Hi,
You might want to read over this insulation test to understand condensation with porous insulations (like Thinsulate) and non porous insulations (like polyiso).
www.buildagreenrv.com
Your setup should perform very close to the test as it also allows free circulation of air behind the wall panels. This assumes that the breathable membrane is fairly open to air circulation.
Basically, the porous insulations will result in condensation in cool/cold weather (whenever the outside air temperature is below the dew point of the air in the van). With your arrangement (allowing more air circulation behind the wall) there will be more condensation, but it will also result in faster drying once the air in the van is dryer.
Non-porous insulation will normally not produce condensation because the inside surface of the insulation will be above the dew point of the van air.
Gary
My opinions closly parallel
@GaryBIS regarding the insulation, condensation, and mould building science.
Many DIYers are kinda “experimenters” by nature. If you do proceed with your design it would be good to record the results over the years & post them on this forum for all of us to learn from. Building Science theory suggests to me there are better ways to insulate a van than your design. I will say it is good to think outside of the box for solutions, vet them, & then prove it good or bad based upon testing.
The following are reasons I believe your design would perform inferior to a total spray foam or polyiso filled in with “Great Stuff” (walls & ceilings) & high load XPS on the floor. I also like the aesthetics of the “teahouse”, however I am not considering the aesthetics here just insulation performance. The conundrum here is a “Double Vapour Barrier” - it is not accepted in buildings & I do not like it in vans, but we inherit this problem somewhat by insulating a van & camperizing it.
insulation performs poorer if air is allowed to migrate thru it
condensation will more easily form on the van skin (open transfer of water vapour)
the breathable layer is on the inside & can not be installed on the outside, where it normally would be
wood is mould food (you need to seal it well & at least it is on the inside, however depending on your interior RH & usage I could see dew forming on the wood & if not completely sealed it could absorb water)
dust & debris can be mould food & with an open breathable design if particles can get into the Thinsulate & then aquire enough moisture then you could have mould growth trapped in the Thinsulate.
Here i another thread on insulation & mould if you have not already read it. Lots of information & opinions there to digest;
Hello DIYers Due to a rather large topic & one of many opinions, I have started this thread in hopes to create logical conversations, sharing ideas, & amicable debate concerning the Van Build Envelope
www.promasterforum.com