets between the plexiglass and Polyiso don’t contain enough water vapor to make visible condensation. I thin it takes an influx of water vapor laden air to get condensation.

      I do plan to do a multi day test to see if each of the insulation panels continues to pick up more and more moisture, or if it reaches an equiliberium — I’ll think about adding a longer drying out period at the end.

      On the first test, the Thinsulate did dry out faster than the wool, so , maybe that is an indication it will dry out pretty well given some time.

      Will think about trying to find some polyester to test.
      I am adding Aerocel closed sell foam to the insulation samples. This seems like a good candidate, but it is pricey.

      Thanks for the feedback.


  7. I have been reading everything I can find on this topic. I appears that everything conflicts everything else. I dont plan to live in my van just weekend – maybe a week at a time. i think the best plan for me is to supply plenty of ventilation so that water vapor never gets into walls but have plenty of DRY makeup heated air to stay warm “enough” and use an electric blanket if needed. I think spray in Iso is probably the only real option for fully insulated ” Live -in”

    • Hi Frank,
      It is confusing to have so many opinions out there on insulation — and, so little data to support those opinions. That’s why I thought some actual data might be helpful to people.

      I have spray urethane foam on my van, and I’m happy with it. It does prevent water vapor from getting to the van walls and condensing. I believe that the same thing can be accomplished with rigid polyiso sheets adhered to the van wall with Great Stuff and then a full bead of Great Stuff around the periphery. That’s what I would likely do on a next van.

      Good ventilation will help minimize condensation, but my feeling is that there will be times when you can’t ventilate enough to prevent condensation. I plan to do some more testing on this in my van in the near term.

      Honestly, I would not overthink this too much. There are lots of van conversions around out there and its rare to hear of anyone having a serious problem caused by a bad insulation job.


  8. Very informative and appreciated! Im debating what insulation to go with in my sprinter conversion. several trips made w o insulation and woke up to a dewy sleeping bag.
    Newbie question, if the insulation is attracting some moisture doesn’t that reduce the level of moisture in the air and what might end up on my bag through the night? My trips are 3-4 nights max with weeks in between, wondering how much i should worry about the moisture level or in my case maybe its a slight positive? thinking of thinsulate so far, ease of use and i liethe black side showing. thank you for any thoughts. Jason

    • Hi Jason,
      The wool insulation folks make the claim that wool absorbs water vapor into it and that this helps to control the humidity, and then when the humidity in the van drops, the wool gives back the water it has absorbed to the van.

      I’m skeptical of this in that the amount of water that the wool can hold is far less than the water vapor generated by people, cooking, coffee making, clothes drying etc. — the write gives a rough estimate of what the water sources add up to. I think that its likely that the only way to really control the humidity and condensation in the van is via ventilation. Pulling in dryish outside air and heating it up so it can hold more water, and letting it pick up moisture in the van, and then exhausting humid air should control humidity in the van — and, some people have reported that this does work. It does mean the furnace will run more to heat up the outside air you bring in.

      One person who’s opinions are usually well thought out is that one technique for reducing humidity in the van is to leave some windows without an inside insulating shade results in lots on condensation on that window, and that this reduces the relative humidity in the van. Not sure about this myself, but worth a try.

      I do plan to do some multi day tests and track how much moisture the insulation picks up at night compared to how much it dries out during the day — does it end up falling behind and getting water logged? But, I don’t think this will change the logic above that says ventilation is the key – but, I’ve been wrong many times in the past 🙂


  9. I am speaking only for a consumer view, but this is tremendously helpful! I look forward to the results of your mold test, since I live in the PNW. Thanks for doing these valuable tests.

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