On the outside of the bales will be two coats of clay render, behind the timber cladding.  This makes the straw air tight (but still breathable), fire proof and moisture resistant.

The first coat is a clay slip, made of clay and water, and any gaps between bales etc are filled with a straw & clay mix at this stage.

The second coat is a mixture of clay, chopped straw and sand.

We first tried out the first coat, shown here.