just bought a foreclosure. i hope to live in it for a year then rent it. I'm looking for a good looking solution that is also idiot proof for a long time.

This is not a photo of my window but the exact same situation. The bottom sill is moldy and soft. There is no fan in the bathroom. I would indeed like to have a conversation with the person who thought this up. I presume its from the water inside but I haven't out it coming from outside.

Would the correct fix be to put in some pvc board on the bottom sill, caulk it and call it a day? The only other solution I can think of are those weird plastic floor transition pieces? I am willing to pay more for a more foolproof solution.

And no I haven't really pulled any boards to see the extent of the damage yet.