Insulating a campervan: materials, thickness and moisture traps

0.036 W/(m·K). That is the thermal conductivity Armacell declares for its AF/ArmaFlex Evo elastomeric foam sheets, from 3 to 32 mm thick [1]. For steel, France’s thermal calculation rules for buildings give 50 W/(m·K) [2]. Do the sum: the steel skin of your van conducts heat about 1,400 times better than the foam you stick to it. The whole business of insulating a van comes down to that ratio, and to a second fact, mentioned less often: steel lets no water vapour through at all.

This guide expands on the winter step of our guide to campervan trips. It goes deeper into insulation and condensation, which our guide to the campervan in winter covers alongside heating, gas and frost. It compares published data, material by material, without naming a winner: at the same thickness, the gaps often come down to a few tenths of a m²·K/W.

Lambda and thermal resistance: comparing like with like

Two figures are all you need. Thermal conductivity (lambda)The heat flow through 1 m of a material for a 1 °C difference between its two faces, in W/(m·K). The lower it is, the better the material insulates. is a property of the material. Thermal resistance (R-value)How well a given layer slows heat down, in m²·K/W. The higher it is, the better the layer insulates. belongs to the layer as fitted: it depends on the thickness. For most insulation materials, notes the OÖ Energiesparverband, the energy agency of the Austrian state of Upper Austria, lambda lies between 0.035 and 0.045 W/(m·K) [3].

An example: 20 mm of elastomeric foam at 0.036 W/(m·K) gives 0.020 ÷ 0.036, or about 0.56 m²·K/W. The table applies the same sum to the materials commonly used in vans, with the value declared by one manufacturer of each family and, alongside it, the Austrian agency’s range.

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Look at the third column: at 20 mm, elastomeric foam, sheep’s wool, cork, mineral wool and extruded polystyrene all sit between 0.4 and 0.65 m²·K/W. Only rigid PIR foam stands apart, at about 0.9. So the choice turns less on lambda than on the last two columns: what the material does with water, and how it behaves in a fire.

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How thick: what each millimetre costs

In a van, you do not choose a thickness the way you would in a loft: every millimetre comes out of the living width, and twice over, since there are two walls. The reverse calculation is more telling: what thickness does it take to reach R = 1 m²·K/W?

Product formats follow these orders of magnitude. Armacell sells its sheets up to 50 mm thick, with a slightly poorer lambda above 32 mm [1]. Amorim publishes the resistance of its cork from 10 mm thick: 0.25 m²·K/W [6]. Thermafleece sells UltraWool in 50, 70 and 90 mm, which are building thicknesses [5].

The point to remember: within the thickness a van wall allows, switching material gains you a few tenths of a m²·K/W. The rest is decided elsewhere, where there is no insulation at all.

Pillars, windows, steel: the thermal bridges

A thermal bridgeA point in the wall where heat finds an easier path than through the insulation: a metal part, a window, a poorly covered joint. is calculated like everything else. A 1 mm steel panel, at 50 W/(m·K), offers a thermal resistance of 0.00002 m²·K/W [2]: next to nothing. Ordinary glass, at 1.00 W/(m·K) [2], does barely better: a 4 mm pane, a thickness taken as an example, offers 0.004 m²·K/W, about 140 times less than 20 mm of elastomeric foam.

The body’s pillars and cross-members are steel, joined to the outer skin. Left bare on the inside, they carry the outside cold right into the middle of the insulation. So cover them too, with a material that does not mind water, since they will stay cold. For the windows, the insulating screens mentioned in our guide to the campervan in winter remain the simplest answer.

Where the water forms: cold steel and the dew point

The water comes from you. The Umweltbundesamt, Germany’s federal environment agency, puts the water vapour given off by a person in light activity at 30 to 40 g per hour, based on figures from the Fraunhofer Institute for Building Physics, and points out that breathing and sweating load a bedroom’s air at night [11]. For two people over 8 hours, that would make 480 to 640 g: an order of magnitude only, since sleep has no line of its own in that table. Cooking and washing drying indoors add to it [11].

That vapour settles on the first surface cold enough. The mechanism, that of the dew point, is the same as in a tent. In a van, the coldest surfaces are the bare steel and the windows. The German agency adds a second threshold: mould can grow from 70 to 80% humidity at the surface of a material, if it persists, without the material looking wet [11].

On a winter night, the outside air often drops below these temperatures, and bare steel puts up almost no resistance to the cold. Vapour that reaches it condenses, and since steel lets nothing through, the water stays on the insulation side. For the Sprinter, Mercedes-Benz sets two rules on this: accumulated moisture and condensation must be able to drain away quickly and freely, and the original drain holes must be left intact [9].

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For insulation fitted to the inside of a wall, the German agency gives an instruction I am happy to carry over to a van’s steel: bond the boards over their whole surface, so that room air cannot circulate behind the insulation, where it cools down [11]. Insulation glued in spots, with gaps behind it, lets exactly that air reach the metal.

Vapour barrier or not: what the two camps say

In a building, the rule is clear. According to the installation guide published in 2023 by the CSTB, France’s building research centre, a vapour barrierA membrane that strongly restricts the passage of water vapour through a wall. always goes on the inside (warm side) of the insulation, as a continuous layer, with the sheets overlapping by at least 100 mm and sealed with compatible adhesive tape [13]. The Austrian agency gives the logic behind it: the vapour resistance of the layers must decrease from the inside out, failing which a vapour control layer is needed on the inside [3].

A van turns that order upside down: the most vapour-tight layer, the steel, is on the outside. Applied as it stands, the building rule would call for a vapour barrier on the living-space side. The question is still debated among converters, though, and the sources we read fall into two camps.

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• The vapour-tight camp. For the Sprinter’s roof and walls, Mercedes-Benz wants insulation materials that are neither hygroscopic nor absorbent, that do not hold water and that repel it [9]. Armacell presents its closed-cell foam as having a built-in vapour barrier [1]. The CSTB notes that most reflective films are highly vapour-tight and can make an effective vapour barrier on the inside [8].
• The breathable camp. Havelock Wool, an American maker of sheep’s wool insulation, writes that in most vans wool does not need a vapour barrier, because it absorbs and releases moisture, while asking you to judge according to your design and climate [14]. Without referring to vans, Thermafleece describes its wool as vapour open [5], and the Austrian agency writes that it absorbs up to 30% of its weight in moisture with no change in lambda, and that it resists moisture in the short term [3].

The manufacturers in both camps sell what they recommend, and none of the pages we read publishes a measurement taken in a van. The two approaches do agree on one point, though: liquid water must not stay trapped against the steel, and it must be able to drain away. My conclusion is one of method: pick an approach and stick to it all the way. A film fitted halfway, pierced by the panel screws, meets neither the CSTB’s requirement for continuity [13] nor the promise of wool that is free to dry.

Thin reflective insulation: what the CSTB says

Multi-layer reflective films have thinness and lightness on their side. The CSTB’s specialist group in charge of these products devoted a note to them in 2007, written for buildings: their own thermal resistance generally ranges from 0.1 to 1 m²·K/W, and from 0.5 to 2 m²·K/W once installed, counting the sealed air gaps next to them [8]. Fitted alone, the note concludes, their performance is often inadequate: they are supplements to insulation [8].

Their selling point, shiny faces that reflect radiant heat, only works facing an air gap: a low emissivity increases the resistance of an air gap by about 0.15 to 0.50 m²·K/W, provided the gap is genuinely airtight [8]. Under a screed, with no air space, the note states that these faces bring no gain [8]. I struggle to see why a face glued straight to the steel would do any better. The note also warns that emissivity can deteriorate quickly with moisture [8], which, in a van, is worth weighing up.

Six mistakes to avoid, with the sources

1. Blocking the body’s drain holes with insulation or glue: Mercedes-Benz requires them to be left intact [9].
2. Pressing a material that holds water against the steel when the vehicle manufacturer asks for the opposite: that is the whole point of the wool debate [9].
3. Leaving gaps behind insulation glued in spots: the Umweltbundesamt has the boards bonded over their whole surface [11].
4. Relying on the shiny faces of a film glued straight to the steel: without an air gap, they add nothing, according to the CSTB [8].
5. Fitting a discontinuous vapour barrier: the CSTB asks for overlaps of at least 100 mm sealed with compatible adhesive tape, and for junctions and penetrations to be properly dealt with [13].
6. Blocking up the vents in the name of insulation: for the Umweltbundesamt, airing remains the most effective way to remove moisture from a home [11]. The ventilation rules for carbon monoxide are in our guide to the campervan in winter.

Insulation is chosen on two figures, its thermal resistance and what it does with water, and fitted so that vapour never reaches the steel without a way back out.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

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