I've long had the idea of external-cladding existing buildings with straw bales, rendered, as the ideal way of super-insulation-with-restyling that's going to have to happen to the whole building stock. But how to support the considerable weight and thickness of the straw-bale skin? Katymac's project has got me thinking about that! The answer (as it often is!) is railway sleepers and/or telegraph poles.

If the strawbales are to be supported off the ground, they'll need
Option A: a 450wide masonry plinth to keep the bales 200 or whatever above GL. Maybe that has to go right down to a new or widened found at normal found depth, 1m below GL - big job, lots of wicked concrete.
Option B: or maybe the Bldg Insp would accept that as the bales are non-structural, and subject to much movement anyway, they could be on a surface strip of concrete? Following topsoil removal, of course. The bales would need to be tied back to the existing wall to prevent toppling outward, but in such a way that the bales wouldn't put eccentric down-load on the walls, as they settle or heave.
Option C: or maybe on railway sleepers resting on rammed no-fines fill (e.g. clean scalpings) in a topsoil-depth trench. The sleepers' laid-flat height would raise the bales 150 above GL and along with the scalpings would form an effective DPC, though there could be a layer of polythene in there too.
If essential to solidly support the strawbale skin in-unit with the existing wall, instead of putting it on equally deep founds and lots of underground masonry (Option A),
Option D: the bales could be supported on a platform cantilevered off the existing wall just above GL. The platform would consist of pairs of sleepers laid flat side by side - about 500 wide x 150 thick vertically. These would span between heavy cantilever brackets fixed to the wall. But actually that's not possible - the overturning force on the wall from a series of such cantilever brackets would be enormous.
Option E: it would be OK if the load was placed on the outer skin as a simple down-load, without the rotational element. The corners (and internal angles) of the building are key to this. Take the southwest corner. The pair of sleepers along the south wall actually lap over the top of those along the west wall, and they're bolted face-to-face together where they lap. Each pair then holds the other pair flat - all rotational effect is contained within the stiff L-shape that you've created, that will stay flat. All that remains is down-load and there's several ways that can be easily transfered to the centreline of the outer skin, both at external angles (corners) and at internal angles. This works where angles are close enough together for sleepers (or extra-long points sleepers) to span between. For longer distances, telegraph poles, but obviously more difficult to bolt rigidly together.
Option F: For yet longer walls, no alternative but ground-bearing support piers on deep founds - but that's nothing compared to a continuous plinth.
Option G: in Katymac's case, if her barn turns out to have steel columns @ 3.5c/cs, then probably true cantilever brackets could be welded, bolted or otherwise hung from them, or strapped round the masonry piers that surround them. Long point sleepers or telegraph poles would be needed. The corners would be done as Option E, the middles of the gable ends as Option F - maybe at the flanks of french doors?

So you'll clear B or C with the Bldg Insp? If he says no to shallow founds, then E and F are the gd option, and sooner or later you're going to need a helpful kind of structural engineer - he'll make it clear to you. If he disputes "there's several ways that can be easily transfered to the centreline of the outer skin" then I'll give him a talking-to!
At last we can write steel columns out of the plot!

Throwing away 26m2 of useful floorspace, which exists, and would cost at least £20,000 to build, at conventional tender prices. Also, the 150mm thick existing blockwork will provide some thermal massiveness if left exposed internally, internal straw bales less so.

If I insulate internally then I have to plaster & render - I can only afford one or the other

Posted By: MartianThere should be no need for more than about 250mm of concrete in the bottom for a lightweight wall like this.

Why use any concrete at all? Haven't you missed the point of strawbales? It's about getting away from this rediculous addiction to Ordinary Portland Cement.

Well, Martian as one of Them, do you think a shallow found, accepting a bit of heave etc, wd be OK for the straw bale skin?
Another way of tieing back, which might make the hazel rods unnecessary, wd be say 300w galv EML or maybe stronger mesh, chicken wire or even D49, screwed and washered to the wall and turmed out horizontal as continuous skirt built into every or every alternate course of straw bale. That's how Cotswold freestone (apparently dry jointed) facings are attached to their blockwork backing - nice and flexible.

Have you thought of rammed tyre foundation? After tests by Aamazon nails, thet discovered the filled tyres to be stronger than concrete. Also, at their build of the meeting room for Eco building society, they managed to get permission for shallow foundations due to the straw bale light loads.
Everyone, be so careful with your external south west facing walls, especially if you have windows and openings. Make sure the overhang is even more tham 500mm and consider cladding with larch that face. The driving horizontal rains I am facing is causing me to look at cladding now.This is Rachel by the way- I couldn't log in!