Think of all the people who have spot welded new sections in their floor and don't have them sealed properly. what is the moisture going to do when it gets trapped in the seams?
From my observations, the older trucks and cars were not really designed well in eliminating migrating moisture. Just about anything that is sandwiched between the floor sheet metal and the final substrate, i.e. carpet will in fact trap moisture. I think the solution is to properly seal the vehicle.
