My energy rater just suggested laying visqueen all over my crawlspace. He said it should be sealed with caulking at all overlaps of pieces and where it meets the walls / supports. This would lock the moisture from the ground under the plastic. You could also seal the cement / concrete walls with a "paint on" moisture barrier, as well. Then you'd likely need to insulate those HVAC ducts better. According to the energy rater, a great deal more energy is wasted through the crawlspace than people realize. So this wouldn't be a wasted effort.

If the joists are rotten, how do you know the flooring isn't? I know it's more work, but if you were to rip out the floor, it would be so much easier for you to work, I assume the crawlspace is fairly cramped.....I can think of nothing better than being able to stand up to work. In order to prevent moisture creeping up into new joists, you need suitable damproof course between the piers and the joists/bearers, but before you totally seal the underneath, shouldn't you ascertain where the moisture comes from? Presumably you're in a cold climate, and I know that dictates different construction methods, but I used to live in a warm, moist climate, (Australia) and the big priority was to VENTILATE sub-floor areas, to keep them dry.

How big is the addition? If the area is sinking it sounds more like a foundation issue. You dig too deep. Back fill and don't compact. Set the footing forms and pour. After the weight of the whole building is assembled, the soil settles. Along with it, you're addition. A competant contractor should be able to direct you. I have a daughter in Charlottesville. Great little city. Ron

I have another opinion. For a house you're only going to live in for 6 years, I wouldn't like any of this. My concern would be that the person/entity fixing the damage is not in a contractual relationship with you. This can lead to accountabililty issues. Sure, the structural engineer you hire has a contract with you, but what is he/she signing off on? That the work meets minimal standards? To me, it might not be the same thing as a contractor offering you solutions of A-quality and B-quality, depending on your budget and that you can make a decision on. I hope I'm explaining this well enough. My secondary concern would be that when you sell this house in 6 years, the market might not be so hot then, and you may have a whole host of items that you will have to disclose... may make the home harder to sell. (It might not). But there's a risk there. Thirdly, for a house you will only have for 6 years, you have no guarantee that you won't be suffering with fix-it items during those 6 years. It just doesn't sound like a great way to spend 6 years-with a money pit.