Condensation ruins more van builds than rust ever starts on its own. Warm air inside a van hits cold steel panels and turns straight to water, and that water sits behind the insulation where nobody sees it until the smell shows up. Caravan and campervan registrations in Australia reached almost 938,000 in 2025, up 32 per cent since 2019, so more builders are hitting this problem for the first time. Getting a vapour barrier van conversion right stops that damage before it starts, but the order you install things in matters just as much as the material itself.
What Order to Install Everything In
- Strip the van back to bare metal and let it dry fully, even overnight if humidity is high.
- Fit rigid or closed cell insulation against the panels first, sealing every gap with tape or spray foam.
- Run the vapour barrier over the top of the insulation, on the warm, living side only.
- Tape every seam and every screw hole, since a single gap lets moist air straight through.
- Fit the interior lining last, once the barrier is fully sealed and checked.
Common Mistakes That Undo the Whole Job
Most failed builds skip step two. Fitting a vapour barrier straight onto bare metal with no insulation behind it does nothing, because the metal is still the cold surface moisture wants to hit. Others tape the barrier loosely, thinking a few staples will hold, then wonder why damp patches show up near the wheel arches six months later. Spray foam gaps first, tape second, barrier last. That order is not flexible. Door seals cause a second, quieter failure. Factory seals were built for a bare cargo van, not one with insulation packed behind the trim, and they compress unevenly once panels go back on. A cheap run of replacement seal strip fixes this in an afternoon and stops the exact spot most builders never think to check.
Materials That Actually Work in a Van
| Material | Job it does |
| Closed-cell foam or XPS board | Insulates and resists water on its own |
| Reflective foil or polyethylene sheet | Blocks vapour once taped and sealed |
| Foil tape, not duct tape | Holds a seal through heat and vibration |
| Spray foam | Fills gaps insulation boards cannot reach |
Why the Climate Still Matters
A van parked in Cairns behaves differently to one parked in Hobart. Hot, humid regions push moisture inward, so a barrier fitted only on the inside can trap it rather than block it. Cooler regions push moisture outward from cooking, breathing and hot showers taken nearby, which is the more common pattern for Australian van builds. Fit the barrier for the climate the van will actually live in, not the climate it was built in. Builders working out of Sydney or Melbourne workshops sometimes forget their finished van is headed north for winter, and that single oversight undoes an otherwise solid build.
Test the Seal Before You Close the Wall
Ventilation still matters even with a perfect barrier fitted. A sealed van with no airflow traps whatever moisture gets in anyway, cooking and breathing included, and a barrier was never meant to replace fresh air. Roof vents, cracked windows on dry days, and a small extractor fan all pull double duty, working alongside the barrier rather than against it. One cheap test catches most gaps before they become a problem. Run a phone torch behind the finished barrier at night while someone checks outside for light leaking through seams. Any glow means a gap, and every gap found this way is one less damp patch found the hard way, months later.
