[kach22i]

The warning I give people on barns is based mostly upon what I have read.

If animals were kept in there, the oils form their fur/skins gets absorbed into the wood. If you turn this into a human occupied space, you turn up the heat which warms up the wood, releasing the sent of long gone animals into the air. Sheep are reported to be the worst.

I can imagine that tobacco leaves would be leaving the same air born oily residue in the structure. You new space may end up smelling like a humidor no mater how many layers of vapor barrier or venting you employ.

I wish I knew of a test which would help you determine if this will be a problem.

Does the barn still smell a little like tobacco on a really hot day?

One of my favorite books on converting barns into living spaces shows a new independent building built within an existing old barn. The contrast is really striking and very pleasing to the eye. I'll scan some images if interested.