Van Vapour Barrier Guide
Every UK van conversion should include a proper vapour barrier. Without one, the warm, moist air inside your van will pass through the insulation and condense on the cold metal panels, leading to rust, mould, and rotting wood within 12-24 months.
This guide explains why, how, and what materials to use.
Why a Vapour Barrier Matters
The air inside your campervan contains moisture from:
- Breathing (a person exhales approximately 1 litre of water per night)
- Cooking
- Boiling water for tea/coffee
- Drying clothes
- Wet dogs and coats
This warm, moist air naturally moves towards cold surfaces — the metal body panels. If it reaches the metal through your insulation, the moisture condenses (like a cold pint glass on a warm day). This condensation runs down the inside of the panels, collects in the bottom of your van walls, and causes rust from the inside out.
The vapour barrier stops this by providing a sealed layer that warm air cannot pass through.
Where to Place It
The vapour barrier goes between the warm living space and the insulation. This is the critical detail: on the warm side of the insulation, not the cold side.
[Outside] → Metal panel → Insulation → Vapour barrier → Interior wall board
Some converters incorrectly place the barrier against the metal. This traps moisture between the metal and the barrier, causing the metal to rust faster.
Best Materials for a Van Vapour Barrier
1. 500-Gauge Polythene Sheet (Best Budget Option)
- Cost: £15-25 for a 10m roll
- Thickness: 125 microns (500-gauge minimum)
- Pros: Cheap, easy to staple, widely available (Screwfix, B&Q, Wickes)
- Cons: Can tear if stretched, must be taped at every seam
How to use: Cut to size, staple to the wooden battens/framing, overlap seams by 15cm, seal all overlaps with aluminium foil tape (not standard parcel tape — the adhesive degrades).
2. Reflectix (Best Premium Option)
- Cost: £15-20 per 10m x 60cm roll
- Structure: Double-layer bubble wrap with reflective foil on both sides
- Pros: Acts as both vapour barrier and radiant heat barrier, tear-resistant
- Cons: More expensive, harder to seal completely
Reflectix is popular in van builds because it does double duty. The foil reflects radiant heat back into the van while the bubble layer adds R-value.
How to use: Staple directly to battens. Seal seams with Reflectix-brand tape or high-quality aluminium tape.
3. Self-Adhesive Vapour Barrier (Best for Curves)
- Cost: £30-50 for 20m x 1m
- Products: DPM (Damp Proof Membrane) tape, Visqueen, or sheet membrane
- Pros: Sticks directly to metal, excellent for wheel arches and irregular shapes
- Cons: Expensive, harder to work with on large flat panels
Step-by-Step Installation
Step 1: Clean the metal
Before fitting any insulation or barrier, clean the metal panels with white spirit or panel wipe. Remove all grease, sticker residue, and loose rust. Treat any existing rust with Kurust or a rust converter.
Step 2: Fit insulation
Fit your chosen insulation between the metal ribs. XPS foam board is standard for flat panels; sheep's wool or recycled polyester for curved sections.
Step 3: Install battens (if using wooden framing)
If you are using wooden battens to create a cavity for wiring and to attach wallboards, fix them to the van ribs using self-tapping screws and rivnuts or Plusnuts.
Step 4: Fit the vapour barrier
Staple your chosen vapour barrier material over the insulation and battens. Overlap all seams by at least 10cm. Pay special attention to:
- Corners (use extra tape)
- Wheel arch curves (cut and overlap small sections)
- Window cutouts (tape to the metal lip before fitting windows)
- Wiring pass-throughs (seal around each cable with grommet tape)
Step 5: Tape everything
Use aluminium tape on all seams, staples, and edges. Aluminium tape has a longer lifespan than standard duct tape, which degrades in 1-2 years and leaves a sticky mess.
Step 6: Test before closing
Before fitting wallboards or plywood, check for gaps by shining a bright light behind the barrier from outside. Any light leaks = air leaks = moisture leaks.
Common Mistakes
Mistake #1: Vapour barrier on the metal side. Some online guides recommend putting the barrier against the metal. This is wrong for UK climates. The insulation must be between the metal and the barrier.
Mistake #2: Using the wrong tape. Standard duct tape fails within 12 months in a van's temperature swings. Use aluminium foil tape designed for HVAC ductwork. Price: about £8-12 per roll.
Mistake #3: No barrier on the floor. The floor also needs a vapour barrier. Fit 500-gauge polythene between the metal floor and your insulation board. Tape it to the wall vapour barrier to create a sealed box.
Mistake #4: Forgetting the ceiling. The ceiling is the most important area for a vapour barrier. Warm air rises, hits the cold metal roof, and condenses. Without a barrier here, water drips onto your ceiling boards and bed.
Do You Need a Vapour Barrier with Closed-Cell Foam?
Closed-cell spray foam (like the 2-part kits from EasyCell or DIY kits) is itself a vapour barrier when applied thick enough. But 10mm Armaflex closed-cell sheet is NOT a vapour barrier — the seams between sheets leak.
If you use closed-cell spray foam >2cm thick, you can skip the separate vapour barrier. For any sheet insulation, you need one.
Conclusion
A vapour barrier is not optional for a UK van build. It costs £20-50 and takes an afternoon to fit. Skipping it will lead to damp insulation, rusty panels, and a van that smells musty within two years.
Budget pick: 500-gauge polythene from Screwfix (£18) + aluminium tape (£10). Premium pick: Reflectix (£40) + Reflectix tape (£15) for added thermal benefit.
