What is the cheapest material to build an outside wall of a house with?

I think its probably straw bale, I am looking into this myself. Only issue is that you'd need to lime plaster it, but if the finish doesnt need to be perfect I am thinking about doing it myself

Around here the cheapest is stone (actually basalt rock), collected for free and built with concrete using 0-10mm aggregate. But it depends what is available locally and how you cost time and transport.

Angon a mo - is tony discussing an outside wall or (a buildings) external wall

In my book an outside wall could be in the garden or boundary wall etc.

If its an external wall to a building then I would modify my answer above to concrete block and EWI, because of ease of construction and the need for flat surfaces, however collected (free) stone would still be cheaper if labour was not an issue

14" isn't unduly thick, for any kind of modern wall. Still, you can have a wall that thick and still minimise 'footprint' (i.e excavated/concreted area) by thin block or stud structural wall inboard, with no found needed under thicker lightweight EWI outboard.

You're right

Posted By: ringiHow do you stop the cold bridge at the floor with bales, as I expect they need to be supported under their full width with concrete, so stopping a passive foundation being used?

I have a bale passivhaus on a passive slab, so QED. There's no 'standard' solution. What we did was to build a passive slab with a separate ring beam - a lot like http://www.viking-house.co.uk/images/LLf.jpg The ringbeam is 260 mm wide and the EPS either side are 125 mm external and 100 mm internal. Then we built a timber ladder structure out of 4x2 that is bolted to the ringbeam and overhangs the EPS on each side to the width of the bales (470 mm). The ladder was topped with 18 mm ply and the bales rest on that. Somebody on the Navitron forum built an experimental rig that demonstrated the 18 mm ply was plenty strong and stiff enough to do the job of supporting a house. I'm embarrassed to say that I forget who! The ladder is filled with technopor foamed glass insulation.

Given enough land, and being allowed enough ridge height, so I can have the “ground” floor above the level the bales start at, they could be a nice system.

Ground floor above the bales? Don't understand. You want the bales above the floor so that water spilled on the floor doesn't track into the bales.

Posted By: ringiHow did you get building control to agree to it?

Engineer's signature - same as any other structural question.

Am I right to assume that the bales provided all the support to the roof and 1st floor?

Not quite. In an area along the south side where ground floor windows are close together they get some help from the 4x2 window framing.

How did you cope with the bales compressing over time while keeping air tightness?

Bales are 'precompressed' during building and don't compress much after that. The plaster skins restrict further movement; the wall is really a composite panel. Indeed I believe the engineer's calculations disregard the support from the bales and only consider the strength of the plaster skins. Given that on some houses there are gaps between the plaster and the foundation, this isn't entirely realistic! But it seems to satisfy bureaucracy.