With proper air circulation timbers will last a long time. The barns you are referring to were probably 200 years old when they finally needed sills, right? PT won't last that long. Most of the damage I have seen at the sill and post bottom level has been from manure packed against it. Or from contact with the earth after a foundation failed. Again, the failure is after a couple of hundred years of frost heaving. I am going to be using a dry laid foundation on my next small shed. I don't know if you could get away with it on a larger building or not. If you look in Jacks book you will see different species of trees and which ones are suitable for decay resistance. Alas, there is no more chestnut, however we still have white oak. I don't even know if the new PT (ACQ?) is contact rated. I don't use any PT, plywood or other "modern" building materials. I did see a barn this summer that had a beautiful chestnut frame with bright green sills and repaired post ends of PT.
Sorry to wander, but I think that nature will provide most of our solutions, we don't need wood that's been drugged.
