Moisture-adaptive vapor barrier for residential/office containers?

We have been able to take over a residential/office container that we want to install as a holiday home. It is heated with gas but sparingly. Now the insulation is causing problems because there are contradictory opinions.

We wanted to create a cassette wall inside the metal wall of the container with wooden slats and fill the spaces with Thermo Jute insulation boards. Then chipboard panels are applied, which are covered with cork as a vaporretarder. Now we hear it should be a vaporbarrier so that the jute insulation does not soak up water. But if moisture does get into the spaces, nothing can escape anymore. This is also warned against.

Now I have read about moisture-adaptive vapor retarders. In particular, the following statement seems suitable for a water vapor-impermeable metal wall: "If the exterior side of the components is impermeable to water vapor (diffusion-tight), the material creates a high potential for drying back, which is often essential for protection against building damage." (energie-fachberater-de) Then a moisture-adaptive vapor retarder would be a MUST in our case.

But regarding the mode of operation, the explanations on the internet seem contradictory:
"This causes that in winter, when the moisture pressure on the construction is greatest, the vapor retarder allows almost no moisture to enter the component." (wissenwiki)
"In winter, with low relative humidity, they are strongly diffusion-retarding" (wissenwiki) –> [high moisture pressure = high humidity or not?]
"If the amount of water vapor ready for diffusion is too large, the resistance of the materials increases and protects the components from being exposed to too much moisture." (sanier-de)

Our scenario is that in winter, with sparing heating, condensation can occur due to exhaled air and cooking/showering. A vaporbarrier would prevent the insulation from soaking up moisture but also would not allow moisture to escape from the wall construction. A moisture-adaptive vapor retarder would then close and only become more open again when it is warmer. That would be the optimal solution in our case. Or is there a flaw in this reasoning somewhere?

Many thanks for any feedback!
Michael

Hello Manfred

Thanks for the info. An external insulation would probably exceed the project scope. I also find relying entirely on a moisture-adaptive vapor retarder risky. But I find an aluminum vapor barrier just as risky with open-cell insulation. We have actually excluded closed-cell insulation like PU because we want it to be as natural as possible.

In the article you linked, there is the variant of ventilated insulation:

This insulation is not applied directly to the wall on the inside, but maintains a distance of up to 5 centimeters from it. An air layer forms there, which circulates with the outside air through slots in the outer shell. This prevents mold, even if the insulation materials are not waterproof.

Would that be an option? That is, to have a 2-3 cm wide air gap between the metal wall and the jute insulation boards and a vapor retarder made of cork between the jute insulation boards and the interior? So roughly like this:

Metal wall | Air | Jute insulation | Cork | Interior

It is basically like cavity wall construction.

Therefore, I would insulate the thing like a normal house with a vapor barrier for the insulation so that it does not absorb moisture from the inside and create ventilation slots at the bottom of the outer wall like with facing bricks to dry the insulation.

Ah thanks for the tip. Instead of Thermo jute insulation boards, we would then have to use Isover Ultimate core insulation boards. They are specially designed for cavity walls and water-repellent. Apparently, the air layer is not even needed then. However, I also found a comment from Dipl.-Ingenieur & Architect (Dieter Ehlers) who says:

I would consider a container like a steel boat hull. Shipyards insulate the inside with a PU foam insulation. It expands to a thickness of 5 - 10 cm. This is followed by a ventilated cladding made of wood, plywood, wood composite materials, or similar. This prevents condensation from forming on the hull, and if moisture forms behind the cladding, it should dry out again when heated. Whether this works well in every case, however, depends greatly on professional installation and ventilation.

There is no mention of a vapor barrier. And we want to do without the plastic foam stuff. Instead of PU foam, we then take water-repellent core insulation boards and maybe Thermo jute insulation boards on the ceiling. They have more than twice the heat capacity, and hardly any moisture accumulates inside the container roof anyway.

The whole thing then looks like this:

Metal wall (with ventilation slots) | Air layer (optional) | Core insulation | Vapor barrier (optional?) | Wall paint/wood/cork | Interior