I have been considering the moisture issue of this system some more, and am concerned. As of right now I intend to use the double plank wall system I have designed for my shop with a natural wood interior and plaster exterior, with exposed frame inside and out.

My biggest concern is that the posts will sweat where the boards are joined into them. This would give great occasion for both the boards and the frame to rot. The greatest concern I have is for interior, rather than exterior, moisture.

The following things I have decided:
1. Abandon the foam skim coat inside the cavity. In this case I think that this would just trap moisture and facilitate rot. The wall should be able to breathe to a certain degree.

2. The clay and plaster on the outside should be more than enough protection for the planks as far as keeping moisture out.

3. Before the outside is plastered, some sort of flexible seal strip should be put into the channel to protect this region should the plaster gap around the posts (which it certainly will) This could be a modern material, or an old-fashioned material like cordage soaked in tar (I doubt I can get whale or seal blubber for the purpose) This idea inspired by old ships, which had the gaps between the wooden planks sealed thusly.

Now what I need to do is figure out how to prevent moisture from condensing around the posts, and how to generally prevent rot in this region and on the wood as a whole.

The best solution I can come up with for the first problem is ventilation. The old Bohlenwand houses in many cases have the original planks that have stood there for in some cases over 400 years, often totally unfinished and unprotected -and generally made out of Swiss Pine, not German oak. The reason I think is that moisture had no real reason to gather there, it had plenty of opportunity to go elsewhere. On the old farmhouses the attics were wide open, because they were used to store the hay. plenty of ventilation there. The moist air could escape between the floorboards and up into the attic.

Now obviously a modern home needs to be a little bit tighter than all this, or at least the average homeowner would want it to be. But with a well and cleverly ventilated house I think the problem could largely be alleviated. But I think that if there was adequate ventilation in key areas such as along the walls, heat loss could be minimized while allowing moisture to escape. A well ventilated attic would be needed for this to work, I doubt an open ceiling with exposed trusses would be very good here.

For the second issue, there are two things I could think of that might help a great deal:
1. The wood on the inside of the cavity is thoroughly sealed with tar.
2. The wood on the inside of the cavity is thoroughly covered with plaster or a lime wash.

The first approach would seal off the wood from moisture, while still allowing it to breath.

The second approach may allow vapor through, but the plaster would make it very difficult for mold or fungus to grow and rot the wood.