Last updated: September 12, 2026
The insulation question in a van is not “what has the highest R-value per inch” — it is “what happens to this material when the steel behind it drops below the dew point and water forms on it.” A metal box with two adults breathing in it makes roughly a litre of water vapour a night. The material that survives that beats the one with the best number on the bag.
That single idea reorders the ranking. Spray foam and polyiso win on paper at roughly R-6 per inch. Denim and fibreglass batts lose in practice not because their R-value is poor, but because they hold water against bare steel and never dry. Below is what each material actually does in a van, where it belongs, and what to leave out.
Quick answer: Our top pick in 2026 is the 3M Thinsulate SM600L — our #1 rated choice. See the full ranked comparison, alternatives and buying advice below.
- Why R-Value Per Inch Is the Wrong First Question for Van Insulation
- Before Anything: Treat the Rust
- Van Insulation Materials Compared
- The Material Picks That Actually Belong in a Van
- What Not to Use: Reflectix and Fibreglass Batts
- Where in the Van Goes What
- Sound Deadening Is a Separate Job
- Getting the Vapour Story Right
- Frequently Asked Questions
Why R-Value Per Inch Is the Wrong First Question for Van Insulation
House insulation lives inside an assembly designed to dry — a vented cavity, a vapour barrier on the warm side, a drainage plane outside. A van has none of that. You have a sealed steel shell that conducts heat brilliantly, flexes constantly, and reaches the dew point on any cold night while you exhale warm humid air into it. So condensation is not a risk, it is a certainty.
The design question is whether the water that forms can dry out, or whether it sits trapped between insulation and metal for three years while rust eats the van from the inside. That is why moisture behaviour and reversibility matter more than the R number, and why a 1.7-inch layer of hydrophobic polyester often beats a thicker, “better” material that sponges.
Two secondary effects decide the rest. Thermal bridging means the structural steel ribs conduct heat straight through your wall — you can insulate every cavity perfectly and still bleed heat through bare ribs touching your panels. And air movement beats R-value: warm moist air circulating behind an unsealed build will condense on cold steel no matter how thick the insulation is.
Before Anything: Treat the Rust
See also: Gifts for Van Lifers: Small, Useful and Actually Wanted • Van Toilets Compared: Composting, Urine-Diverting, Cassette and Bucket
This is not optional. Once insulation and panelling go in, you will not see that corner of the van again until you tear the build out. Any surface rust, chipped paint or scratch from removing old shelving needs wire-brushing back, treating with a phosphoric acid converter or rust encapsulator, and painting — especially the floor pan, wheel arches, the bottom of the ribs where water collects, and any hole you have drilled. If you are still choosing a vehicle, our notes on planning a van conversion from an empty shell cover what to check before you buy.
Van Insulation Materials Compared
Figures below are typical published values for the product forms van builders use. Treat them as a ranking, not a promise — installed performance in a rib-bridged steel shell is always lower.
| Material | R per inch (approx.) | Moisture behaviour | Relative cost | Install difficulty | Reversible? |
|---|---|---|---|---|---|
| 3M Thinsulate SM600L | ~R-3.2 | Hydrophobic fibres; does not absorb, dries out, breathes | High | Easy — cut with scissors, glue up | Yes |
| Polyiso rigid board (foil-faced) | ~R-6 to 6.5 (falls in deep cold) | Closed cell, foil is a vapour retarder; edges must be sealed | Low to medium | Medium — cut and fit to curves | Yes |
| XPS rigid board | ~R-5 | Closed cell, very low absorption; stable | Low | Medium | Yes |
| Sheep wool batt | ~R-3.5 | Absorbs and releases moisture; needs airflow to dry | High | Easy | Yes |
| Recycled denim batt | ~R-3.5 | Absorbs and holds water; slow to dry in a sealed cavity | Medium | Easy | Yes |
| Closed-cell spray foam | ~R-6 to 7 | Air and vapour sealing when adhered; traps water if it debonds | Medium to high | Hard — messy, temperature-sensitive | No |
| Fibreglass batt | ~R-3.1 | Holds water, slumps, loses R when damp | Very low | Easy | Yes |
| Reflectix / bubble foil | ~R-1 as a material | Foil is a vapour barrier; condenses on the cold face | Low | Easy | Yes |
The Material Picks That Actually Belong in a Van
3M Thinsulate SM600L — best default for walls and ceiling
Thinsulate is a polyester/polyolefin batt that does not absorb water, dries if it gets wet, and compresses into the curved, awkward cavities a van is full of. The trade-off is honest: at roughly R-3.2 per inch it is not the best insulator per inch, and it is expensive by the roll. Skip it if your budget is tight and your build is simple — rigid board on the flats out-insulates it for less money.
Polyiso and XPS rigid board — best R per inch and best for floors
Foil-faced polyiso gives the most thermal performance for the thickness on large flat panels, and half-inch board under the plywood is the standard floor build. The catch is that a van is not flat: you spend real time cutting to fit, and every gap and edge must be sealed with foil tape or you have made a condensation channel. Polyiso also loses R-value at very low temperatures, so cold-climate builders often prefer XPS. Either way, rigid board alone leaves the ribs bridging heat — plan a thin layer over them.
Sheep wool — best for people who want a natural, forgiving material
Wool buffers humidity: it takes on a substantial share of its weight in moisture without feeling wet, then releases it when conditions change. In a van with a roof vent that actually runs, it works well and smells better than any foam. In a sealed, poorly ventilated build it stays damp. Skip it if your van sits shut up for weeks, and never use it without solid roof vent and airflow planning.
Closed-cell spray foam — highest performance, real downsides
Two-part closed-cell foam air-seals and insulates in one pass, adheres to the steel, and gives the best R per inch available. Professionals do it well and some of those builds are excellent. Be clear-eyed about the trade-offs, though. Van panels flex and drum constantly; foam that debonds creates a gap you cannot see, and any moisture reaching it is trapped against bare steel. It permanently hides the metal, so a rust spot you missed is now invisible and untreatable, and it is effectively impossible to remove — grinding it off is days of miserable work. DIY kits are fussy too: off-ratio or cold-day application gives crumbly, under-performing foam. Prep the metal obsessively and accept that the build is permanent.
What Not to Use: Reflectix and Fibreglass Batts
Reflectix and similar bubble-foil products are the most misused material in van builds. As bulk insulation they are around R-1 — the impressive packaging numbers depend on a sealed reflective air gap that almost no van wall provides. Worse, the foil is a vapour barrier: press it against a cold panel and you have simply moved the condensation surface. It has one good job in a van, radiant blocking in window covers where it faces an air gap and can be dried.
Fibreglass batts are cheap, which is the whole of their appeal. They hold water, slump under vibration until the top of the cavity is empty, lose most of their R-value when damp, and shed irritant fibres into a small living space. There is no van scenario where they are the right answer.
Where in the Van Goes What
| Location | Best choice | Why |
|---|---|---|
| Walls (cavities between ribs) | Thinsulate or wool | Conforms to curves, tolerates the damp cycle, no gap-cutting |
| Wall ribs / furring | Thin Thinsulate or 1/4-inch foam strip | Breaks the thermal bridge through structural steel |
| Ceiling | Thinsulate, or thin polyiso between furring | Warmest surface, where condensation shows first |
| Floor | 1/2-inch XPS or polyiso under plywood | Compressive strength; rigid board handles the load |
| Doors | Thinsulate only, loosely | Drain paths must stay clear; never pack absorbent batt in a door |
| Wheel arches | Sound deadener + thin closed-cell foam | Road noise and spray dominate here, not heat loss |
| Cab / windscreen | Nothing permanent — removable window covers | Glass is the real loss; insulation you can take down beats insulation you cannot |
| Rocker channels, sills, factory drain paths | Skip entirely | These are designed to drain; filling them causes rust |
Note the last two rows: knowing where to leave insulation out is half the skill. Every factory drain hole and sill channel should be left alone. Glazing is always the weakest thermal point in a van, which is why removable covers deliver more comfort per dollar than any wall upgrade — see our guide to making insulated window covers.
Sound Deadening Is a Separate Job
Builders conflate the two, then wonder why expensive insulation did not quiet the van. Sound deadening is constrained-layer damping — butyl mats such as Noico, Kilmat or Dynamat stuck to large flat panels to stop them resonating. Full coverage is wasted weight; roughly 25 to 30 percent on the middle of each panel kills most of the drumming.
Insulation slows heat transfer. Fibrous materials like Thinsulate and wool absorb some airborne noise, but nothing stops a steel panel ringing except damping it. Do the butyl first, on bare treated metal, then insulate over it. If road noise is your main complaint, spend on deadening and a properly built floor before another roll of batt.
Getting the Vapour Story Right
There are two defensible strategies and one that fails. A sealed system — closed-cell foam or fully taped rigid board, every seam and penetration sealed so warm moist air never reaches the steel — works only if the sealing is genuinely complete; one gap concentrates all the condensation in a single hidden spot. A drying system uses hydrophobic or moisture-buffering material with no vapour barrier, so anything that condenses can evaporate back out.
What fails is the hybrid: absorbent batt behind a plastic or foil layer, trapping moisture on the cold side with nowhere to go. Whichever you pick, ventilation does the heavy lifting — a roof fan that runs while you cook and sleep removes more moisture than any material choice. Pair it with a low-humidity heating setup, since unvented propane heaters add water vapour as they warm, and revisit moisture when you plan your layout and cabinetry so nothing sits tight against a cold outer wall.
Frequently Asked Questions
What is the best insulation for a van conversion overall?
For most builders, 3M Thinsulate SM600L on the walls and ceiling plus half-inch rigid XPS or polyiso under the floor is the best balance. Thinsulate handles the curves and the condensation cycle without absorbing water, rigid board gives the compressive strength and R-value the floor needs, and the whole build stays reversible if you need to inspect or repair the shell later.
Is Thinsulate or polyiso better for van walls?
Polyiso has roughly double the R-value per inch, so on a large flat panel it insulates more for the same thickness. Thinsulate wins on installation and moisture tolerance: it conforms to curved ribs with no cutting, leaves no gaps to seal, does not absorb water, and dries out if it gets damp. Many builders use both — rigid board on the big flats, Thinsulate over the ribs and in the awkward cavities.
Is spray foam a bad idea in a van?
It is not automatically bad, but it is permanent and unforgiving. Done well by someone who preps the metal properly, it air-seals and insulates better than anything else per inch. Done badly, it debonds from flexing panels, traps moisture against bare steel, hides rust you can never inspect again, and cannot realistically be removed. If you are not confident in your rust prep, choose something reversible.
Do I need a vapour barrier in my van?
Only if you are committing fully to a sealed assembly with closed-cell foam or completely taped rigid board. Adding a plastic vapour barrier over absorbent batt insulation is the worst of both worlds: moisture still gets past and then cannot dry. If you build with hydrophobic or moisture-buffering material, skip the barrier and rely on ventilation to carry the water out instead.
Can I use Reflectix as my main van insulation?
No. Reflectix performs at roughly R-1 as bulk insulation; its advertised numbers require a sealed reflective air gap that a van wall does not provide, and the foil acts as a vapour barrier that simply relocates your condensation problem. It has one legitimate use in a van build: removable window covers, where it blocks radiant heat, faces an air gap, and can be taken down and dried.
Ready to decide? Our #1 pick for 2026 is the 3M Thinsulate SM600L.
Live price & availability on Amazon.
