ᐅ Moisture-adaptive vapor retarder for residential/office containers?

We were able to take over a residential/office container that we want to set up as a holiday home. Heating will be done with gas but used sparingly. Now the insulation is causing problems because there are conflicting opinions.

We planned to build a cassette wall inside, attaching wooden battens to the metal wall of the container and filling the gaps with thermal jute insulation boards. Then plywood panels would be installed, covered with cork to act as a vapor retarder. Now we hear it should be a vapor barrier instead to prevent the jute insulation from absorbing moisture. But if moisture does get into the gaps, then nothing can escape anymore. This is also being warned against.

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

However, the explanations about how they work seem contradictory online:
"This means that in winter, when the moisture pressure on the construction is greatest, the vapor retarder allows almost no moisture to enter the building component." (wissenwiki)
"In winter, with low relative humidity, they are strongly diffusion-resistant." (wissenwiki) –> [Does high moisture pressure mean high humidity or not?]
"If the amount of water vapor ready to diffuse 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 minimal heating, condensation can occur from breathing and cooking/showering. A vapor barrier would prevent the insulation from absorbing moisture but would also not allow moisture to escape from the wall construction. A humidity-adaptive vapor retarder would close up and only open again when it is warmer. That would seem to be the optimal solution in our case. Or is there a flaw in this reasoning?

Thank you very much for any feedback!
Michael

Hello Manfred

Thanks for the information. External insulation would probably exceed the scope of the project. Relying entirely on a humidity-adaptive vapor retarder also seems risky to me. But an aluminum vapor barrier seems just as risky with open-cell insulation. We have basically ruled out closed-cell insulation like PU because we want to keep things as natural as possible.

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

This insulation is not applied directly on the wall from the inside, but maintains a distance of up to 5 centimeters (2 inches) from it. This creates an air gap, which circulates with the outside air through slots in the external cladding. This prevents mold even if the insulation materials are not waterproof.

Would that be an option? That is, to have a 2–3 centimeter (1 inch) air gap between the metal wall and the jute insulation panels, and a cork vapor retarder between the jute insulation panels and the interior? Something like this:

Metal wall | Air | Jute insulation | Cork | Interior

It is basically like cavity wall construction.

Therefore, I would insulate it like a regular house, using a vapor barrier on the insulation side to prevent moisture from inside being absorbed, and create ventilation slots at the bottom of the outer wall, similar to brick veneer, to allow the insulation to dry.

Ah, thanks for the tip. Instead of thermo jute insulation boards, we would then have to use Isover Ultimate core insulation boards. These are specifically designed for cavity walls and are water-repellent. Apparently, an air gap might not even be necessary. However, I also found a comment from a certified engineer and architect (Dieter Ehlers) who says:

I would consider a container like a steel boat hull. Shipyards line the interior with insulation made of PU foam. This expands to a thickness of 5–10 cm (2–4 inches). Then a ventilated cladding made of wood, plywood, wood composite materials, or similar follows. This prevents condensation on the hull, and if moisture builds up behind the cladding, it should dry out again when heated. Whether this works well in every case, however, depends heavily on professional workmanship and proper ventilation.

There is no mention of a vapor barrier there. And we want to avoid plastic foam materials. Instead of PU foam, we will use water-repellent core insulation boards, and perhaps thermo jute insulation boards on the ceiling. These have more than twice the heat capacity, and hardly any moisture accumulates from the inside on the container roof.

The whole setup then looks like this:

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