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A YouTube viewer on one of  my vids has challenged my method of insulation.

Him:
So you have created a semi-sealed micro-climate where the water vapour can repeatedly condense and vaporise ? - I don't understand how you think this is better than filling the space with close cell (ie. impervious to water) foam. How can condensation exist if there is no space?? I think you are actually going to end up with standing water collecting in the ceiling.

Me:
Even if you had 2 inches of rigid r10 foamboard eventually warmth is going to permeate thru to the cold metal... condensation will form where the two meet. If you don't have air flowing through there moisture will build up and become a problem. That is my understanding. If you can share a link that refutes that I would be interested to read it. All the better if it's from a reputable source and not just some dudes blog. Thank you. ☺

Him:
Well I will try and find some references, but what has to permeate is warm wet air not just warmth and if you attach reflectix to the metal then closed cell foam then reflectix all of which are waterproof, in reality water penetration is never going to happen - whereas you are leaving 2-3cm of moist air beneath a cold roof unprotected. This is not some abstract theory - many including me have done this. Whilst you still have a naked roof you could try an experiment - just stick a 6" square of reflectix on the ceiling and experience the difference between the covered portion and the naked roof on a cold night.

***
He has me second guessing myself on this topic. 

Thoughts?
I responded to his comment on you page with this:

So a few things here:
1) Reflectix is a radiant barrier and using it without an airspace is both incorrect and makes using the product pointless. Used as you describe below is incorrect.
2) While using foam to displace air volume (and potential for moisture) is good theory, the only way to accomplish this in a van is using 2-component spray foam (not the cans). Short of that you aren't going to be able to create a truly airtight pocket in a mobile setting with the materials you describe.
3) When faced with choice between "attempting" to make an airtight pocket, or accepting that it isn't going to happen with the materials we're talking about, Inch has the better approach: i.e. accept permeability and allow ventilation so moisture can dry out.
4) The question then becomes "what is adequate ventilation" and I don't have an answer for that. I would say that gap between ribs and ceiling might be enough, but perhaps more would be better.
Brad has it right.

It's a metal box, you need to let it breathe.
Talking about insulation is akin to religion, politics or motor oils.

There are basic parameters and testing results but that is mixed in with personal experiences, conflicting advice, biases (real,or imagined) and simple Stubborn bull headedness.

With metal, be it a car or a building, the Sun quickly warms metal and the cold quickly cools metal.
As a result you have quick massive temperature swings to deal with. If you have a cold surface and a sudden influx of warm moist air, you will end up with a wet dripping surface.

Ideally you would use a radiant barrier that is NOT fastened to the metal. You want an air space so the radiant barrier can work.  
In an ideal situation that airspace would be sealed with an inert moisture free gas, not unlike double insulated windows are.
Sadly, you cannot do this in a vehicle so you want to leave a space of air. Ideally this air space is allowed to ventilate itself as well.
But honestly, this air space will end up somewhat stagnant.

There are reflective barriers that offer about zero insulation capability such as reflectix,
There are barriers that do offer insulation value such as Prodex sold by insulation4less.com

What you do between the reflective barrier and the interior Van space is up to you. Ridgid foam covered by wood or fabric? Skip the foam and do your final finish?
Educate yourself and come up with a solution that YOU are comfortable with. There is no 100% correct answer in a moving box that can be moved from a humid tropical area to an arid area to a artic area in a matter of days.
A building stays put and you insulate in a manner which works for your locale.

Personally, I'd favor a product such as Prodex that provides both attributes and cover it with a ridgid decorative panel.

Do the best you can and deal with whatever consequences arise.

Whatever you do, shun fiberglass batt insulation, you might just as well hang wet sponges inside your Van. Even the wrapped Batts will be trouble prone.

Your greatest moisture source will be YOU living in your Van. Cooking, breathing, cleaning, heating, etc. 
This human moisture is the one you really want to mitigate and control. Choosing your appliances wisely and ventilation are your best methods.  Run from unvented propane heaters and spend a little coin on a proper RV furnace. Cook outside if you can. Stop breathing too. 

The natural environment you will deal with the best you can by education and planning. 

Dave
Thanks for the responses guys. Much appreciated. 

One of my biggest concerns is about the particular area where the van's metal would meet the insulation if no air gap is left. From my understanding warmth will permeate through the insulation and eventually meet the cold metal.  If that's the case, won't condensation form where the two meet?
Condensation can only happen where there is air with moisture in it completely preventing that is the idea of the guy who posted on your video. I don't believe it is realistic/possible to completely make a space airtight for good... at least not in a van
Plus moistureless air sounds like an abundance of bloody noses.
When the outside temperatures drop below the inside dew point temperature you will get condensation on the inside of your windows.

Condensation on windows does not necessarily mean your getting condensation elsewhere in the vehicle As long as the temperatures in "spaces" within your vehicle remain above the dew point there shouldn't be condensation. But closed up cabinets and closets against outer walls can get quite chilly.

Sent from my iPhone
Anyone who is aiming for perfection in this matter isn't going to get it. Just do the best you can.