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I used 15mm poplar plywood panels for the complete construction. These are quite light, dimensionally stable and look nice
12.5mm beech plywood was used for the floor
Image Source: Opitec
Image Source: https://vanue.world/de/produkt/vanue-insulate-platte-19-mm-6-m²/
If you want to do a van conversion, you will find 20 youtube videos and 20 different opinions on how best to do it. Everyone has a different opinions on how to insulate a van or whether to insulate it at all. As a trained carpenter, I understand very well how to build a house, how to ventilate the wood, how to build physically so that the wood does not mold.
The problem is that a van is not a house and you can't compare the structure and insulation. The sheet metal shell is far too tight to allow moisture to escape like in a house. This means that any moisture that is in the van remains in the van. Either in the insulation, in the air or as condensation on the sheet metal. This moisture must now be removed from the van and there are different opinions on this.
You allow the moisture to condense on the sheet metal, run down and drain off to the outside at designated points. Many people install inorganic insulation that cannot become moldy. I used 19 mm Vanue insulation for this. On the floor and as the first layer on the walls. If the insulation gets damp, it's not a problem because the moisture can't get into the insulation.
The second option is to let the moisture into the insulation (As in classic house construction). here the moisture is absorbed by the insulation and released again later when the air is drier. I used sheep's wool for this. Sheep's wool can absorb moisture very well, keep it and release it back into the air at the right time. In addition, sheep's wool does not mold, detoxifies the air and is resistant to fungi and insects.
Now I have installed Vanue insulation as the first layer and sheep's wool as the second layer. My complete van conversion is designed in such a way that as much air as possible reaches the insulation. In this way, the insulation can absorb a lot of moisture, but release it much better later. For this purpose, I have always left 8mm of air between the wood in the ceiling cladding and stapled jute fabric in front of the sheep's wool in the complete conversion. This is the only separation between the insulation and the interior space