If you are building up against the hillside agree with need for drainage and damp-proof membrane.
I wouldn’t leave a gap, as nature will try to fill it over time, and you'll probably get damp.
An ideal solution would be to build up a drainage layer, by lining with a silt-filtering “geomembrane” (otherwise the drainage gravel will silt up over time), porous drain at the bottom, gravel to the top; insulation layer; building your inner wall against that. So, you'd have several layers to build up at the same time, from the outside in:
silt-filtering wrap + gravel
insulation
damp-proof membrane
wall (tyres, or other mass structure)
This construction is therefore a multi-barrier approach – trying to intercept and direct water away from the building AND installing a barrier against the damp. A bit of a pain, but if you are using your building much, you really don't want it to be damp.
Damp-proof membrane can be pretty inconvenient as its is usually difficult to work with (big, heavy gauge plastic sheet) and you don't want to damage it while you are building each layer up and gradually unwrapping it. Have tape to fix tears...
Earth-filled tyres are a useful thermal mass building block, and turn a fairly tricky-to-sensibly-dispose of waste product into something useful (a building block).
I find filling them very hard work. Other people have told me its about technique rather than simply strength, but I don't seem to have picked that up.
You could build a mass wall using rammed earth in formers, wacked in layers. Can be quicker (e.g. I think its quicker & less exhausting to use a manual wacker plate, on earth layers, than a sledge hammer in individual tyres – I've personally tried the tyres but not wacking a rammed earth wall, so that's just my guess).
As you probably realise, Ben Law's house is timber frame with straw-bale infill. This is a different principle to the earthship principle: insulated building walls vs: mass walls. I believe Ben's walls are lime rendered; his roof has a large over-hang to keep water off.
What is your building orientation / aspect? - could you be on for an “earthship-type” passive
solar heating, heat storage in a massive back wall design? Check:
http://www.groundhouse.com/ (its called “groundhouse as it is not a “Mike Reynolds-approved earthship design)
http://www.lowcarbon.co.uk/earthship-brighton
Mike-Reynolds “approved” earthship design (in France):
http://web.me.com/gilliantrott/Site/The_Construction.html
the earth-ship originator (Mike Reynolds):
http://earthship.com/
I don't think the Americans usually use damp-proof membranes, as their original structure were above ground (with an earth bank at the back, in relatively dry climates).
The Brighton and Brittany buildings are both in temperate / wet climates, so both had complete damp-proof membrane wraps, heavy-guage polythene, which completely lies under the floor, up the back wall, to the roof (which is at ground level at the back), all in one sheet.
Daren & Adi have produced a brief book:
http://www.groundhouse.com/groundhouse-build-cook-book/
(Yes, there is a review by me, for a
permaculture magazine - its a nice, quick, book). There is a lot of detail tucked away on their website though + videos, so tuck in! (I'm credited somewhere for the
solar “plumping”, which sounds rather nice, but is supposed to be “plumbing”. I did the underfloor heating too, which we thought would be sensible, since were not sure how the passive nature of the structure would perform in that climate.
The Brighton Earthship & Brittany Groundhouse also have insulative “break” layers at the back, so that you have a mass wall on the inside and are not trying to heat up the whole hillside behind. The material used was rigid sheets made from spun recycled glass, “foamglass”. High compressive strength.
In a drier climate, southern Spain,
http://www.earthship.es/welcometoearthship, I think Dave & Laura didn't use a damp-proof membrane. Time will tell!
The “render” used on the tyres, all over in fact, is adobe (i.e., mud, sand and binder, such as chopped
straw). Check internet for “recipes”. Quite a thick layer, adding to wall mass, useful for sucking up heat in the summer, and releasing it in the winter.
A green roof can be pretty heavy as will soak up a lot of moisture at times.
Both Earthship Brighton & Groundhouse Brittany included their water storage in containers in the hillside in the back – so that rain water from the roof could gravitate to them.
To keep the roof structure light, and ensure less silty water (which is potentially used for drinking), one used steel, the other a membrane over plywood.
The opening sky-lights at the back are sensible (& multifunctional). Obviously allow light in, but also allow air flow (when open). So, on sunnier days, hot air generated by the large sough-facing windows naturally ventilates though the building out through the sky-lights, creating a coiling breeze.
Hope that helps.
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