Install the black 6mil plastic first. Check out the grade of your yard to see where the water is draining from. Make sure your gutters and downspouts are in good shape with no clogs and rain diverters are still there. Its odd for such an old floor to just start moving like that, normally by that age the floor is pretty stable. Have you changed anything to the house in the past two years. Sounds like you could also have a leak somewhere as well. If your subfloor is 2" off the floorjoist then you definatly have a serious problem.
Thanks Kevin. Is there a special kind of plastic to use? Does this just get laid down on top of the ground and get held in place with bricks/rocks or are there any kind of special considerations that need to be made?
The yard is almost entirely flat. We have high quality gutters in the front of the house that divert water through pipes out to the ditch. However we don't have any gutters in the rear and haven't installed any as because of our flat yard, there is no good place to divert the water to...unless of course I route the water all the way around the house and back out to the ditch through more pipes. Also, two newer homes have been built around us over the last 2-3 years. With both homes, they raised the grade for the foundations. One home is about 50 yards from the rear of our house and the second is 10 yards away. With the closest one, a shallow diversion ditch was created to help route runoff to the ditch.
As far as the old floor moving that much, the real problems seem to be in the rooms where the new h/w flooring (and OSB subfloor) was installed. I'm not sure if they pulled up the original h/w floors and put new OSB down before reinstalling the h/w, but I would guess not.
Leaks - we have none that we are aware of, and have had both a plumber and an insurance adjuster look and neither could find a leak.
So what type of person/company would you advise that I reach out to? Flooring expert, a foundation person, etc? Just not really sure who would be the best to seek advice from.