I would be curious to see the engineering numbers on the compression of the sprayed in foam. Will it really compress? You can put a whole building on foam, under a slab.

I think I recall reading about this similar topic, somewhere too, whether it was a full bent raising with no top plate or a building with a top plate. Each side had positive points and it came to a stand still. Basically it was a personal choice as to what type of structure you wanted to deal with.

Use a natural building material, like clay, that shrinks more than the timber and don't worry about it.

Tim