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0.036 W/(m·K). That’s the thermal conductivity Armacell declares for its AF/ArmaFlex Evo elastomeric foam sheets, from about 1/8 to 1 1/4 in (3 to 32 mm) thick [1]. For steel, France’s thermal calculation rules for buildings give 50 W/(m·K) [2]. Run the numbers: your van’s sheet metal conducts heat about 1,400 times better than the foam you glue to it. Insulating a van comes down to that ratio, plus a second fact that gets mentioned less often: steel lets no water vapor through at all.
This guide expands on the winter step of our camper van trip guide. It digs deeper into insulation and condensation, which our guide to the camper van in winter covers alongside heating, propane, and freezing. It compares published data, material by material, without naming a winner: at the same thickness, the differences often come down to a few tenths of a m²·K/W, a point or two of R-value.
Lambda and R-value: compare at the same thickness
Two numbers are all you need. The heat flow through 1 m of a material for a 1 °C (1.8 °F) difference between its two faces, in W/(m·K). We keep it in metric, as European spec sheets do. The lower it is, the better the material insulates. is a property of the material. How well a given layer slows heat flow, in ft²·°F·h/Btu in the U.S.; the metric version, RSI, is in m²·K/W, and 1 m²·K/W equals R-5.68. The higher it is, the better the layer insulates. belongs to the layer as installed: it depends on the thickness. For most insulation materials, notes the OÖ Energiesparverband, the energy agency of the Austrian state of Upper Austria, lambda falls between 0.035 and 0.045 W/(m·K) [3].
Example: 20 mm (just over 3/4 in) of elastomeric foam at 0.036 W/(m·K) gives 0.020 ÷ 0.036, or about 0.56 m²·K/W: R-3.2. The table runs the same calculation for materials commonly used in vans, with the value declared by one manufacturer in each family and, next to it, the Austrian agency’s range.
| Material | λ (W/(m·K)) | R-value for 20 mm (RSI in m²·K/W) | Against water and vapor | Fire rating |
|---|---|---|---|---|
| Closed-cell elastomeric foam (Armacell AF/ArmaFlex Evo) | 0.036 | R-3.2 (0.56) | μ of at least 7,000; a built-in vapor barrier, according to the manufacturer [1, 4] | B-s2,d0 (sheets) [1] |
| Sheep’s wool (Thermafleece UltraWool) | 0.035 (0.036 to 0.040 according to the Austrian agency) | R-2.8 to R-3.2 (0.50 to 0.57) | Vapor-open according to the manufacturer; μ of 1 to 2, and absorbs up to 30% of its weight in moisture with no change in λ, according to the Austrian agency [3, 5] | Not stated |
| Mineral wool (fiberglass, stone wool) | 0.032 to 0.045 | R-2.5 to R-3.5 (0.44 to 0.63) | Vapor-open: μ of 1 to 2 [3] | Not stated |
| Expanded cork (Amorim) | 0.039 (0.041 to 0.05 according to the Austrian agency) | R-2.3 to R-2.9 (0.40 to 0.51) | Water absorption under 0.5 kg/m² (0.1 lb/ft²) in the EN 1609 test, μ of 20; relatively insensitive to moisture, according to the Austrian agency [3, 6] | E [6] |
| Rigid polyisocyanurate (polyiso) foam board, PIR (Recticel Eurothane Mur) | 0.022 | R-5.2 (0.91) | Polyurethane boards are among the insulation materials for damp areas of buildings, according to the Austrian agency [3] | F (product alone) [7] |
| Extruded polystyrene (XPS) | 0.031 to 0.042 | R-2.7 to R-3.7 (0.48 to 0.65) | μ of 80 to 200; used in damp areas of buildings [3] | Not stated |
| Reflective foil insulation | Rated by its R-value alone | R-0.6 to R-5.7 (0.1 to 1) for the product alone | Most foils are highly vapor-tight [8] | Not stated |
λ: value declared by the manufacturer named [1, 5, 6, 7], or the OÖ Energiesparverband’s range for the whole family [3]. R-value: calculated by us, thickness divided by λ, multiplied by 5.678 for U.S. units, and rounded. μ: water vapor diffusion resistance factor, 1 for air. Fire rating: European classification for building products (EN 13501-1). These are building values, not measurements taken in a vehicle. No R-value per inch: the Federal Trade Commission’s R-value Rule bars per-inch claims on insulation labels unless tests prove the figure doesn’t drop as the product gets thicker [15]. No prices: we didn’t find dated, comparable figures.
Read the third column: at 20 mm, elastomeric foam, sheep’s wool, cork, mineral wool, and extruded polystyrene all fall between R-2.3 and R-3.7 (0.4 to 0.65 m²·K/W). Only rigid PIR foam board stands out, at about R-5.2. So the choice comes down less to lambda than to the last two columns: what the material does with water, and how it behaves in a fire.
How thick: what every fraction of an inch costs
In a van, you don’t pick a thickness the way you would for an attic: every fraction of an inch comes out of the interior width, and twice, since there are two walls. Working backward is more telling: how thick does each material need to be to reach R-5 (0.88 m²·K/W)?
| Material | Thickness needed | Based on |
|---|---|---|
| Rigid PIR foam board | 0.8 in (19 mm) | Recticel’s λ [7] |
| Sheep’s wool | 1.2 to 1.4 in (31 to 35 mm) | Thermafleece’s λ, then the top of the Austrian range [3, 5] |
| Elastomeric foam | 1.2 in (32 mm) | Armacell’s λ [1] |
| Expanded cork | 1.4 in (34 mm) | Amorim’s λ [6] |
| Mineral wool | 1.1 to 1.6 in (28 to 40 mm) | Austrian range [3] |
Calculated by us: thickness = RSI × λ, with R-5 equal to 0.88 m²·K/W, rounded to the tenth of an inch and to the millimeter.
Product formats follow these orders of magnitude. Armacell sells its sheets up to 2 in (50 mm) thick, with a slightly worse lambda above 1 1/4 in (32 mm) [1]. Amorim publishes the resistance of its cork starting at 10 mm (about 0.4 in) thick: 0.25 m²·K/W, or R-1.4 [6]. Thermafleece sells its UltraWool in 50, 70, and 90 mm (about 2, 2 3/4, and 3 1/2 in), which are building thicknesses [5].
The takeaway: within the thickness a van wall has room for, switching materials gains you a point or two of R-value. The rest is decided elsewhere, where there’s no insulation at all.
Ribs, windows, sheet metal: the thermal bridges
A A spot in the wall where heat finds an easier path than through the insulation: a metal part, a window, a poorly covered joint. works out the same way. A 1 mm (0.04 in) steel panel, at 50 W/(m·K), has a thermal resistance of 0.00002 m²·K/W, about R-0.0001 [2]: next to nothing. Ordinary glass, at 1.00 W/(m·K) [2], does barely better: a 4 mm (0.16 in) pane, a thickness taken as an example, offers 0.004 m²·K/W, or R-0.02, about 140 times less than 20 mm of elastomeric foam.
The body’s pillars and cross members, the ribs, are steel, tied to the outer skin. Left bare on the inside, they carry the outdoor cold right into the middle of your insulation. So cover them too, with a material that doesn’t mind water, since they’ll stay cold. For the windows, the insulated covers in our camper van winter guide are still the simplest fix.
Where the water forms: cold sheet metal and the dew point
The water comes from you. The Umweltbundesamt, Germany’s federal environment agency, puts the water vapor a person gives off during light activity at 30 to 40 g (1 to 1.4 oz) per hour, citing the Fraunhofer Institute for Building Physics, and notes that breathing and sweating load a bedroom’s air overnight [11]. For two people over 8 hours, that would come to 480 to 640 g, roughly 17 to 23 oz: a ballpark only, since sleep doesn’t get its own line in that table. Cooking and laundry drying indoors add to it [11].
That vapor settles on the first surface that’s cold enough. The mechanism, the dew point, is the same as in a tent. In a van, the coldest surfaces are the bare sheet metal and the windows. The German agency adds a second threshold: mold can grow once the humidity at a material’s surface reaches 70 to 80%, if it lasts, without the material looking wet [11].
| Air inside the van | Droplets (dew point) | Mold possible (80% at the surface) |
|---|---|---|
| 59 °F (15 °C), 60% humidity | 45 °F | 51 °F |
| 59 °F (15 °C), 70% humidity | 49 °F | 55 °F |
| 68 °F (20 °C), 50% humidity | 49 °F | 55 °F |
| 68 °F (20 °C), 60% humidity | 54 °F | 60 °F |
| 68 °F (20 °C), 70% humidity | 58 °F | 64 °F |
Calculated by us with the National Weather Service’s vapor pressure formula [12], rounded to the nearest °F. A surface colder than the second column gets covered in water; colder than the third, it stays at 80% humidity or more, the threshold cited by the Umweltbundesamt [11].
On a winter night, the outside air often drops below these temperatures, and bare sheet metal puts up almost no resistance to the cold. Vapor 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: built-up moisture and condensation must be able to drain away quickly and freely, and the factory drain holes must be left intact [9].
For insulation installed on the inside of a wall, the German agency gives an instruction I’m happy to apply to sheet metal: glue the boards down over their entire surface, so room air can’t circulate behind the insulation, where it cools [11]. Insulation glued in spots, with gaps behind it, lets exactly that air reach the metal.
Vapor barrier or not: what each side says
In a building, the rule is clear-cut. According to the installation guide published in 2023 by the CSTB, France’s national building research center, a A membrane that greatly slows the passage of water vapor through a wall. always goes on the interior (warm) side of the insulation, as a continuous layer, with the sheets overlapping by at least 100 mm (about 4 in) and sealed with compatible tape [13]. The Austrian agency explains the logic: the vapor resistance of the layers should decrease from the inside out, or else you need a vapor retarder on the inside [3].
A van flips that order: the most vapor-tight layer, the sheet metal, is on the outside. Applied as is, the building rule would call for a vapor barrier on the interior side. Still, van builders debate the question, and the sources we read fall into two camps.
- The vapor-tight camp. For the Sprinter’s roof and walls, Mercedes-Benz wants insulation that isn’t hygroscopic or absorbent, doesn’t hold water, and repels it [9]. Armacell presents its closed-cell foam as having a built-in vapor barrier [1]. The CSTB notes that most reflective foils are highly vapor-tight and can make an effective vapor barrier on the interior side [8].
- The breathable camp. Havelock Wool, an American maker of sheep’s wool insulation, writes that in most van builds, wool doesn’t need a vapor barrier, because it absorbs and releases moisture, while telling you to assess your own design and climate [14]. Without referring to vans, Thermafleece describes its wool as vapor-open [5], and the Austrian agency writes that it absorbs up to 30% of its weight in moisture with no change in lambda, and resists moisture in the short term [3].
The manufacturers on both sides sell what they recommend, and none of the pages we read publishes a measurement taken in a van. Both approaches agree on one point, though: liquid water must not stay trapped against the metal, and it has to be able to drain. My takeaway is about method: pick one approach and follow it all the way through. A barrier installed halfway, punctured by the panel screws, meets neither the CSTB’s continuity requirement [13] nor the promise of wool that’s free to dry.
Reflective foil insulation: what the CSTB says
Multilayer reflective foils have thinness and light weight going for them. The CSTB’s specialist group for 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 (R-0.6 to R-5.7), and from 0.5 to 2 m²·K/W (R-2.8 to R-11.4) once installed, counting the sealed air spaces next to them [8]. Installed alone, the note concludes, they often fall short: they’re add-ons to insulation [8].
Their selling point, shiny faces that reflect radiant heat, only works facing an air space: low emissivity raises the resistance of an air space by about 0.15 to 0.50 m²·K/W (roughly R-0.9 to R-2.8), as long as it’s truly airtight [8]. Under a floor screed, with no air space, the note says those faces add nothing [8]. I don’t see why a face glued straight to the sheet metal would do any better. The note also warns that emissivity can degrade quickly with moisture [8], which, in a van, is worth thinking about.
Six mistakes to avoid, with sources
- Plugging the body’s drain holes with insulation or adhesive: Mercedes-Benz requires them to be left intact [9].
- Pressing a material that holds water against the metal when the vehicle manufacturer asks for the opposite: that’s the crux of the wool debate [9].
- Leaving gaps behind insulation glued in spots: the Umweltbundesamt calls for boards glued down over their entire surface [11].
- Counting on the shiny faces of a foil glued directly to the sheet metal: without an air space, they add nothing, according to the CSTB [8].
- Installing a discontinuous vapor barrier: the CSTB calls for overlaps of at least 100 mm (about 4 in) sealed with compatible tape, and for junctions and penetrations to be properly dealt with [13].
- Sealing off the vents in the name of insulation: for the Umweltbundesamt, airing out remains the most effective way to get moisture out of a home [11]. The ventilation rules for carbon monoxide are in our camper van winter guide.
You choose insulation on two numbers, its R-value and what it does with water, and you install it so that vapor never reaches the sheet metal without a way back out.
FAQ
What’s the best insulation for a camper van?
None wins across the board. At 20 mm (just over 3/4 in), elastomeric foam, sheep’s wool, cork, and mineral wool come in at R-2.3 to R-3.6, and rigid PIR foam board at about R-5.2, based on manufacturers’ spec sheets. The real difference is how they handle water: Mercedes-Benz asks for materials that don’t hold water in the Sprinter, while wool makers count on its ability to absorb moisture.
Do you need a vapor barrier in a camper van?
The sources disagree. In a building, the CSTB puts the vapor barrier on the warm side, as a continuous layer; in a van, the vapor-tight sheet metal is on the cold side. Armacell presents its closed-cell foam as a built-in vapor barrier, and Havelock Wool writes that wool can do without one in most vans, depending on the design and climate. Either way, water must not stay trapped against the metal.
How thick should camper van insulation be?
Start from the R-value you’re aiming for: R = thickness ÷ lambda, converted to U.S. units. For R-5, you need about 0.8 in of rigid PIR foam board, 1.2 to 1.4 in of sheep’s wool, 1.2 in of elastomeric foam, or 1.4 in of expanded cork. Every fraction of an inch comes out of the interior width, and the steel ribs and windows remain thermal bridges to deal with separately.
Is reflective foil insulation enough to insulate a van?
On its own, no, according to a 2007 note from France’s CSTB written for buildings: these products’ own R-value ranges from R-0.6 to R-5.7, and their shiny faces only work facing a sealed air space. The CSTB classes them as add-ons to insulation. They can, however, act as a vapor barrier on the interior side, since most foils are highly vapor-tight.
Why is there water behind the insulation in a van?
Because vapor from inside the van reaches the sheet metal, which is colder than its dew point. In air at 59 °F (15 °C) and 70% humidity, a surface below about 49 °F (10 °C) gets covered in water. Since steel lets nothing through, the water stays on the insulation side. That’s why Mercedes-Benz asks for it to drain freely and for the factory drain holes to be left intact.
Sources
- Armacell, AF/ArmaFlex Evo, technical data sheet, accessed November 8, 2026. armacell.com https://www.armacell.com/sites/default/files/2025/05/01/AF-ArmaFlex%20Evo%20-%20Technical%20Datasheet%20-%20en-EE.pdf ↩
- French Ministry for the Ecological Transition, Règles Th-bât, fascicule Matériaux, France’s thermal calculation rules for buildings, materials section, published December 20, 2017 (in French), accessed November 8, 2026. rt-re-batiment.developpement-durable.gouv.fr https://rt-re-batiment.developpement-durable.gouv.fr/IMG/pdf/2-fascicule_materiaux.pdf ↩
- OÖ Energiesparverband (State of Upper Austria), Umweltfreundlich gedämmt. Ökologische Dämmstoffe, brochure, January 2026 (in German), accessed November 8, 2026. land-oberoesterreich.gv.at https://www.land-oberoesterreich.gv.at/files/publikationen/ESV_Oekologische_Daemmstoffe.pdf ↩
- Armacell, Sanitär, Heizung und Klima (SHK), catalog, December 2024, technical data for AF/ArmaFlex Evo (in German), accessed November 8, 2026. armacell.com https://www.armacell.com/sites/default/files/2025/02/19/20241216_Armacell_Catalogue_SHK_de-DE_web.pdf ↩
- Thermafleece (Eden Renewable Innovations), Thermafleece UltraWool, fact sheet V5, December 2024, manufacturer’s document posted by a UK retailer, accessed November 8, 2026. celticsustainables.co.uk https://www.celticsustainables.co.uk/content/Thermafleece/2025/Thermafleece-UltraWool-Factsheet-V5-December-2024.pdf ↩
- Amorim Cork Solutions, Expanded insulation corkboard, technical data sheet CTP-465/002, May 2026, accessed November 8, 2026. amorimcorksolutions.com https://amorimcorksolutions.com/media/10945/tds_expanded-insulation-corkboard_en.pdf ↩
- Recticel Insulation, Eurothane Mur, product page (in French), accessed November 8, 2026. recticelinsulation.com https://www.recticelinsulation.com/fr/product/eurothane-mur ↩
- Commission chargée de formuler des Avis techniques (CSTB, France’s national building research center), specialist group no. 20, Performances des produits réfléchissants opaques utilisés dans l’enveloppe des bâtiments en tant que complément d’isolation thermique, information note no. 1, version 4 of December 12, 2007, posted by the Swiss canton of Vaud (in French), accessed November 8, 2026. vd.ch https://www.vd.ch/fileadmin/user_upload/themes/environnement/energie/fichiers_pdf/GS20_note.pdf ↩
- Mercedes-Benz, Body and Equipment Guideline Sprinter, Series Model 907, edition 2026-9, pp. 154 and 155 (U.S. website), accessed November 8, 2026. mbvans.com https://www.mbvans.com/content/dam/mb-vans/us/upfitter/10092026_BEG_Sprinter.pdf ↩
- Volkswagen Commercial Vehicles, Aufbaurichtlinie Der neue Crafter, body builder guidelines, November 2024 edition, p. 240 (in German), accessed November 8, 2026. umbauportal.volkswagen-nutzfahrzeuge.ch https://umbauportal.volkswagen-nutzfahrzeuge.ch/download/Technische%20Informationen/Aufbaurichtlinien/Crafter/Aufbaurichtlinie-Crafter-DE-48-2024.pdf ↩
- Umweltbundesamt, Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden, guide to preventing and remediating mold in buildings, as of April 2024 (in German), accessed November 8, 2026. umweltbundesamt.de https://www.umweltbundesamt.de/system/files/medien/4031/publikationen/240513_uba_fb_schimmelleitfaden_0.pdf ↩
- National Weather Service (NOAA), Vapor Pressure, calculation sheet, accessed November 8, 2026. weather.gov https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf ↩
- CSTB, Guide de détails de pose de membrane pare-vapeur dans le cadre des travaux d’isolation par l’intérieur, installation guide for vapor barriers in interior insulation work, version 2 of July 2023, Profeel program (in French), accessed November 8, 2026. media.proreno.fr https://media.proreno.fr/storage/media/shares/pdf/10051/guide-pose-pare-vapeur-travaux-isolation_V2.pdf ↩
- Havelock Wool, Wool Insulation for Vans, application page and FAQs, accessed November 8, 2026. havelockwool.com https://havelockwool.com/pages/applications-vans ↩
- Federal Trade Commission, 16 CFR § 460.20, R-value per inch claims, R-value Rule (Labeling and Advertising of Home Insulation), text from the Legal Information Institute at Cornell Law School, accessed November 8, 2026. law.cornell.edu https://www.law.cornell.edu/cfr/text/16/460.20 ↩
- Federal Trade Commission, 16 CFR § 460.5, R-value tests, R-value Rule (Labeling and Advertising of Home Insulation), text from the Legal Information Institute at Cornell Law School, accessed November 8, 2026. law.cornell.edu https://www.law.cornell.edu/cfr/text/16/460.5 ↩
Vans, RVs, biking and calculators
Julien de Jurquet writes for Outfarer about camper vans, RVs, biking, and calculators. He reads manuals cover to cover and never lets a number slide without its unit, so your choices rest on math, not promises.
Guillaume Hoarau