Straw Bale Floor
This was a completely experimental technique which seems to have worked well. Since straw bales in walls can double as both insulation and structural support then why not the same in a floor... Building a conventional timber floor with joists uses a lot of sawn timber, a fair bit of work and joists can create 'thermal bridges' through insulation placed between them.
The ground was roughly leveled and compacted with an small excavator before construction started. The base of this hole was covered with builders damp proof plastic (i'm not certain that this was really needed, maybe gravel would have been better). On top of the plastic the ground was covered with scrap wooden pallets. On top of the pallets a single thickness of straw bales was laid right up to the walls and gaps at the edges were stuffed with split bales.
The floor boards were laid directly on top of these straw bales. In my case the floorboards were reclaimed crate lids, made up in 6' x 4' (2m x1.5m) panels. I used odd scraps of wood under the corners and edges of the pallets and screwed the palettes down onto them. This just means that when you step on one palette and it sinks very slightly, the ones next to it move together.
If you were doing this with regular floorboards rather than pallets I'd suggest something like laying 2"x1" (50 x 25mm) battons flat every 2'(60cm) or so. Lay the floorboards on top and fix down with some fairly heavy duty screws.
This was very easy and cheap to do. It made a floor that is not completely flat or level although it is completely practical. It is also just slightly springy, which I quite like. Maybe it moves at most 1/2" (12mm) under a person's weight. The only problem with this seems to be by the walls the join between floorboards and plaster on the wall is a bit prone to cracking, which just loks a little bit untidy. The palettes under the straw are intended to provide an air gap, though it could potentially be a popular hang out for rodents. We didn't have a problem with this though. They don't like to burrow through lime plaster so being careful to plaster everywhere up and having a cat do the job fine for me.
Visionary architect Hundertwasser thought that it was unhealthy for us to walk of perfectly level floors as that wasn't what our feet and beings evolved for !