The real goal: manage moisture
Every person breathes out roughly a liter of water a day, and cooking and showering add more. In a sealed metal van, that moisture finds the coldest surface, the bare steel, and condenses into liquid. Trapped against the metal behind your walls, it feeds mold and rust you cannot see.
So a good build does three things at once: slows heat moving through the walls (insulation), removes moisture from the air (ventilation and dry heat), and keeps any moisture that does form able to dry out rather than get trapped.
Cold is a comfort problem you can solve with a heater. Trapped condensation is a structural problem you cannot see until it has already done damage. Design for moisture first.
Thermal bridging: the hidden leak
A van’s steel ribs run from the outer skin to the inner surface. Steel conducts heat extremely well, so those ribs act like little highways that bypass your insulation, called thermal bridges. They are the coldest interior spots and the first places condensation appears.
You cannot eliminate them, but you can reduce them: insulate over the ribs where possible, avoid screwing furring strips straight through to bare metal without a thermal break, and pay attention to the large flat panels and the ribs alike.
Insulation materials
There is no perfect material, only trade-offs between R-value, moisture behavior, cost, and how easily it fills the van’s odd curved cavities.
- Sheep wool (e.g. Havelock) is popular for vans because it manages moisture gracefully: it can absorb and release water without losing much performance, and it is easy to stuff into cavities. Higher cost.
- Rigid foam board (polyiso, XPS) gives excellent R-value per inch on the big flat panels, but is fiddly around curves and needs gaps filled.
- Wool-like synthetic batts (3M Thinsulate) are hydrophobic, easy to work with, and glue up well, a favorite for curved walls and ceilings.
- Closed-cell spray foam has high R-value and seals air, but is messy, hard to remove, can trap moisture if misapplied, and can crack with vehicle flex.
Many builders mix materials: foam board on flat panels, wool or Thinsulate in the ribs and curves. Whatever you choose, do not leave large empty cavities where humid air can circulate against cold metal.
Skip household fiberglass batts. They hold water like a sponge, lose R-value when damp, and stay wet against the steel, exactly the failure mode you are trying to prevent.
The wall assembly
Think of the wall as a system, from steel to interior:
- Rust prevention on any bare or scratched metal first.
- Optional sound deadening (butyl mats) on large panels to cut drumming.
- Insulation filling cavities without big voids.
- A considered approach to vapor. Vans are debated territory: a hard vapor barrier can trap water if it forms on the wrong side, while an assembly that can dry in both directions is more forgiving. Many successful builds prioritize drying ability and constant ventilation over a perfect sealed barrier.
- Interior wall (paneling) that leaves the assembly able to breathe.
Ventilation is not optional
You cannot insulate your way out of moisture; you have to remove it. A roof fan is the single most important tool: run it while cooking and showering, and keep a low background level of air exchange even in cold weather. Cracking a window opposite the fan creates cross-flow.
Pair that with a dry heat source. A diesel or gas air heater burns fuel in a sealed chamber and vents exhaust outside, so it adds heat without adding water to the air, unlike a stovetop flame or an unvented propane heater, which dump moisture indoors.
The cheapest dehumidifier is a habit: fan on when you cook, wipe condensation off cold windows in the morning, and do not dry wet gear inside a closed van overnight.
Do not forget the floor, windows & gaps
- Floor: a thin layer of foam board under the subfloor cuts the cold coming up and the condensation forming under your bed. Watch total height if you have a garage or a standing-height goal.
- Windows: factory glass is a huge thermal hole. Insulated reflective covers (Reflectix-style) make a big comfort difference and cut morning condensation on the glass.
- Gaps and penetrations: every wire and bolt hole is a potential leak for air and water. Seal them.
Common mistakes
- Chasing maximum R-value while ignoring ventilation and moisture. Balance matters more than raw R.
- Fiberglass batts that trap water against steel.
- Sealing moisture in with a barrier on the wrong side and no way for the wall to dry.
- Skipping the floor and windows, then wondering why the van is cold and damp.
- Heating with an unvented flame, which adds the very moisture you are fighting.
Quick checklist
- Treat any bare metal against rust before insulating
- Fill cavities with a moisture-tolerant material, no big voids
- Reduce thermal bridging at the ribs where you can
- Insulate the floor and make insulated window covers
- Install a roof fan and use it; add cross-ventilation
- Heat with a sealed, vented (dry) source
- Build so the wall can dry rather than trap water