Wall cladding in the bathroom

So it is not SO deadly if the vapor barrier is not 100% tight. Of course, you should pay attention to it, but you can build it in a way that mistakes are forgiven. At least when it comes to mold. Much worse is in regard to wind and heat protection..

Best regards Joe

So it is not THAT deadly if the vapor barrier is not 100% airtight. Of course, you should pay attention to it, but you can build it in such a way that mistakes are forgiven. At least when it comes to mold. Much worse is it regarding wind and heat protection..

Regards Joe

Do you know that for sure, meaning are you an expert in this or do you rather believe it?

I spoke with a roofer who told me that warm (rather humid) air creates a slight overpressure and therefore escapes through a hole just like in a balloon at the hole. If the layer behind it is rather cold (e.g. in winter), the moisture from the warm air condenses. Condensate + insulation is already a worrying combination. In winter it may not dry out or not dry well afterwards and so mold could develop over time.

Is this scaremongering or quite justified?

Consider it justified. I have dealt with this topic myself for a long time. Heat protection has absolutely nothing to do with the vapor barrier...

In terms of airtightness, it is rather correct, and yes, you can design a construction so that it is less prone to mold. But that is not always that simple, especially not for non-experts. You also don’t necessarily choose the structure just so you can be sloppy with the vapor-retarding layer.

I think it is not possible for a vapor barrier to be 100% airtight. In my opinion, 100% is practically not achievable, hardly (durably) doable, BUT the whole thing must be almost 100% airtight. Any carelessness can be expensive later. The airtightness layer MUST be created as perfectly as possible. Better too meticulous than too sloppy.

How big a leak can be and what size is still acceptable cannot really be answered. It depends entirely on the room conditions.

I would not risk anything.

The vapor barrier is indeed an airtight layer with regard to heat protection. So it is related to that.
So if you have a ventilation gap behind the insulation, it is not too critical if you have small holes in the vapor barrier. Here the moisture carried in can be "vented" to the rear through the air. It becomes problematic if it is airtight outward after the insulation as well. Then the moisture cannot escape and the insulation gets soaked.
But there are also moisture-variable membranes here, which allow diffusion back to the room side again to prevent mold.
It is often calculated that a 100% vapor barrier is installed to lay OSB boards in the top floor ceiling. But they are also vapor-tight in the broadest sense. Here the moisture cannot pass further and problems are preprogrammed.

Overall it is correct that the vapor barrier (whether as a foil or otherwise) should be as airtight as possible. However, there are assemblies that have fault tolerance and others that do not.

Regards Joe

I would probably stick drywall to the wall with bonding adhesive. However, the wall must not be too damaged for that.

In my case, it is exclusively about interior walls.