The metal ribs, doors and curves are where a campervan insulation plan gets awkward. Covering the large wall panels is only part of the job; the material also has to fit those smaller spaces without trapping moisture.

PIR board may suit a broad flat panel but cannot follow every curve. Flexible insulation can fit awkward cavities more easily, while exposed steel remains a route for heat to escape.

The useful way to plan it is by location. Flat panels, ribs and curves, doors, deep cavities and the floor each have different practical constraints.

A mixed system usually makes more sense than forcing one material into every part of the van.

What are you actually trying to achieve?

Campervan insulation has three jobs:

• slow heat loss when it is cold outside;
• slow heat gain when the sun is beating down on the van;
• reduce how much cold bare metal is exposed to warm cabin air and help keep the room-facing lining warmer.

That last point is important. Insulation does not remove moisture from the air. Two people sleeping in a van, cooking and making tea will still add water vapour. You still need ventilation.

Our campervan condensation guide explains the moisture side in more detail, and the roof fan and ventilation guide covers airflow.

The weak spots matter as much as the big panels

It is tempting to concentrate on the large wall and roof panels because they are easy to see and easy to cover.

But the steel ribs, pillars, door frames and areas around windows can remain direct paths for heat to travel through the shell. These are thermal bridges.

You probably cannot eliminate every thermal bridge in a van, but you can avoid making the problem worse by leaving large areas of bare metal around otherwise well-insulated panels.

This is one reason flexible closed-cell foam is popular with converters: it can follow curved metal and continue across awkward shapes where rigid board cannot.

Closed-cell foam – very useful against the metal shell

Flexible closed-cell elastomeric foam is one of the easiest materials to use directly against curved van bodywork.

Products such as ArmaFlex are designed to resist water-vapour ingress through their closed-cell structure. They also bend around curves and can be cut into strips for ribs, pillars and smaller panels.

Some of the product links in this article are Amazon affiliate links. If you buy through one of them I may earn a small commission, at no extra cost to you.

A current example is this Armacell ArmaFlex AF 19mm self-adhesive insulation. Amazon currently lists it as a 6m² rubber insulation panel suitable for vehicle insulation. It is the sort of product I mean when I talk about proper closed-cell elastomeric foam rather than generic shiny bubble wrap.

If you specifically want a campervan-oriented peel-and-stick option, Dodo Mat Thermo Pro 10mm is another current example. It is a 5m² closed-cell foam roll with a reinforced foil face. I would still treat 10mm as one part of the insulation build rather than assuming a thin layer solves every thermal bridge.

The main advantages are:

• it conforms well to curved steel;
• it does not rely on fluffy loft staying open;
• it can create a fairly continuous layer over ribs and awkward shapes;
• good closed-cell products resist moisture far better than open-cell foams or absorbent fibre.

The downside is cost. Covering a complete large van in a thicker closed-cell product can become expensive.

That is why I would seriously consider using it where its flexibility and moisture resistance are most useful, then combine it with another insulation in deeper, flatter cavities.

PIR board – excellent performance where it actually fits

Rigid PIR board such as Celotex or Kingspan gives a lot of thermal resistance for its thickness. That makes it attractive in a campervan where every millimetre of interior width matters.

On broad flat sections it makes a lot of sense. Cut it carefully, fit it closely and it can provide a very effective layer without taking up huge amounts of space.

But a van body is not made from neat rectangular stud bays.

If a rigid board leaves big gaps behind it because the steel is curved, the theoretical performance on the datasheet is not the same thing as the performance of the finished wall. It also becomes fiddly around ribs, pressed panels and complex door cavities.

So I see PIR as a flat-panel material, not something that has to be forced into every part of the van.

It can also be useful in a floor build-up where a rigid board is properly supported and protected. Our campervan flooring guide is worth reading before deciding how much floor height you can afford to lose.

Thinsulate and other automotive fibre insulation

Automotive fibre insulation is particularly useful where a rigid board is awkward.

3M Thinsulate SM600L, for example, is made for vehicle applications. It is lightweight, compressible and can be fitted into trim cavities and curved areas. 3M also describes the fibres as hydrophobic, so they resist absorbing water.

If you want to use Thinsulate, check the exact product code and manufacturer data before buying. Similar-looking listings can use different product codes and specifications, so I would not choose one from the word “Thinsulate” alone.

That does not mean you should stuff it into a soaking wet door and forget about it. Factory drain holes still need to work and any actual water leak needs fixing.

But for irregular wall and roof cavities, a purpose-made automotive fibre material can be much easier to fit neatly than cutting dozens of tiny pieces of rigid board.

There is another benefit: these products are also useful acoustically. Thermal insulation and sound deadening are not exactly the same job, but a quieter panel van is a pleasant side effect.

What about sheep wool?

Sheep-wool insulation gets a lot of attention in self-build vans because it is pleasant to handle, fits irregular spaces and can buffer moisture.

Current Thermafleece data puts its thermal conductivity in roughly the same general range as many conventional fibrous insulations, depending on the exact product. It is vapour-open rather than waterproof, and it can take up moisture at high humidity.

That distinction matters.

Moisture buffering does not mean you can allow rainwater to run into the insulation. If a cavity is known to get wet, fix the source of the water or use a material appropriate for that location.

I would be much happier using wool in a protected wall or roof cavity than packing it into the bottom of a door where water is designed to drain through.

Also remember that “natural” does not mean non-combustible. Check the actual fire classification of the product you are considering rather than relying on assumptions.

Is foil bubble insulation enough on its own?

This is one of the most confusing areas of campervan insulation.

Reflective insulation can be useful, but the reflective part needs an adjacent air space to do its job properly. Manufacturers of reflective systems specify air gaps as part of their tested assemblies.

If a thin shiny bubble product is simply glued flat to the steel and covered immediately with ply, it should not be treated as though it has suddenly become the equivalent of a thick PIR board or foam layer.

That does not make foil useless. It means it has to be used as part of a sensible build-up rather than because the silver surface looks technical.

Whenever you see an impressive R-value or thermal-resistance claim for a reflective product, check the installation used to achieve it. If the tested system includes one or two air gaps, those air gaps are part of the insulation system.

Do you need a vapour barrier in a campervan?

This is where internet arguments usually begin.

In a house wall it is possible to design a relatively predictable build-up. In a van you have doors, factory holes, ribs, wiring routes, windows, fixings and a steel shell that is difficult to seal perfectly.

A perfectly continuous vapour-control layer sounds attractive, but a badly detailed one can still allow moist air behind it through every screw hole, edge and opening.

For that reason I would not give a blanket rule that every campervan must have a separate plastic vapour barrier, or that every vapour barrier is wrong.

Instead, think about the whole assembly:

• Is the insulation itself highly vapour-resistant, like a properly installed closed-cell foam?
• Are seams and edges reasonably continuous?
• Are you leaving large bare-metal voids behind the lining?
• Can any cavity get wet from outside?
• Does the van have enough ventilation when occupied?

Getting those things right is more useful than adding another layer simply because somebody on a forum says every van needs one.

What would I use in a DIY panel van?

If I were planning a conversion today, I would not try to use one material everywhere.

A sensible approach would be something like this:

Large flat wall and roof panels: rigid PIR where it can be fitted closely, or a suitable automotive fibre insulation where the shape makes rigid board awkward.

Ribs, curves and difficult metal areas: flexible closed-cell foam to reduce exposed cold metal and maintain continuity.

Deep irregular cavities: a suitable automotive fibre or treated wool product, provided the cavity is dry and factory drainage is not blocked.

Doors: be particularly careful. Understand where rainwater can enter and where the factory drains are before adding anything absorbent.

Floor: use a build-up that provides enough insulation without creating an unnecessarily tall floor. Rigid insulation is often easier to support here than fluffy material.

The exact thickness depends on the van, your climate and how much interior space you are prepared to sacrifice. There is no magic number that suits a weekend summer van and a full-time winter camper equally well.

Windows and the cab can undo a lot of good work

You can do a beautiful job on the walls and roof and still lose a lot of heat through the windscreen, cab windows and other glazing.

This is especially relevant with minibuses and vans that have a lot of factory glass. Our minibus camper conversion guide talks about that compromise in more detail.

Good curtains, insulated screens and sensible cab separation can sometimes make a more noticeable difference than obsessing over another few millimetres of insulation in an already well-filled wall.

Insulation and ventilation have to work together

A well-insulated van can still be wet inside if it is sealed up with two sleeping people and a boiling kettle.

The aim is not to make the van airtight and hope moisture disappears. The aim is to reduce unwanted heat transfer while still giving damp air a controlled way out.

That means roof vents, opening windows or powered extraction still matter. Fixed ventilation should not be blocked just because it feels draughty on a cold night.

In winter, insulation reduces the amount of heat you need to replace, while ventilation removes the moisture you do not want to keep. You need both.

Common campervan insulation mistakes

A few mistakes come up repeatedly:

• covering the big panels but leaving large bare-metal ribs and voids;
• treating thin reflective foil as high-performance bulk insulation without the required air gap;
• compressing fluffy insulation so tightly that it loses the loft it relies on;
• filling door bottoms without checking drainage paths;
• insulating over rust or damp instead of repairing the problem first;
• assuming insulation means you no longer need ventilation;
• chasing the highest theoretical R-value while creating gaps everywhere because the material does not fit the van.

The neatest-looking material on a spreadsheet is not automatically the best material once you try to fit it around a Transit, Ducato, Boxer or Sprinter shell.

Choosing the insulation mix

For a DIY conversion, the material should follow the job it has to do.

That gives a practical mix:

• closed-cell foam where you need flexibility, continuity and moisture resistance;
• PIR board where the surface is flat enough to fit it properly and space efficiency matters;
• automotive fibre insulation for curved panels and awkward cavities;
• treated sheep wool where you want a flexible, vapour-open fibre and can keep it away from actual water ingress;
• reflective products only as part of an installation that gives them the air space they need.

Then pay just as much attention to the bits that are easy to forget: ribs, doors, window surrounds, the cab and ventilation.

A campervan will always be a compromise. It is still a metal vehicle with doors and glass. But a carefully fitted mixed insulation system can make it far more comfortable without wasting space or hiding a future condensation problem behind the lining.