I've been researching this very thing for about 6 months now. Here's more or less what I've settled on. (Started my build this week, so insulation is soon.)
*There's a summary at the bottom.
Things to keep in mind:
- Brand new vans are not likely to rust as the paint is fresh, and you'll be covering it. The paint can withstand moisture. Bare metal is what you need to worry about.
- There are multiple types of insulation, as heat can move three different ways: Radiation, Convection, Conduction.
- What climate will you be in? I'm building mine for mixed, but if you're in the desert you'd care less about moisture and more about radiant heat. Make sense?
- I have over a decade of paint and coating experience and work part-time as a handyman. I've also consulted a government PhD space and aeronautics engineer. And I've watched probably 200 hours of van build videos on youtube. So hopefully this info helps!
Things not to do:
- I would NOT adhere any insulation directly to the steel walls. For me, there is great value in the ability to remove panels and check the the painted metal for failures.
- DON'T use Reflectix without an air gap... or at all. Aluminium foil is cheaper per sqft... and the bubble wrap in the center does nothing but add slight rigidity. Don't adhere this to the steel. The plastic of the bubble wrap will 100% degrade over time in the heat. The walls can get to 170°F in the direct desert sun which will not bode well for your expensive bubble wrap.
- DON'T use fiberglass. It causes cancer when you breath it in often. Given the van will be moving around a lot, fiber dust will work it's way in, unless you have a hermetic seal, which no one does... ALSO, and maybe more direct to this thread, it very very very easily traps moisture. This is why Closed Cell Foam is key... also Hydrophobic Melamine is a good alternative.
Remember! -- The engineering around insulation is different for a moving metal can house than it is a wood stationary house. Making walls in a van like you do in a house doesn't make sense.
Layers of progression:
MOISTURE BARRIER FOR STEEL WALLS
First I removed anything along the floor and walls that will ultimately be behind a wall or floor. I sanded down any surface rust to the best of my ability. If you're van is new you'll just need a light scuffing with ~120 grit sandpaper and a soft hand, DON'T break through paint, just dull it.
I am then applying RustOX from Encore Coastings. No VOCs, non-toxic, durable, flexible, waterproof paint made for painting tugboat bottoms and industrial deep-sea oil rigs. Made to stand up to salt water environments, underwater, for years. You'll probably want 2-3 gallons, which should run you around $200-300. You can apply it with a brush, roller, or spray it.
I did this because it's way way cheaper than Line-X or Rhino Liner, I don't need the tread, I wanted to do it myself, and I wanted something made for preventing water, not for adding grip and ding-resistance to a truck. There are other options... (look up EcoPoxy or EcoDur), both are more expensive and may require a pro to do it, but would perform even better.
NEXT IS A RADIANT BARRIER
After researching thermodynamics and consulting my scientist buddy, we decided to apply a layer of aluminum foil --- Not your kitchen variety, but it'd still work... --- I'll be getting mine from AtticFoil. It's thick, durable, etc.
Adding a layer of foil direction to the painted metal will not only add more protection from moisture and elements but also reduce the radiant emissivity of the steel from TERRIBLE, to ALMOST PERFECT. Aluminum blocks 97% of radiant heat. Considering a van is a steel box, that is where the majority of heat enters your vehicle, after windows (which can also get a shiny aluminum barrier and/or Ceramic Window Tint.
You then NEED to have an air gap. I'm using the 1-2inch gap that is already available to me from the ribbing in the van.
** EVERYTHING that touches the aluminum will counteract the barrier effect, since Aluminium conducts heat almost perfectly. So only apply to areas that aren't going to touch other things.
NOW A CONDUCTIVITY BARRIER
Ok, so now I'm adding Closed Cell Foam (won't absorb water, blocks vibration, reduces the conductivity from steel to wood) to the ribbing, and then adding a firing strip.
To the firing strips I'm using 1" Polyiso rigid foam panels. I'm using this because it has the highest R-value of a rigid panel (R-6 to R-6.5 per inch which is better than fiberglass or mineral wool). They come with foil on them. I'll face that foil to the inside for a radiant internal barrier as well.
Polyiso also doesn't hold moisture, is super light, can withstand 270°F, durable when the van is bumping along and lightly flexible. (2" panels will not be flexible).
I'll tape these with reflective duct tape to create a semi-vapor barrier.
INTERIOR RADIANT/CONDUCTIVE/VAPOR BARRIER
Finally, I'll add a ~1/2" firing strip (like a Lath strip for plaster) to create a small air gap for the polyiso foil side, and then to that I'll adhere my wood paneling for the inside of the van. On the back of the paneling I might add a layer of thinsulate or some sort of closed cell foam or Mass Loaded Vinyl as a vapor barrier, not sure yet.
One thign to consider is that a moisture barrier is less useful when you've already sealed the steel, and all your components don't absorb moisture. If you're in a place that's damp or moisture otherwise becomes a problem, you might actually want some air flow to allow it to dry out.
*** To accomplish this, I consider convection. I might stick a small 12v or solar boat vent fan in my roof. If the air gap between the steel/aluminium and my polyiso is moderately connected from side to side and to the ceiling, I can vent that air space which will both eliminate moisture AND use convection to elminiate pretty much ALL heat in the summer. (You'd want the fan to turn off in the winter at night so you don't loose built up thermal energy in the wall...)
SUMMARY
-Steel
-RustOX
-Aluminum
-Air ~2"
-Closed Cell Foam on connection points to dampen sound and conductivity.
-1" Polyiso Rigid Foam
-Aluminium (already on the polyiso)
-Air ~1/2"
-Thinsulate
-Wood
This should give you an R-Value of about R-12 and should pretty much eliminate Radiant Heat gain or loss. Few people talk about or really design for Radiant heat, but when you're living in a metal box, it really ought to be front and center, especially given you only have a few inches to work with.
Hope this helps! I'll be posting my build out either here or on Expedition Portal this week, stay tuned!