Moisture damage to the sill in the basement - How to fix

[Jalkesen]

Hello.

I live in a split-level house built in 1989.
I am currently renovating a room that is against the outer wall, which is not below ground.

The basement consists of leca brick walls, but interior walls of wooden studs and glass wool insulation with gypsum as the surface layer.
I have now torn down the interior wall and realize that there is quite a high moisture content in the sill. I measure the relative humidity to be about 17-19%. I have also removed a piece of the sill and measured underneath with roughly the same values. Part of the sill was rotten but had the same moisture content as the rest of the sill. The attached picture shows more clearly how the construction looks where the wall and floor meet.

My belief is that the moisture coming under the platon mat is not ventilated out behind the baseboard but migrates into the sill.
The question now is, if the studs are to be redone, is it enough to fold up the tar paper on the vertical edge of the sill to protect it from this moisture? The bottom layer of drywall screws was rusty, which indicates that even the drywall (has/had) moisture in it.

Does anyone have tips and ideas on how to build this wall now that it is going to be rebuilt anyway?

 

[SueCia]

if the sill is rotten, it will probably need to be fixed anyway. Then can you put under the sill paper then or?

 

[Jalkesen]

if the sill is rotten, it will need to be fixed anyway. Then you can insert under sill adhesive then or?

There is sill adhesive under the sill, but I think the moisture is coming in on the vertical edge of the sill, not from underneath.