The UK Van Insulation Guide: Damp is the Enemy, Not Cold

(and the honest answer to the vapour barrier argument that never ends)

There's a particular kind of van build video, usually filmed somewhere with reliable sunshine, where a tanned man in a vest explains how to keep your van warm for the Alpine winter. Lots of talk about R-values. Lots of talk about cold. It's all very aspirational and almost entirely useless to anyone building a van in Britain, where the genuine threat to your conversion isn't a frosty night in the mountains — it's the slow, patient, year-round project of the weather trying to turn the inside of your van into a terrarium.

Because here's the thing nobody mentions when they're stapling sheep's wool to a van in Arizona: in the UK, insulation is not really about keeping warm. Cold is easy. Cold is a solved problem — you buy a diesel heater, you turn it on, you are warm. Job done.

Damp is the one that gets you. Damp is the mould creeping across the back of your ply lining. It's the condensation that drips off the ceiling onto your face at 3am. It's the smell — that faint, mushroomy, never-quite-gone smell that you stop noticing but your passengers absolutely do not. Damp is what quietly destroys a conversion over three winters while you're busy admiring your worktop. So this guide is built around keeping the damp out, and everything else serves that goal.

The enemy: condensation, not cold

A quick bit of physics, in plain English, because it's the single thing that makes the rest of this guide make sense.

Warm air holds more moisture than cold air. You, breathing, are a moisture machine. So is your kettle, your one-pot pasta, your damp coat, and — if you have one — your wet dog, who is the single most effective humidifier ever invented. All that warm, moist air drifts around the cabin until it meets something cold. In a van, the coldest thing is the metal skin, sitting there at the same temperature as the rainy car park outside.

When warm wet air hits cold metal, the moisture condenses out as water. This is the same reason a cold pint glass sweats in summer. Now picture that happening across the entire inside surface of your van, behind the lining, where you can't see it and it can't dry out.

So the real job of insulation isn't just slowing heat loss — it's keeping that metal skin warm enough that water doesn't condense on it in the first place. And critically, insulation alone cannot do this. You also need ventilation, to physically remove moist air before it has the chance to condense anywhere. In a UK build, ventilation is not optional and not secondary. It is the equal partner of insulation, and I'd argue most failed conversions failed on ventilation, not insulation.

Materials

Here are the main options, judged honestly on UK availability and the UK damp problem rather than how they look on Instagram.

Celotex / Kingspan (rigid foam board). The workhorse of the British van build, mostly because you can buy it at any builders' merchant on a wet Tuesday. Excellent R-value per millimetre, cuts easily with a bread knife, cheap enough. The catch: it's completely non-breathable, which is exactly why the vapour barrier argument below exists. Common spec is 50mm in the walls and roof, dropping to 25mm in the floor where every millimetre costs you headroom.

Thinsulate (3M). Flexible synthetic batting, thin, and brilliant for the awkward bits — door cards, pillars, the fiddly curved cavities where rigid board simply won't go without a fight. Less R-value per millimetre than foam, but it leaves zero gaps in the places foam struggles, and gaps are where the trouble starts. Importantly, it's breathable, which puts it on a completely different side of the vapour barrier debate.

Sheep's wool. UK-grown, naturally breathable, and genuinely clever about moisture — it absorbs and releases water vapour without rotting or losing performance. It's the most forgiving material for a breathable build. Downsides: it costs more, and sourcing it in the right thicknesses for a van (rather than for a loft) takes some hunting.

Spray foam. Fast, fills every gap, sticks to everything. Including, permanently, the things you might one day want to unstick. It's near-impossible to remove if you ever need to get back to the metal — to chase a leak, say, or refit a window — and it's a known headache for resale and van finance, as lenders and buyers have learned to be wary of it. I don't recommend it for a conversion.

Sound deadening — different job, different material. Worth saying clearly: butyl mats (Dynamat, Noico, STP Gold) are not thermal insulation. They're heavy bitumen-backed pads you stick to bare metal to stop the panels resonating like a drum. They do nothing meaningful for warmth or damp. Do this job first and separately, and don't let anyone tell you it replaces insulation.

Installation order

1. Sound deadening on the bare metal — optional but recommended. You don't need full coverage; 30–50% of each panel kills most of the resonance.
2. Insulation layer.
3. Vapour barrier — if you're using one, and that's the whole next section.
4. Ply lining and fit-out.

The vapour barrier debate — and an actual answer

Right. This is the bit you came for, and it's the bit every other guide goes strangely quiet about.

It is genuinely remarkable how much human passion can be generated by a sheet of polythene. Forum threads about vapour barriers run to twelve pages and end in people calling each other dangerous amateurs over the correct way to tape a seam. Marriages have probably been tested. So let me lay out what's actually being argued, and then — unlike most guides — actually tell you what to do.

What the debate is. A vapour barrier (VB) is an airtight membrane installed on the warm side of the insulation, between the insulation and your ply lining. The idea is to stop warm, moist cabin air from ever reaching the cold insulation and metal behind it, so it can't condense there. One camp says: of course you fit one, this is basic building physics. The other camp says: a vapour barrier that isn't perfectly sealed is worse than no barrier at all, because instead of keeping moisture out, it traps the moisture that inevitably sneaks in — and then that moisture has nowhere to go.

The honest UK position. Both camps are right, which is why they argue forever. A perfect vapour barrier — every seam taped, every penetration sealed, fully airtight — works beautifully. It's standard practice in proper house construction, and there's nothing wrong with the theory.

The problem is the word perfect. In a house, the warm-side membrane is a big continuous sheet behind plasterboard with relatively few things poking through it. In a van, your VB has to survive being pierced by every single cable, every water pipe, every gas line, every self-tapping screw holding your battens on, and every bolt for every cupboard you'll add later and the three you'll add after that. Achieving true airtightness is, realistically, a fantasy. And a 95%-sealed vapour barrier may genuinely be worse than none, because you've created a sealed envelope with a few small leaks — exactly the conditions to let moisture in and then trap it against cold metal with no way to escape.

The pragmatic recommendation. So here's what I'd actually do, split by which insulation you chose:

• If you went with rigid foam (Celotex/Kingspan): fit a vapour barrier and do it properly. Tape every seam, seal every penetration you can reach, use proper foil tape rather than whatever's in the drawer. The rigid foam is your primary defence here; the VB is a useful second line. Accept from the start that it won't be perfect, and build in good ventilation precisely to cover for the gaps you can't seal.
• If you went breathable (Thinsulate or sheep's wool): skip the plastic VB entirely. The whole point of breathable materials is that they handle moisture passively — letting it move through and dry out rather than trapping it. Bolting an airtight membrane onto a breathable system gives you the worst of both. Put your energy into ventilation instead.
• Either way, the bit that matters most: fit two roof vents, one toward the front and one toward the rear, so air actually flows across the cabin rather than sitting still. Cross-flow ventilation does more real-world good in a UK van than any amount of forum debate about which brand of tape is vapour-tightest.

And one more thing about the failure mode, because it's sneaky. A bad vapour barrier doesn't fail dramatically. Nothing leaks on day one. Instead, twelve to eighteen months later, mould quietly appears behind the ply lining — out of sight, where you won't find it until you smell it or until you pull a panel for some unrelated reason and discover a small ecosystem. Then comes the part where you strip out work you were rather proud of. That's the cost of getting this wrong: not a disaster, just a slow, deflating refurb.

Thermal bridging

Your van's metal ribs and frame run straight through your lovely insulation and connect the cold outside to the warm inside — a thermal bridge, conducting heat out and cold in, and very often the coldest interior surfaces where condensation shows up first. You won't eliminate it, but you can deal with the worst of it:

• Stuff the hollow ribs with offcuts of foam — there'll be plenty.
• Run a thin layer of foam (6–10mm) across the rib faces before the main insulation goes on, so the metal isn't sitting directly against the lining.
• Make peace with the fact that a van is not a Passivhaus. Some bridging is unavoidable; covering the ribs removes most of the problem and that's a fair place to stop.

Floor, walls, roof — where to spend your effort

• Roof: top priority. Heat rises, so this is where most of it leaves, and the roof takes the worst of both the sun and the freezing rain. Do this properly.
• Walls: second. Conveniently also the easiest large surface to insulate well on a standard panel van.
• Floor: don't skip it. It gets neglected because every millimetre of floor insulation is a millimetre off your headroom. 25mm of rigid foam under the ply floor is the accepted compromise — thin enough to live with, thick enough to matter.
• Doors: the forgotten frontier. Easy to ignore, easy to do. Thinsulate pushed into the door cavities makes a real, noticeable difference to both temperature and road noise for very little effort.

Common mistakes

• Leaving gaps in the insulation. A gap is a cold bridge. Uniform thinner insulation beats thick insulation with holes in it every time.
• Gluing rigid foam to dirty metal. Van panels come coated in a film of factory grease. Skip the degreasing step and your beautifully cut foam will quietly peel off in the dark and slump into the sills.
• Forgetting the floor. See above. People do this constantly and feel it every winter.
• Fitting a vapour barrier and then drilling forty holes through it for your electrical runs. If you're going to seal it, plan your cable routes first.
• Skipping sound deadening and then spending every motorway journey listening to individual stone chips ping off the wheel arches like you're inside a snare drum.

Get the materials sensible, the gaps filled, and — above all — the ventilation right, and the vapour barrier question becomes a lot less frightening than the forums make it sound. It's a sheet of plastic. Important, occasionally, but a sheet of plastic. The damp is the enemy. Build for that.