What insulation materials for converted vans?
Answering my own question as I did quite a lot of online research on that topic.

I did this across different languages (English, French &amp; German) which was interesting: habits/technics seem to vary from one country to another.



Criterias &amp; vocabulary

Criterias


Thermal insulation: must be excellent
Resistance to humidity: must be excellent
Compression potential: your vehicle vibrates a lot, over time the insulating material might get very compressed (i.e. rockwool). So you might want to avoid materials that compress a lot, at least for the walls.
Sound insulation: a great plus


Insulation-related scientific terms


Thermal Conductivity: the lower the value, the better
Thermal Resistance: the higher the value, the better
Specific Heat Capacity: the higher the value, the better
Thermal Diffusivity: the lower the value, the better


Eco-friendliness values:


Embodied Carbon &amp; EPDs: carbon associated with making the material, read more




Insulating materials list

Cork sprayed



Thermal conductivity at 0 °C: around 0.043W/(m K)

Price: in France, suber-spray.com sells a bucket of 12Kg for 215€ which can cover around 6 square meters if applying 2 layers (recommended). So a cost of 36€ per square meter if applying 2 layers (without adding the cost of renting/buying the gear to apply it).

Pros: 


can reach any tiny part, great to avoid thermal bridges
fast to apply
apply the thickness you want by doing as many layers as you need
cork has many eco-friendly features, from production to destruction (recycling)
good sound insulation
can be the colour you like


Cons: 


usually more expensive than other insulating materials
need of gear to apply it: spray gun, compressor, drill with mixer


Example of usage on vans: suber-spray.com/spip.php?rubrique10

Tip: it's expensive, so you might want to use a cheaper isolating material for the flat parts to be isolated, and this for the smaller hard-to-reach/hard-to-insulate parts.

Note: This insulating material is often use in sailing boats.

Buy sprayed cork:


in USA: vipeqindustries.com 
in Germany: kork.net, see shop.korkmosaik.com/Korkgranulat
in Canada: sprayedcorkresolution.ca
in Italy: diasen.com
in France: suber-spray.com
in France: alsacork.fr




Cork rolls/sheets





Thermal conductivity at 0 °C: around 0.043W/(m K)

Exists in different thicknesses, but the ticker the less flexible.

Price: 2 mm thickness, 150 x 50 cm = 10€

Pros: 


good sound insulation
easy to buy (your local DIY shop)
cork has many eco-friendly features, from production to destruction (recycling)


Cons: 


mostly good for flat parts, otherwise time consuming (1 month, 4h per day for this Mercedes Sprinter youtube.com/watch?v=jAmyxwGx7aU)




Reimo X Trem isolator





Thermal conductivity: 0,020 - 0,034 W/mK (maybe around 0.03 at 0°C?).

Made of expanded polyethylene.

Sold as roll.

A product of Reimo, a German shop specialized in converted vehicles.

Data sheet - ihr-business-im-internet.de/anwendungen/ms-visucom/bilder/firma70/EN/zubehoerprofi/276.pdf

Price: 15€ per sq meter of 20mm thick - on campingshopwagner.de/product_info.php?info=p4340

Pros: 


simple to put
good sound insulation


Cons: 


may be difficult to obtain outside of Germany
mostly good for flat parts, otherwise time consuming


Buy Reimo X Trem isolator:


in Germany: campingshopwagner.de
in France: h2r-equipements.com
in France: reimo.com/fr
in UK: reimo.com/en




Aeroflex FIRO





Thermal conductivity at 0 °C: 0,034 W/mK

Sold as roll.

Data sheet: aeroflex.nl/en/products/aeroflex-firo.

Price: usually around 10€ per square meter when 19mm thick.

Pros: 


simple to put
good sound insulation
different thicknesses available
self glueing available
clean &amp; aesthetically pleasing


Cons:


mostly good for flat parts, otherwise time consuming


Tip: buy self gluing version to make things simpler.

Example usage on a Ford Transit : verpiss-dich-von-meiner-veranda.de/post/188/wohnmobil-isolierung/

Buy Aeroflex FIRO:


in Germany: ebay.de
in Germany: amazon.de
in Germany: hood.de




Armaflex (product of Armacell)





Thermal conductivity at 0 °C: 0,036 W/mK

Price: 10€ per square meter - ebay.de/itm/like/271583864528

Sold as roll.

Pros: 


simple to put, just cut to the shape you like
good sound insulation
different thicknesses available
self glueing available
clean &amp; aesthetically pleasing


Cons:


mostly good for flat parts, otherwise time consuming


Armaflex "AF series" - armaflex.com/www/armacell/INETFAQ.nsf/IDP/478F84DCC0A98ED6C1256CD700600264

Buy Armaflex:


in Germany: amazon.de
in Germany: ebay.de




Polyurethane foam



Thermal conductivity at 0 °C: around 0.025W/(m K)

Pros: 


good sound insulation
can expand 50 times more, i.e. a 1L of product can produce 50L of foam
can "fill" an empty area, filling even the smallest cavities


Cons:


can deform the car body when added
prevent future car body work (in case of accident) on parts having that foam
compresses over time? (TBC)


Polyurethane on Wikipedia 



Thin multi-layer insulation





Price: 5 à 15€ per square meter, according to number of layers &amp; materials used.

Pros: very thin.

Cons: 
 - poor sound insulation
 - fragile, so must covered by something almost straight away to avoid damaging it

Example usage: isere-evasion.com/isolation.htm

Example product: castorama.fr/store/Isolant-mince-multicouche-17-prod250073.html

Which country use it in converted van: 


France, no idea why. There are big debates on French van conversion forum about the actual effectiveness of this material.
USA, I think they use something similar called "Refletix".




Denim isolation





Pros: eco-friendly.

I just came across that material &amp; thought it probably was worth mentioning it as it's very eco-friendly. Further research on its characteristics is needed though.

hurriedyear.com/2016/02/10/insulating-a-sprinter-van/

ecohabitation.com/actualite/nouvelles/isolation-coton-recycle-choix-ecolo



More insulating materials


"Trocellen"
"KFLEX"
Sheep's wool




My biased opinion

I think the best is to use Reimo X Trem isolator (or Armaflex or Aeroflex FIRO) and afterwards add some sprayed cork in the parts you could not cover. 

By adding sprayed cork on these parts that couldn't be covered, you'll reduce the thermal bridges, or completely eliminate them if you put enough layers.



Resources


greenspec.co.uk/building-design/insulation-materials-thermal-properties/
trafic-amenage.com/physique/thermique-isolation.html (FRENCH)