Gabel,

When dealing with a half timbering system, I am naturally assuming that one will be copying some old world European style of building such as those pictured in a previous post. In this case the boards will have to interface with very long braces, and are going to have to be keyed into them. It does appear however that in the European Bohlenwand the planks are not keyed into any horizontal members, and the top faces of horizontal members, or at least the ground sills, are sloped outward to let water drain off.

I like the idea of the borads forming a 'floating panel' between the posts. They would be free to slide back and forth and so would not be compromised in any way by post movement.

On the subject of the foam anti-draft layer of spray foam:
I have thought about this, and decided that any sprayed or plastered on substance that is intended to resist draft is going to crack and break as the wood moves seasonally. What I am now considering instead is using housewrap on the inside of the cavity instead. It would stapled up against the outer plank layer, against the sides of the posts, and could wrap around into the rabbets for the from planks to ensure no draft gets through. Or, alternately, foam could be sprayed along the edge of the wrap on the post to do the same. The housewrap would be able to move freely with seasonal changes in the wood without loosing its seal, and it doesn't seem quite as risky concerning moisture issues as the foam.

Gabel you also mentioned about building regulations, that of course is the big question isn't it. What problems do you foresee that should be addressed?

The diagram I made uses 7" deep walls, leaving a 3-1/4" cavity for insulation. 3" of extruded polystyrene foam board would be about R-15, compared to a fiberglass which for 2x4 is r-13 and 2x6 is r-19. There is less thermal bridging in this than is a stud wall, and when you factor in the insulation and thermal mass properties of the planks and clay you will probably still come to an average of about r-15. I haven't done any real calculations, that's just a rough guess. This system would perform at least as good as a standard 2x6 stud wall, and of course making the posts deeper would push the r-value up even more.

I will do some figuring and put up an r-value comparison later