This paper deals with the development of bio-based thermal insulation panels from sunflower pith particles, which are an agricultural by-product, in the view to compete with the current petroleum-based counterparts. Sunflower pith is glued to a native starch, then moulded and dried. Three different starchy binders were tested: potato starch, wheat starch and pea starch. The results showed that the pea starch blocks were more resistant to water than the others. Formula optimisation showed that the right balance between efficiency and savings was achieved with a 7.5% (w/w) binder dilution in water, and a 10% (w/w) binder in the block. To further protect the bio-based material from liquid water and ensure its natural breathing, a waterproof coating was also investigated by selecting an alcoholic pine resin solution at 100 g/L and a bio-based commercial aqueous solution at 170 g/L, which did not adversely affect the water vapour diffusion properties of the pith blocks. The pith block now has very good thermal, mechanical and waterproof properties, close to those of a commercial expanded polystyrene panel.