Consider a high and wide concrete wall of say 30 cm thickness, covered with a sheet of vapor barrier on one side, and a sheet of say 30 cm thick and effective insulation on the other side.
The vapor barrier completely blocks water vapor. The insulation lets water vapor through and leads away any condensed water downwards where it is drained away. The concrete lets vapor vapor through, albeit slowly.
The air on the vapor barrier side (the inside of a hypothetical building) is 25°C with 50% Relative Humidity, and the air on the insulation side (the outside of a hypothetical building) is 10°C with 100% RH. See illustration above which shows the wall from above.
According to some vendors of basement insulation, a setup like this will cause the concrete wall to become relatively dry due to vapor pressure towards the insulation in which the water vapor will condense and allow the water to drip down and then be drained away.
But I can't make myself understand how this process works from a physical point of view. I thought an infinite source of water vapor spreads out until all air is saturated with water, so I would expect the wall to end up having 100% RH.
Will the wall end up relatively dry, at say 70% RH? If yes, how exactly?