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Polypropylene rice bags or feed bags filled with clay/ soil or insulation. They last under the plaster for a very very long time. The construction is decades old with these bags, and similar structures have lasted hundreds of years with less modern material. If plastered well, maintained and done properly, there will be no water damage.

Thankyou. Very interesting concept. I've looked at hay bale & mudbrick but this looks better in several ways. Your designs also look great.
Local regs would be a prob here, but many parts of the planet it would work.
I wonder about hemp chaff as bag filler? maybe mixed with clay etc?
my mind is racing :-0

yaya!! Inspiration Come! <3