This paper investigates the fire resistance and insulation properties of earth plaster applied to corncob thermal insulation boards. The corncob boards were produced using a hydraulic heat press with a water-starch ratio of 10%, providing a unique composite material for thermal insulation. The study focuses on the fire resistance of the earth plaster when applied to corncob boards and the thermal performance of the boards themselves. The plastered corncob boards were subjected to flame exposure tests, in which the impact of fire on the plaster layer and the underlying insulation was analysed. The thermal degradation, material strength, and heat transfer properties were carefully monitored to understand how the earth plaster influences the fire behaviour of the corncob-based insulation. In addition to fire resistance, the insulation properties of the corncob boards were evaluated. The thermal conductivity and insulation efficiency were measured under controlled conditions, comparing the performance of corncob boards both with and without the earth plaster layer. The findings demonstrate that even a thin layer of earth plaster can significantly enhance fire resistance by delaying or preventing ignition and reducing heat transfer. The compressed corncob boards and the loose corncob exhibited enhanced thermal insulation performance, making them promising options for circular building materials. This research demonstrates the potential of utilising agricultural by-products, such as corncob and combined earth plaster, to develop sustainable, fire-resistant insulation solutions for the construction industry.