Why can't humidity from the living space enter the insulation, but humidity from outside is allowed to enter the insulation?
Warm air holds more water than cold air. Everyone notices this in winter. The values or the amount of water that air can hold at x degree can be looked up. I will take example numbers for explanation. Your living space is at 20°C and the air contains 1 liter of water per m³ at 60% humidity. Now there is a leak in the vapor barrier/vapor retarder and the warm air from the living space can escape into the insulation. On the way outside, the warm air eventually meets the cold air. The temperature of the warm air keeps going down until it balances out. Now you have to remember that the air still contains 1 liter of water. But because cold air cannot hold as much water as warm air, the water has to go somewhere. This water then precipitates as condensation. Depending on the outside temperature and humidity, about half a liter or more or less precipitates as water.
Moist-cold air entering the insulation from outside is not a problem. Why? -> Because air can only get warmer when it moves from outside to inside, it can only absorb more water. That means the water does not even think about precipitating as condensation because there is always more space for water in the air.
The same can be observed with thermal bridges. For example, warm air passes along the cold wall and cools down. But it has stored too much water for the temperature and so water precipitates on the wall (the coldest point). That is how mold forms. If you have walls that are too cold and tend to mold, it is best to insulate these walls from the outside. Because then the problem of the cold wall on the inside is usually solved. That means the wall is no longer so cold inside that water can precipitate on it.
Why do we use vapor retarders more nowadays than vapor barriers? Because nature always seeks absolute balance, the water vapor in the insulation sometimes wants to return to the apartment. It is nice if the foil allows this. Therefore, today more emphasis is placed on allowing some diffusion. But one should be aware that this can by no means compensate for leaks or gross processing errors. It is only somewhat less sensitive and more forgiving.
The adhesive tapes hold for 25 years+ with proper installation. Some manufacturers even specify 50 years.
If that is too risky, you can look for alternatives. There are insulation materials, for example, that do not require vapor retarders. Whether that really works and is good, I do not know. One would have to look into it.
You could also use a vapor retarder made of paper. These are often actively used in timber-framed houses.
Often OSB boards are used as vapor retarders or foil. They are joined with tongue and groove (I do not know if glued or not) and then taped again at the joints with adhesive tape.
- Have you ever consulted an independent building expert about this?