Damp sill in garage

[joano]

Hello!

Similar questions have been asked before, but I'm asking for some help anyway.

I'm planning to insulate my garage, but the construction isn't the best, with only a pressure-treated bottom plate on a concrete slab with construction plastic in between. The wind barrier before the panel is in the form of plasterboards. See picture. These boards are clearly moisture-damaged (deteriorated paper at the bottom). I've measured the moisture content in the bottom plate in various spots with a hobby meter, and it varies between 15-25%. The stud/batten that holds the panel isn't as damp, perhaps because it dries much more easily even though it's at the same height as the bottom plate.

So I have a few questions. Where is the moisture coming from? How long does it take for a bottom plate to dry in an uninsulated garage?



Question 1. The previous owner claimed that snowy cars were the cause, and it looks like the moisture on the plasterboard is coming from the inside, but shouldn't the bottom plate be dry now? There hasn't been a snowy car in the garage for a long time. Hence the next question: Could it be "old" moisture that is still in the bottom plate and hasn't had time to dry out? Could it be that the bottom plate is "sucking moisture" from outside? The concrete slab raises only about a decimeter from the paving stones, where snow obviously accumulates from time to time during the winter even if one shovels, and it's easy to shovel snow under the panel.

How would you waterproof this to a reasonable level?

Thanks in advance!

Best regards,
Jocke